CHEM 351@ INORGANIC POLYMERS AND ELECTRON DEFICIENT COMPOUNDS 2.pdf

Polymer

1. CHEM 351
INORGANIC POLYMERS AND
ELECTRON DEFICIENT COMPOUNDS
10/21/2014 1

2. COURSE OUTLINE
1.Electron-Deficientcompoundse.g.Boron
compounds.
2.InorganicPolymers,Rings,Cagesandsilicates
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3. REFERENCES
1.ConceptsandModelsofInorganicChemistrybybodiee.
Douglas,DarlH.McDanielandJohnAlexander
2.ConciseInorganicChemistrybyJ.D.Lee.
3.ChemistryoftheElementsbyN.N.Greenwood,A.Earnshaw.
4.Non-MetalRings,CagesandClustersbyJ.DerekWoollins
5.AdvancedInorganicChemistrybyF.A.Cotton,Geoffrey
Wilkinsonetal.
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4. INTRODUCTION
•Theformulasandstructuresofseveralkindsofcompoundscan
bepredictedwiththeaidofthevalencebondtheory,molecular
orbitaltheory,theoctetrule,andthe18-electronrule.However
theelectronicandmolecularstructuresofonelargeclassof
compoundscannotbeunderstoodintheseterms.
•AttheverytimeGN.Lewisproposedtheelectron-pairbond,
AlfredStockwaspreparingaseriesofcompoundswhose
formulasgavenohintastotheirstructuresandwhose
structure-oncedeterminedcouldnotbeaccommodatedbya
simplevalence-bondmodel.
•StockwasabletoprepareandcharacterizeB
2H
6,B
4H
10,B
5H
9,
B
5H
11,B
6H
10andB
10H
14.Thesecompoundscouldbedividedinto
twogroupshydrogen“rich”ofgeneralformulaB
pH
p+6;anda
hydrogen“poor”offormulaB
pH
p+4.Athirdseriesofverystable
anionsB
pH
p
2-
(whichcanbethoughtofasderivedvia
deprotonationofB
pH
p+2)hasbeenprepared(p=6to12)
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4

5. ELECTRON DEFICIENT COMPOUNDS
•Thesearecompoundswithtoofewelectrons
foraLewisstructuretobewrittenwithanoctet
aroundthecentralatome.g.compoundsof
group1,2and13elementsoftheperiodictable
especiallycompoundsofboron.
•Electron-deficientcompoundsarecompounds
inwhichthenumberofvalenceorbitalsexceed
thenumberofvalenceelectrons.(e.g.BH
3,
B
2H
6,AlH
3)
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6. ELECTRON PRECISE COMPOUNDS
•Compoundswiththecorrectnumberofelectronpairs
forbondformationwithnoneleft-overasnon-bonding
electronpairsonthecentralatom;i.e.allvalence
electronsofthecentralatomareengagedinbond
formation.E.g.carbonandcarbongroupofcompounds
(group14)(CH
4,C
2H
6,SiH
4,SnH
4)
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7. ELECTRON RICH COMPOUNDS
•Compoundswhichhavemoreelectronpairsthanare
neededforbondformationwiththeextraelectronpairs
beingpresentasnon-bondingelectronpairsonthe
centralatom.E.g.compoundsofgroup15,16,17
elements(NH
3,H
2Se,H
2O,H
2Te,H
2S,PH
3,HCl,AsH
3,HF,
SbH
3,HBr,HI)
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8. BORANES
•Boranesarecompoundscomposedsolelyofboronand
hydrogenandmaybeneutraloranionic.
•STRUCTURE
•Boranesfallintofivestructurecategoriesthemost
importantofwhichare:
(a)Closo-B
nH
n
2-
derivedfromtheGreekword
meaning“closedorcaged”.
(b)Nido-B
nH
n+4derivedfromtheLatinwordmeaning
“nest”.
(c)Arachno-B
nH
n+6derivedfromtheGreekword
meaning“spider’sweb”.
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9. CLASSIFICATION OF HIGHER BORANES
(Electron counting)
•Forboranes,thebuildingblocksfromwhichthe
deltahedronisconstructedisassumedtobeaB-Hunit.
•TheelectronsintheotherB-Hbondareignoredinthe
countingprocedurebutalltheothersareincluded
whetherornotitisobviousthattheyhelptoholdthe
skeletontogether.
•Byskeletonwearereferringtotheframeworkofthe
clusterwitheachB-Hgroupcountedasaunit.
•IfBatomhappenstocarrytwoterminalhydrogenatoms
(H
T)onlyoneoftheB-Hbondsistreatedasaunit.
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10. E.g.1 B
4H
10≡ (BH)
4H
6
If the structure or shape is
4(B-H) units 4 x 2e-= 8e-s 4 BHB’s
6H 6 x 1e-= 6e-s 2 additional BHTs
14e-s1 BB
7 bonds
E.g.2. B
5H
11≡ (BH)
5H
6
5(B-H) units5 x 2e-= 10e-s
6H Atoms 6 x 1e-= 6e-s
16e-s
8 bonds
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11. WADE’S RULE
1.Boranesofformula[B
nH
n]
2-
willbefoundtohavethe
CLOSO(caged)structurewithaBunitateachcorner
ofacloseddeltahedronandnoBHBbondsinthe
closostructure.
Suchstructureareknowntohave(n+1)skeletal
electrons.
Theseseriesofanionsisknownforn=5to12
Trigonalbipyramidal[B
5H
5]
2-
Octahedral [B
6H
6]
2-
Icosahedral [B
12H
12]
2-
NB:Theclosohydroboratesandcarboranesareoften
thermallystableandfairlyunreactive.
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12. 10/21/2014 12

13. 2.BoraneswiththeformularB
nH
n+4(BH)
nH
4
theNIDO(nest)structure(i.ecanbeviewed
asaclosostructurewhichhaslostonevertex
orcornerandmayhaveaBHBoraBBbond.
Thecompoundsinthisseriescontain(n+2)
skeletalpairsofelectronse.g.B
2H
6,B
5H
9,
B
6H
10,B
10H
14etc.
Ingeneral,thethermalstabilityofNIDO
boraneisintermediatebetweenthatofcloso
andArachnobaranes.
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14. 10/21/2014 14

15. 3.TheboraneswithformulaB
nH
n+6≡(BH)
nH
6the
Arachno(spider)structureandarelikeclosoboranes
lesstwoverticesandmayalsohaveBHB’sbonds.They
have(n+2)corneredpolyhedronrequiring(n+3)
skeletalelectrons.Theyarethemostunstable.E.g.
B
4H
10,B
5H
11,B
6H
12,B
8H
14,n-B
9H
15,i-B
9H
15
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16. 10/21/2014 16

17. 4.TheboranesofformulaB
nH
n+8≡(BH)
nH
8the
HYPHO(net)structurehavethemostopenstructure
inwhichtheBatomoccupythencornersofan
(n+3)–corneredpolyhedronrequiring(n+4)skeletal
electronpairs.
Noneutralboraneshasyetbeendefinitely
establishedinthisseriesbutknowncompoundsof
B
8H
16andB
10H
18mayprovetobehypho-boranes
andseveraladductsareknowntohavehypho-
structures.
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18. 5.TheboraneswithformulaB
nH
n+10≡(BH)
nH
10theKlado
structure.Theyhave(n+4)corneredpolyhedronrequiring
(n+5)skeletalelectrons.
Linkagebetweentwoormoreofthesepolyhedralborane
clustersisindicatedbytheprefixCONJUNCTO-(Latinname
for“Ijointogether”).TheyhavetheformulaB
nH
m.Atleast
fivedifferenttypesofinterconnectedboraneclustershave
beenidentifiedandhavethefollowingfeatures;
(a)FusionbysharingasinglecommonBatome.g.
B
15H
23.
(b) Formation of a direct 2-centre B-B σ–bond between 2
clusters e.g. B
8H
18i.e. (B
4H
9)
2,
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19. (c) Fusion of two clusters via 2B atoms at a
common edge e.g. B
13H
19, B
14H
18, B
14H
20
(d) Fusion of two clusters via 3B atoms at a common
face; no neutral boraneor boraneanion is yet known
with this conformation but solvated complex
(MeCN)
2B
20H
16.MeCN has this structure.#
(e) More extensive fusion of 4 B atoms in various
configurations e.g. B20H16, B20H.182-
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20. SUMMARY
TYPE FORMULASKELETAL
ELECTRON PAIRS
CORNERS OF
POKLYHEDRON
EXAMPLES
Closo [B
nH
n]
2-
n+1 n [B
nH
n]
2-
to[B
12H
12]
2-
Nido B
nH
n+4 n+2 n + 1 B
2H
6, B
2H
6, B
2H
6
ArachnoB
nH
n+6 n+3 n + 2 B
4H
10, B
5H
11
Hypho B
nH
n+8 n+4 n + 3 B
8H
16, B
10H
18
Klado B
nH
n+ 10 n+5 n + 4
ConjunctoB
nH
m B
8H
18, B
15H
23 etc.
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21. Using Wades Rule
E.g.
(i)[B
5H
5]
2-
Closostructure
5(B-H) 5 x 2e = 10e
-s
overall charge 2e-= 2e
-
s
(5+1)e-pairs 12e
-
s
i.e. From the formula [B
nH
n]
2-
with (n+1) pair skeletal
electrons
(ii) B
5H
9 ≡(BH)
5H
4 Nidostructure
5(B-H) 5 x 2e = 10e
-s
4H 4 x 1e = 4e-s
overall charge 2e-= 2e
-
s
(5+2)e
-
pairs 14e
-
s
From B
nH
n+4with n+2 skeletal electron pairs
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22. STRUCTURAL CORRELATION
VeryusefulstructuralcorrelationbetweentheNido
andArachnocompoundsisbasedontheobservation
thatclustershavingthesamenumberofskeletal
electronsarerelatedbyremovalofsuchB-Hgroups
andtheadditionoftheappropriatenumberof
electronsandHatoms.
Thistypeofprocessrelatestheoctahedralcloso
[B
6H
6]
2-
aniontothesquarepyramidalnido-B
5H
9
boranewhichisinturnrelatedtothebutterfly-like
arachno-B
4H
10.
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23. 10/21/2014 23

24. NOMENCLATURE
1.Neutralboronhydridesarenamedborane;aGreekprefixindicates
thenumberofBatoms;andArabicnumberisomittedifonlyborane
containingaparticularcountisknownB
2H
6isusuallyreferredtoas
diborane.
2.Anionic species are named as hydroborates.
Greek prefixes separately indicate the number of H and the charge
on
the anion is given a parentheses following.
E.g. B
5H
8
-
is octahydropentaborate(1-).
The structural type sometimes is specified when anions. i.e. B
5H
8
-
is also
octahydro-nodo-pentaborane.
E.g.
B
5H
9 pentaborane(9)
B
5H
11 pentaborane(11)
B
5H
10 pentaborane(10)
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25. THE BONDING PROBLEM IN BORANES
Localized Bonding picture
Retainingthevalencebondconceptoftherelationship
betweenbonddistanceandbondorderaproblemis
immediatelyencounteredonexaminingtheknown
structuresofboronhydrides.
Thecoordinationnumberofeachboron(andsomeofthe
hydrogens)exceedthenumberoflow-energyorbitals.
Ideallyabondingpictureforelectron-deficientcompounds
wouldallowthesamestraightforwardpredictionof
geometry,reactivity,stoichiometry,redoxproperties,
acidityetc.thatthevalencebondapproachpermits“for
regularcompounds”.
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26. Earlyattemptstoaccountfortheelectronic
structureofdiboranesthesimplestmemberofthe
class,includedtheobservationthatB
2H
6is
isoelectronicwithethaneC
2H
4.
InthisviewwecouldregardthetwobridgingH’sin
thestructureasprotonatingthedoublebondof
B
2H
4
2-
.
Subsequentresearchhasconfirmedtheacidic
natureofbridgeH’sintheboranes.
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27. Howeverthisbondingpictureisdifficulttoextentto
thehigherboranes.
Astraightforwardapplicationofvalencebond
theorytotheelectronicstructureofdiboranes
requiressome20resonancestructures.
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28. PFIZER –BONDED (SP
3
) MODEL
•TwooftheSP
3
-hybridorbitalsofeachboronatomareusedin
bondingwiththeterminalhydrogenandalltheseareinvolvedin
2c,2ebond.
•ThetwopointstowardsthebridgingHandinteractwiththe1S-
orbitalofHtoform3c,2ebonds(HencethebondinginB
2H
6is
diamagneticduetotheabsenceofunpairedelectrons).
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29. BANANA (SP
2
hybrid) Model
This model is better suited to the observation that H
TBH
T
angle ≡ 122
◦
far apart from the that they are also2c, 2e.
Generally bonding in boranesconsist of the following:
(i)BHB (3c, 2e) ≡
(ii)BBB can be in two forms.
(a) closed –3c –BBB (b) Opened –3c –BBB bond
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30. MOLECULAR ORBITAL APPROACH
Simplecovalentbondingtheorymolecularorbitals(MOs)
areformedbythelinearcombinationofatomicorbitals
(LCAO);e.g.twoatomicorbitalscombinetogiveone
bondingandanti-bondingMOsandorbitalsoflower
energywillbeoccupiedbytheelectronpairs.
Thisisaspecialcaseofamoregeneralsituationinwhicha
numberofAOsarecombinedtogetherbytheLCAO
methodstoconstructanequalnumberofMOsofdiffering
energies,someofwhichwillbebonding,somepossibly
non-bondingandsomeanti-bonding.
Inthisway2-centre,3-centreandmulticentreorbitalcan
beenvisaged
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31. Inboranechemistrytwotypesof3-centrebondfinds
considerableapplication:B-H-Bbridgebondsandcentral3-
centreB-B-Bbonds.
Thefigurebelowshowstheformationofa3-centreB-H-B
orbital
1fromanSP
x
hybridorbitaloneachofB(1),B(2)andH
1Sorbital,
H.
ThethreeAOshavesimilarenergyandappreciablespatial
overlap,butonlythecombination;(B
1)+(B
2)hasthecorrect
symmetrytocombinelinearlywith(H).
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32. The three normalized and orthogonal MOs have
the approximate form:
Bonding : 
1½[(B
1) + (B
2)] + 1/2 (H)
Non-bonding (anti-bonding):

21/2 [(B
1) -(B
2)]
Anti-bonding : 
3½[(B
1) + (B
2)] -1/2 (H)
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33. Formationofabondingcentral3-centrebond
1andschematic
representationoftherelativeenergiesofthe3molecularorbitals

1,
2and
3.
TheapproximateanalyticformsoftheseMOsare:
Bonding:
1[(B
1)+(B
2)+(B
3)]/3
Anti-bonding:
2[(B
1)-(B
2)]/2
Anti-bonding:
3[(B
1)+(B
2)-2(B
3)]/6
Forclosoandforlargeropenclusterboranesitbecomes
increasinglydifficulttowriteasimplesatisfactorylocalized
orbitalstructure,andfullMOtreatmentisrequired.
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34. MO Description of bonding in B
2H
6
•TheMOschemeforoneoftheB–H–Bbridging
threecentertwoelectronbonds.
•Thenon-bondingorbitalisactuallyofslightly
lowerenergythanshownandsohasslight
bondingcharacter.
•Thisarisesfromthefactthattheorbitals
involvedintheterminalB–Hbondinghavethe
correctsymmetrytooverlapwiththebridging
bondorbitals,resultinginastabilizationofthe
‘nonbonding’orbital.
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35. 10/21/2014 35

36. MO Description of bonding in closo-B
6H
6
2-
ClosoB
6H
6
2-
hasaregularoctahedralclusterof6
boronatomssurroundedbyalargeroctahedronof
radiallydisposedHatoms.
FrameworkMOsfortheB
6clusterareconstructed
(LCAO)usingthe2S,2P
x,2P
yand2P
zboronatomic
orbitals.
Thesymmetryoftheoctahedronsuggeststheuse
ofSPhybridsdirectedradiallyoutwardand
inwardsfromeachboronalongthecartesianaxes
and2pureporbitalsatrightanglestothese(i.e
orientedtangentiallytotheoctahedron).
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37. Six inward-pointing (SP)
orbitals used for a
1g
framework bonding
molecular orbitals
Components for one
of the t
2gframework
bonding molecular
orbitals –the other
two molecular orbitals
are in the yzand zx
planes
Six outward
pointing (SP)
orbitals Used for σ-
bonding to 6H
t
Components for one of
the t
1uframework
bonding
molecular orbitals-the
other two molecular
orbitals are in the yzand
zxplanes
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38. Thesesetofatomicorbitalscombinewithdueregardto
symmetrytogivetheMOsshown.Inall24AOsonthe6B
atomscombinetogive24MOsofwhich7(n+1)are
bondingframeworkMOs,6areusedtoformB-H
Tbonds
andtheremaining11areanti-bonding.
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39. •Thediagramsalsoindicatewhyneutralcloso-boranes
B
nH
n+2areunknownsincethe2anionicchargesare
effectivelylocatedinthelowlyinginwardlydirecteda
1g
orbitalwhichhasnooverlapwithprotonsoutsidethe
clusteri.e.abovetheedgesorfacesoftheB
6octahedron.
•ReplacementoftheH
tby6Bfurtherbuildsupthebasic
threedimensionalnetworkofhexaboridesMB
6justas
replacementofthe4H
tinCH
4beginstobuildupthe
diamondlattice.
•Thediagramsalsoserve,withminormodificationto
describethebondinginisoelectronicspeciessuchas
closo-CB
5H
6
-
,1,2-closo-C
2B
4H
6,1,6-closo-C
2B
4H
6etc.
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40. •Similarthoughmorecomplexdiagramscanbederivedforall
closo-B
nH
n
2-
(n=6-12).
•Thesehavethecommonfeatureofalowlyinga
1gorbitalandn
otherframeworkbondingMOs:ineachcase,therefore(n+1)
pairsofelectronsarerequiredtofilltheseorbitalsasindicated
byWade’srules.
•ItisatriumphforMOtheorythattheexistenceofB
6H
6
2-
and
B
12H
12
2-
werepredictedbyLonguet-Higginsin1954-5adecade
beforeB
6H
6
2-
wasfirstsynthesizedandsomefiveyearsbefore
the(accidental)preparationofB
10H
10
2-
andB
12H
12
2-
were
reported.
•Itisgeneralfeatureofcloso-B
nH
n
2-
anionsthattherearenoB-
H-BorBH
2groupsand4nboronatomicorbitalsarealways
distributedasfollows:
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41. n in the n(B-H
t) bonding orbital
(n+1) in framework bonding MOs
(2n-1) in non-bonding and anti-bonding
framework MOs.
AseachBatomcontributesoneelectrontoitsB-H
tbondand
twoelectronstotheframeworkMOs,the(n+1)framework
bondingMOarejustfilledbythe2nelectronsfromnB
atomsandthetwoelectronsfromtheanioniccharges.
10/21/2014 41

42. TOPOLOGICAL APPROACH TO BORON HYDRIDE
STRUCTURE –styxnumbers
•Lipscombetalestablishedasystematicprocedurefor
obtainingthevalencestructureofmorecomplexboron
hydridesincorporatingthree-centrebonding.
•Theprocedureinvolveessentiallydeterminingthetotal
numberoforbitalsandelectronsavailableforbonding.
•ThenumberofB-HbondsandB-H-Bthreecentrebondis
thencountedandtherequisiteorbitalsandelectronsare
assigned.
•Theremainingorbitalsandelectrons,consideredtobe
availableforframe-workbonding,aredistributedamong
two-centreB-Bbondsandthree-centreB-B-Bbonds.
•Asystematicprescriptionforaccomplishingthisis
outlined.
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43. •Consideraneutralboranewhoseformulacanbe
writtenasB
pH
p+q.
•Themoleculeconsistsofp(BH)groupsandq
“extra”hydrogensdistributedbetweenbridging
positionsandBHgroups(convertingthemtoBH
2
groups.
s=numberofB-H-Bbonds
t=numberofB-B-Bbonds
y=numberofB-Bbonds
x=numberofBH
Tbonds
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44. •Severalrelationscanbeformulatedbetweenstructuralfeatures
andavailableorbitalsandelectronscalledequationofbalance:
Forhydrogenbalance: q=s+x………(1)
i.e.Allthe“extra”HydrogensmustbeinB-H-BorBH
Tunits
Fororbitalbalance: p=s+t……..(2)
Thestructurecontainspboronatoms,eachmustparticipatein
onethree-centrebondifitistoattainacompleteoctet.
ThiscanbeeitheraB-H-BorB-B-B
Forelectronbalance:
Thetotalnumberofelectronpairsavailableforframework
bondingispfromtheBHgroupsplus½qfromtheextra‘H’s.
Thesemustbejustenoughtooccupythes+t+yframework
bondsandthexBH
2bonds.
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45. Hence p + 1/2q= s + t + y + x ………..(3)
Substituting (1) and (2) into (3) we obtain
p –1/2q=t + y……………………..(4)
y=½(s –x)…………………..(5)
In general;
s x
but s q
also s q/2
q/2 s q
NB. These equations are diophantineequations
10/21/2014 45

46. •Applyingtheequationofbalancetoacompoundofgiven
compositionallowsustodetermineasetofstyxnumbersthat
specifyavalencestructure.
eg1. For B
2H
6 (BH)
2H
4 p = 2; q = 4
Andwehave:4=s+x;2=s+t;0=t=yy=-t
Theonlypossiblesolutioniss=2,t=0,y=0,x=2
(written2002)andthestructurecorrespondsto
s t y x
4 -2 2 0
3 -1 1 1
2 0 0 2
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47. eg2. For B
5H
11(BH)
5H
6 : p = 5; q = 6
This formulation gives (4) different styxnumbers
6 = s + x; 5 = s + t; 2 = t + y y = 2 –t; y = ½(s–x)
i.e. (3203),(4112), (5021), (6-130),
s t y x
6 -1 3 0
5 0 2 1
4 1 1 2
3 2 0 3
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48. •For 3203 5B, 3BHB, 2BBB, 0 BB, 3BH
T
•
•
OR
•For 4112 5B, 4BHB, 1BBB, 1BB, 2BH
T
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49. •For 5021 5B, 5BHB, 0BBB, 2BB, 1BH
T
10/21/2014 49

50. •Inchoosingthebeststructuresthefollowingadditional
considerationsmustbekeptinmind:
1.EverypairofadjacentB’smustbebondedtoeachother
throughaB-Bbond,B-H-B,orB-B-Bbonds.
2.PairsofBatomsbondedbyaB-Bbondmaynotbebonded
tooneanotherbyB-B-B,orB-H-Bbond.
3.NonadjacentpairsofBatomsmaynotbebondedby
frameworkbonds.
4.Otherthingsbeingequalthepreferredstructureistheone
withthehighestsymmetry.
Theseconsiderationseliminatestructures(4112)and(5021),
leavingthestructure(3203).
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51. SYNTHESES AND REACTIVITY OF NEUTRAL
BORON HYDRIDES
•ThebestwaytosynthesizeB
4H
10,B
5H
9,B
5H
11,and
B
10H
14,istopyrolyzediborane,B
2H
6undercarefully
controlledconditions.
•Thefactthatsuchanapproachisfeasiblebecomes
clearerifweorganizetheboranefamilysomewhat
differently.
•Thechemicalrelationshipsbetweenboranesmaybe
seenbetterwhenwecombinetheB
pH
p+qformulation
withanothergeneralformula(BH)
n(BH
3)
x,wherencan
assumevaluesbetween0and10inclusivelyandx
assumesthevalues1,2or3.
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52. 10/21/2014 52

53. •Inthetablewehavesomeknownandunknown
boranesandthesearelistedaccordingtotheirn,
qandxnumbers.
•Theusefulnessofthistabularformbecome
apparentwhenitisrecognizedthatitshowsthat
twoboranesmaybeconvertedtooneanotheror
higherboranescanbemadefromsimpleronesby
theapplicationofoneormoreofthefollowing
reactionsintheirpropersequence.
10/21/2014 53

54. (A)GainorlossofBH
3
•Thisisusedtoconvertaboraneofagivennandx
toaboraneofthesame‘n’butdifferent‘x’.
(B)gainorlossofH
2
•Thisisusedtoconvertaboraneofagiven‘n’and
‘x’tooneofthenexthigherorlower‘n’and‘x’.
(C)GainorlossofaBHunit
Thisisusedtoconvertaboraneofagiven‘n’and
‘x’toaboraneofhigherorlower‘n’butthesame
‘x’.
10/21/2014 54

55. GENERAL SCHEME
10/21/2014 55

56. 10/21/2014 56

57. PROPERTIES OF BORON HYDRIDES
PHYSICALPROPERTIES
•Boranesarecolourless,diamagnetic,molecular
compoundsofmoderatetolowthermalstability.
•Thelowermembersaregasesatroomtemperature
butwithincreasingmolecularweightstheybecome
volatileliquidsorsolids.
•Theirboilingpointsareapproximatelythesameas
thoseofthehydrocarbonsofsimilarmolecular
weights.
10/21/2014 57

58. •Theboranesareallendothermicandtheirfree
energyofformationG
f
◦
isalsopositive:
•Theirthermodynamicinstabilityresultsfrom
exceptionallystronginteratomicbondsinboth
elementalBandH
2ratherthantheinherent
weaknessoftheB-Hbond.
•ThebondenergiesoftypicalboranesareB-H
T-
380,B-H-B–440,B-B–330andB-B-B–380kJmol
-
1
comparedtothebondenergyof436kJmol
-1
for
H
2andheatofatomizationofcrystallineboronof
555kJmol
-1
ofBatoms(ie.1110kJmol
-1
of2B
atoms).
10/21/2014 58

59. •Physical properties of some boron compounds
Nido-boranes Arachno-boranes
Compoundmp bp H
f
◦/kJm
ol-1
Compoundmp bp
B
2H
6 -164.9
◦
-92.6
◦
36 B
4H
10 120
◦
18
◦
58
B
5H
9 -48.8
◦
60
◦
54 B
5H
11 122
◦
65
◦
67 (93)
B
6H
10 -62.3
◦
108
◦
71 B
6H
12 -82.3
◦
85
◦
(extra)
111
B
8H
12 Decompose
above -35
◦
- B
8H
14 Decompose
above 30
◦
-
B
10H
14 99.5
◦
213
◦
32 n-B
9H
15 120
◦
120
◦
-
10/21/2014 59

60. CHEMICAL PROPERTIES
•Boranesareextremelyreactiveandseveralare
spontaneouslyflammableinair.
•Arachno-boranestendtobemorereactive(less
stabletothermaldecomposition)thannido-
boranesandreactivityalsodiminisheswith
increasingmolecularweight.
•Closo-boraneanionsareexceptionallystableand
theirgeneralchemicalbehaviorhassuggested
theterm”threedimentionalaromaticity.
10/21/2014 60

61. •Boronhydridesareextremelyversatile
chemicalreagentsbuttheverydiversityof
theirreactionsmakeageneralclassification
undulycumbersome.
•Instead,therangeofbehaviorwillbe
illustratedbytypicalexamplestakenfromthe
chemistryofthethreemoststudiedboranes:
B
2H
6,B
5H
9andB
10H
14.
10/21/2014 61

62. Chemistry of diborane, B
2H
6
•B
2H
6occupiesaspecialplacebecauseall
otherboranesarepreparedfromit.
•Itisalsothemoststudiedand
syntheticallyusefulreagentinthewhole
ofchemistry.
10/21/2014 62

63. PREPARATION
(i)B
2H
6gasismostconvenientlypreparedinsmall
quantitiesbythereactionofI
2onNaBH
4in
diglyme[(MeOCH
2CH
2)
2O]orbythereactionofasolid
tetrahydroboratewithananhydrousacid:
(ii)WhenB
2H
6isusedasreactionintermediatewithoutthe
needforisolationorpurificationthebestprocedureisto
addEt
2OBF
3toNaBH
4inapolyethersuchasdiglyme.
10/21/2014 63

64. (iii)Ontheindustrialscalebythedirectreductionof
BF
3withNaHat180Candtheproductformed
trappedoutasitisformedtopreventsubsequent
pyrolysis:
10/21/2014 64

65. REACTIONS OF B
2H
6
(i)Combustion
CareshouldbetakeninthesereactionsbecauseB
2H
6is
spontaneouslyflammable.Hasahigherheatofcombustion
perunitweightoffuelthananyothersubstanceexceptH
2,
BeH
2andBe(BH
4)
2
(ii)Pyrolysis
B
2H
6undergoescomplexpyrolysisinsealedtubesat
temperaturesabove100Cformingavarietyproducts
dependingontheconditions.Theinitiatingstepisthe
unimoleculardissociationequilibrium;
10/21/2014 65

66. •Initiatingstep:
•StableintermediateB
4H
10isthenfollowedby
B
5H
11
•AcomplexseriesoffurtherstepsgivesB
5H
9,B
6H
10,
B
6H
12andhigherboranesculminatinginB
10H
14as
themoststableendproducttogetherwithpolymeric
materialsBH
xandatraceoficosaboraneB
20H
26.
10/21/2014 66

67. •Cleavagereactions:Bridgebondsarereadily
cleavedevenbyweakligandstogiveeither
symmetricalorunsymmetricalcleavageproducts.
•Symmetrical products (Homolytic)
•Unsymmetrical products (Heterolytic)
10/21/2014 67

68. •Thefactorsgoverningthecourseofthesereactionsare
notfullyunderstoodbutstericeffectsplaysomeroleeg.
NH
3,MeNH
2andMe
2NHgiveunsymmetricalcleavage
productswhereasMe
3Ngivesthesymmetriccleavage
productMe
3NBH
3.
•symmetricalcleavageisthemorecommonmodeand
thermochemicalandspectroscopicdataleadtothe
followingsequenceofadductstabilityforLBH
3.
•PF
3COEt
2OMe
2OC
4H
8SEt
2SMe
2Spy
Me
3NH
-
•Therelativestabilityofsulphideadductsismorenotable
andmanyothercomplexeswithN,P,O,Setc.donor
atomsareknown.
10/21/2014 68

69. •TheH
-
isaspecialcasesinceitgivesthe
symmetricaltetrahedralionBH
4
-
isoelectronic
withCH
4.
•TheBH
4
-
ionitselfprovidearareexampleofa
ligandthatcanbeunidentate,bidentateor
tridentate(eg.[cu
1
(
1
-BH
4)(PMePh
2)
3],[Cu
1
(

2
–BH
4)(PPh
3)
2];[Zr
IV
(
3
–BH
4)
4]).
•Inadditiontopyrolysisandcleavage
reactions,B
2H
6undergoesawiderangeof
substitution,redistributionandsolvolytic
reactions:
10/21/2014 69

70. 10/21/2014 70

71. Hydroboration
•AdditionofB
2H
6toalkenesandalkynesinethersolventat
roomtemperature.
•Hydroborationisregiospecific,theboronatomshowing
preferentialattachmenttotheleastsubstitutedcarbon
atom(anti-Markovnikov).
•Protonolysisoftheresultingorganoboranebyrefluxingit
withananhydrouscarboxylicacidyieldsthealkane
correpondingtotheinitialalkene.
•Oxidativehydrolysiswithalkalinehydrogenperoxideyields
thecorrespondingprimaryalcohol:
10/21/2014 71

72. Thermal isomerization
•Diboraneisanelectrophilicreducingagentwhich
preferentiallyattacksamoleculeatapositionofhigh
electrondensity.
•Internalalkanescanbethermallyisomerizedtoterminal
organoboranesandhencetoterminalalkenes(by
displacement)ortoprimaryalcohols;
10/21/2014 72

73. •Inthecaseofheteropolardoubleandtriplebondsthe
borylgroupBH
2normallyaddstothemoreelectronrich
atomi.e.OatomincarbonylandNatominCNandCN.
•Thusafterprotonolysisaldehydesyieldprimaryalcohol
andketonesyieldsecondaryalcohols,althoughinthe
presenceofBF
3completereductionofCOtoCH
2may
occur.
•Likewisenitrilesarereducedtoamines,oximestoN-
alkylhydroxylamines,andSchiff’sbasestosecondary
amines.
10/21/2014 73

74. (VI)Reductivecleavage:
Reductivecleavageofstrainedringssuchasthosein
cyclopropanesandepoxidesoccurreadilyandacetals(or
ketals)arealsoreductivelycleavedtoyieldanetherand
analcohol:
10/21/2014 74

75. (VI)Removalofatoms:
RemovalofOatomsoccureitherwithorwithout
additionofHatomstothemolecule.
Thusphosphineoxidesgivephosphinesand
pyridine-N-Oxidesgivespyridinewithoutaddition
ofHatoms,whereasaromaticnitrosocompounds
arereducedtoaminesandcyclicdionescanbe
successivelyreducedbyreplacementofCObyCH
2
eg.
10/21/2014 75

76. CHEMISTRY OF NIDO-PENTABORANE, B
5H
9
•Preparation:
(i)B
5H
9canbepreparedbypassinga1:5mixtureofB
2H
6
andH
2atsubatmosphericpressurethroughafurnaceat
250Cwitha3-sresidencetime(orat225Cwitha15-s
residencetime)thereisa70%yieldand30%
conversion.
(ii)PyrolysisofB
2H
6fordaysinahot/coldreactorat180C.
(iii)Apex-substitutedderivatives1-XB
5H
8canbereadily
preparedbyelectrophilicsubstitution(eg.Halogenation
orFriedelCraft’salkylationwithRXoralkenes)
10/21/2014 76

77. (iv)2-XB
5H
8resultswhennucleophilicreactionisinduced
byaminesorethers,orwhen1-XB
5H
8isisomerizedinthe
presenceofalewisbasesuchashexamethylenetetramine
orether:
(v)Furtherderivativescanbeobtainedbymetathesiseg.
10/21/2014 77

78. REACTIONS
(i)Lewisbases
B
5H
9reactswithlewisbases(electronpairdonors)toform
adductsegwithPMe
3togive[B
5H
9(PMe
3)
2].
(ii)WeakBronstedacid:
B
5H
9actsasaweakacid.Theacidityincreaseswith
increasingsizeoftheboraneclusterandarachno-boranes
aremoreacidicthannido-boranes.
Nido:B
5H
9B
6H
10B
10H
14B
16H
20B
18H
22
Arachno:B
4H
10B
5H
11B
6H
12andB
4H
10B
6H
10
10/21/2014 78

79. B
5H
9canbedeprotonatedatlowtemperaturebylossofH
togive
B
5H
8-providingasufficientlystrongbasesuchasalithiumalkyloralkali
metalhydrideisused.
(iii) Cluster expansion :
(iv) Cluster degrading :
10/21/2014 79

80. (v)Subrogationofa{BH}
Subrogationofa{BH}unitinB
5H
9byan‘isoelectronic’
organometallicgroupsuchas{Fe(
5
-C
5H
5)}canoccurandthis
illustratesthecloseinterrelationbetweenmetalloborates,
metal-metalclustercompoundsandorganometalliccomplexes
ingeneral.Eg.[1-{Fe(CO)
3}B
4H
8];[1-{Co(
5
-C
5H
5)}B
4H
8];[2-
{Co(
5
-C
5H
5)}B
4H
8].
10/21/2014 80

81. Nido-decaborane, B
10H
14
•Decaboraneisthemoststudiedofall
polyhedralboranesandonetime(mid1950s)
wasmanufacturedonamulti-tonscaleinthe
USasapotentialhigh-energyfuel.
•Itisnowobtainedinresearchquantitiesby
thepyrolysisofB
2H
6at200Cinthepresence
ofcatalystofMe
2O.
10/21/2014 81

82. Physical Properties
•Itisacolourless,volatile,crystallinesolidinsolublein
waterbutreadilysolubleinawiderangeoforganic
solvents.
•Itsstructureisregardedasderivedfromthe11Batom
clusterB
11H
11
2-
byreplacingtheuniqueBHgroupwith
twoelectronsandappropriateadditionof4H
.
10/21/2014 82

83. •Molecularorbitalcalculationsgiveasequenceof
electronschargedensitiesatvariousBatomas2,4,1,
3,5,7,8,106,9thoughthetotalrangeofdeviation
fromchargeneutralityislessthan0.1
S = 4
t = 6
y = 2
x = 0
10/21/2014 83

84. Chemical properties
•ThechemistryofB
10H
14canbeconvenientlybe
discussedundertheheadings
(a)Protonabstraction,
(b)Electronaddition,
(c)Adductformation,
(d)Electrophilicsubstitution:
(e)Nucleophilicsubstitution
(f)Clusteradditionreaction
10/21/2014 84

85. (a)Protonabstraction
•B
10H
14canbetitratedinaqueous/alcoholicmedia
asamonobasicacid:pka2.70.
•Protonabstractioncanalsobeeffectedbyother
strongbasessuchasH
-
,OMe
-
,NH
2
-
etc.
•X-raystudieson[Et
3NH]
+
[B
10H
13]
-
establishedthat
theionisformedbylossofabridgeprotonas
expectedandthisresultsinconsiderable
shorteningoftheB(5)-B(6)distancefrom179pm
into165pminB
10H
13
-
.
10/21/2014 85

86. •UndermoreforcingconditionswithNaHasecond
H
canberemovedtogiveNa
2B
10H
12;B
10H
12
2-
;
theanionactsasanormalbidentate(tetrahapto)
ligandwithmanymetals.
(b)Electronaddition
•ElectronadditiontoB
10H
14canbeachievedby
directreactionwithalkalimetalsinether,
benzeneorliquidNH
3.
10/21/2014 86

87. •AmoreconvenientpreparationoftheB
10H
14
2-
anionusesthereactionofaqueousBH
4
-
inalkaline
solution.
•Calculationsshowthatthisconversionofnido-
boranetoarachno-clusterreversesthesequence
ofelectronchargedensityatthe2,4and6,9
positionssothatforB
10H
14
2-
thesequenceis6,9,
1,3,5,7,8,102,4,thisisparalleledbychangesinthe
chemistry.
10/21/2014 87

88. (c)Adductformation
•B
10H
14
2-
canformallyberegardedasB
10H
12L
2for
thespecialcaseofL=H
-
.
•CompoundsofintermediatestoichiometryB
10H
13L
areformedwhenB
10H
14isdeprotonatedinthe
presenceoftheligandL:
10/21/2014 88

89. •The adduct can be prepared by direct reaction of
B
10H
14with L or by ligand replacement reactions:
•LigandsLandL’canbedrawnvirtuallyfromthe
fullrangeofinorganicandorganicneutraland
anionicligandsandindeed,thereactionseverely
limitstherangeofdonorsolventsinwhichB
10H
14
canbedissolved.
10/21/2014 89

90. •Theapproximatesequenceofstabilityis:
SR
2RCNASR
3RCONMe
2P(OR)
3pyNEt
3
PPh
3.
Bis-ligandadductsofmoderatestabilityplayan
importantroleinactivatingdecarboraneforseveral
typesofreactionseg:
1.Substitution
10/21/2014 90

91. 2.Clusterrearrangement;
3.Clusteraddition;
4.Clusterdegradation
--Intheclusterdegradationreactionitisthe
coordinatedBatomatposition9thatissolvolytically
cleavedfromthecluster.
10/21/2014 91

92. (d)Electrophilicsubstitution:
•ElectrophilicsubstitutionofB
10H
14followsthe
sequenceofelectrondensitiesintheground
statemolecule.
•HalogenationinthepresenceofAlCl
3leadsto1-
and2-monosubstitutedderivativesand2,4-
disubstitution.
•FriedelCraftsalkylationwithRX/AlCl
3(orFeCl
3)
yieldsmixturessuchas2-MeB
10H
13,2,4-and1,
2-Me
2
B
10H
12,1,2,3-and1,2,4-Me
3
B
10H
11and
1,2,3,4-Me
4
B
10H
10.
10/21/2014 92

93. (e)Nucleophilicsubstitution
•Thisoccurspreferentiallyatthe6(9)position;eg
LiMeproduces6-MeB
10H
13asthemainproduct
withsmalleramountsof5-MeB
10H
13,6,5(8)–
Me
2
B
10H
12and6,9-Me
2
B
10H
12.
(f)Clusteradditionreaction
•B
10H
14undergoesnumerousclusteraddition
reactionsinwhichBorotheratomsbecome
incorporatedinanexpandedcluster.
10/21/2014 93

94. •AmoreconvenienthighyieldsynthesisofB
10H
12
2-
isbythedirectreactionofamineboraneswith
B
10H
14intheabsenceofsolvents.
•Heteroatomclusteradditionreactionsare
exemplifiedbythefollowing:
•Thestructureofthehighlyreactiveanion
[AlB
10H
14]
-
isthoughttobesimilartothenido-
B
11H
14
-
withonefacialBatomreplacedbyAl.
10/21/2014 94

95. •(Theopenfacecomprisesoffluxionalsystem
involvingthethreeadditionalHatoms)
•Themetalalkylsactsomewhatdifferentlytogive
extremelystablemetalloboraneanionswhichcan
bethoughtofascomplexesofbidentateligand
B
10H
12
2-
.
10/21/2014 95

96. •Manyothercomplexes[M(B
10H
12)
2]
2-
and[L
2M(B
10H
12)]
areknownwithsimilarstructuresexceptthat,whereM
=Ni,Pd,Pt,thecoordinationaboutthemetalis
essentiallysquare-planarratherthanpseudo-
tetrahedralasforZn,CdandHg.
10/21/2014 96

97. CARBORANES
•Closelyrelatedtothepolyhedralboronhydrides
isalargefamilyofcarboranes(carbaboranes),
whichareclustersthatcontainbothBandC
atoms.
•TYPE1:methyldiboranes(MeB
2H
6-n)wheren=1,
2,3,4not5or6.
•Similarderivativesofdiboraneinwhichthe
boranegroupreplacestheterminalhydrogenof
theparentboranearealsoknown.
10/21/2014 97

98. TypeII
TheseareactualcarboraneswherebothBandC
featureintheelectrondeficientmolecularskeleton.
Closo-carboranes–C
2B
n-2H
n(n=5ton=12)
•Dicarba-closo-boranes
Theseareneutralspeciesandisoelectronicwith
B
nH
n
2-
Eg.C
2B
3H
5
10/21/2014 98

99. C
2B
4H
6
NIDOCARBORANES[C
2B
n-2H
n
2-
]
Thesehavencageatomsandn+2pairsofcagebonding
electronsandn+1corners.
Structurallytheyadoptanincompletecagestructure,nidoor
neststructurei.eonecagecornerisleftvacantthoughwhen
obtainedasmetalsalts,themetalcationmayoccupythe
vacantsiteinthecrystal.
Isoelectronicspecieswhichmightallbeexpectedtohavethe
nidostructuresare:B
nH
n
4-
,CB
n-1H
n
3-
,C
2B
n-2H
n
2-
,C
3B
n-3H
n
-
,
C
4B
n-4H
n.
10/21/2014 99

100. HEXABORANE (10)
B
6H
10isthelimitingmemberofaseriesoffive
compoundscontainingsixcageatomsand8pairof
bondingelectrons.
10/21/2014 100

101. ARACHNO CARBORANES
•Veryfewoftheseareknown.Theseriesare:B
nH
n
6-
,
CB
n-1H
n
5-
,C
2B
n-2H
n
4-
,C
3B
n-3H
n
3-
,C
4B
n-4H
n
2-
,C
5B
n-5H
n
-
and
C
6B
n-6H
n.
•TheparenthypotheticalanionB
nH
n
6-
areeffectivelythe
skeletonsofa(BH)
pH
qinwhichq=6,eg.B
4H
10andB
5H
11.
•Thesehavencageatomsandn+3pairsofcagebonding
electronsi.etherightnumberforacagewithn+2corners.
•Theyaccordinglyadoptthearachnostructureinwhich
twocagecornersareleftvacanteg.C
2B
7H
13,C
2B
8H
10,
C
2B
3H
7
2-
10/21/2014 101

102. Preparation and Reactions
of carboranes
•Themostimportantpreparativerouteisthe
reactionofboraneswithacetylenes
•TheisomersoftheclosocompoundC
2B
10H
12have
icosahedralgeometryandexhibitextremelyhigh
kineticandthermodynamicstability.
10/21/2014 102

103. •The1,2-;1,7-;1,12-isomershavethecommon
nameso-m-andp-carboranesrespectively.
•o-carboranehasbeengiventhefollowingsymbol
inliterature
10/21/2014 103

104. •ReactionsatBcenterincarboranesparallelthose
ofboranesbridgeprotonabstractionand
electrophilicsubstitution,includinghalogenation.
TheterminalHattachedtoelectrophilicCare
relativelyacidic.
•HencetheseCcenterscanbemetallated
10/21/2014 104

105. •Themetallatedproductsretainstructuralintegrity
andcanreactwithnucleophilestoproducealarge
numberofC-substitutedderivatives.
•Thermalisomerizationoccur.Thediamond-
square-diamondmechanismhasbeenproposed
fortheisomerizationofthe1,2-to1,7-by
Lipscombbutthe1.12-isomercannotbe
generatedbythismechanism.
•Moreover,theactivationenergyrequiredtopass
throughthecubo-octahedraltransitionstateis
likelytoberathertoohigh.
10/21/2014 105

106. •Thediamond-square-diamondmechanismconsistsofa
pairoftriangularfacesatrightangleswhichopenintoa
squareandrejoinwithadifferentpairofvertices
connected.
10/21/2014 106

107. •Analternativeproposalwhichcanleadtoboththe1,7-
and1,12-isomers,isthesuccessiveconcertedrotationof
the3atomsonatriangularface.
•Yetathirdpossiblemechanismthathasbeenenvisaged
involvestheconcertedbasaltwistingoftwoparallel
pentagonalpyramids,comprisingtheicosahedron.
•Itisconceivedthatthevariousmechanismoperatein
differenttemperaturerangesorthattwo(orallthree)
mechanismsareactivesimultaneously.
10/21/2014 107

108. •O-carboraneisattackedbyabasethatexcisesaBH
group,generatinganidoanionthatretains
structuralintegrity.
•The1,7-isomercanbeobtainedbythermal
rearrangementoftheanionorbystartingwithm-
carborane.
•NaHintetrahydrofurandeprotonatestheanion,
givingadianionB
9C
2H
11
2-
.
•Assumingforconveniencesp
3
hybridizationofthe
fiveatomsontheopenface,asetofMO’s
reminiscentoftheCp
-
anionmaybeconstructed.
•TheMO’sareoccupiedbysixelectrons.
10/21/2014 108

109. •Hawthornehasexploitedtheanalogybetween
B
9C
2H
11
2-
(thedicarballideion)andCp
-
toprepare
metaldicarballidecomplexesrelatedtothe
metallocenes.
10/21/2014 109

110. •Ageneraltechniqueforpreparingsuchcomplexes
involvesexcisionofaBHgroupthroughbase
degradationandreactionoftheresultinganionwitha
metalhalide.
10/21/2014 110

111. Nomenclature for Heteroborates
•Verticesofcloso-nido-andarachno-polyhedraaregiven
numbersbasedbyconventiononplanarprojectionsof
polyhedralstructures.
•Thenumberingisbyzones(planes)perpendiculartothe
majoraxis.
•Interiorverticesontheprojectionarenumberedfirst,
thenperipheralones.
•Thiscorrespondstonumberingapicalverticeswithlower
numbers.
•Thenumberingproceedsclockwisestartingfromthe
twelveo’clockpositionoratthefirstpositionclockwise.
Thelocationofheteroatomscanbespecifiedbynumbers.
10/21/2014 111

112. •PB
11H
12is phospha-closo-dodecaborane(12). There is no
need to specify the position of P since all icosahedral
vertices are equivalent.
10/21/2014 112

113. Generalorganizationalschemefortheneutralboron
hydrides,thecloso-polyhedral,hydroborateions,andthe
carboranes
•Giventhemolecularformula,amethodisneededfor
predictingtheprobablestructuralclassificationofaboron
compoundi.e.eitheracloso-,nido-orarachno-.
•Asdescribedearlieron,the(BH)
pH
qsymbolismisbestfor
structuralpurposesandtheseschemehasbeenextendedby
WadeandRudolph.
•Theformulaofanyneutralborane,hydroborateorcarborane
canbewrittenas:
•Wherethenumberofverticesofthepolyhedralfragmentis
(a+p)=nandtheqHsareinvolvedinaBHBorextraBH
T’s.
10/21/2014 113

114. •Assuming that the number of electron pairs
contributed by CH is 3/2
•The number of framework electrons is given by :
3/2a + p +1/2(q + c)
= 3a + 2p + q + c
=2n + a + q + c
Since n = a + p, 2n = number of vertices
When, a + q + c = 2 closo
a + q + c = 4 nido
a + q + c = 6 arachno
10/21/2014 114

115. •Eg. [B
6H
6]
2-
(BH)
6
2-
= (6x3e
-s
) + (6x1e
-
) + 2e
-s
= 26e
-s
6B-H = (6x2e
-s
) = -12e
-s
2n + a + q + c = 14e
-s
2n = -12e
-s
Closoa + q + c = 2e
-s
•B
5H
11
(BH)
5= (5x3e
-s
) + (11x1e
-
) = 26e
-s
5B-H = (5x2e
-s
) = -10e
-s
2n + a + q + c = 16e
-s
2n = -10e
-s
Arachnoa + q + c = 6e
-s
10/21/2014 115

116. Nidoboranes (BH)
pH
4
Arachnoboranes (BH)
pH
6
•Thusanidoboraneisafullyprotonated(BH)
p
4-
andthearachnoboraneisafullyprotonated
(BH)
p
6-
•Whenwelookat(BH)
p
c-
formulationthissuggest
thatagreatrangeofcompoundscontainingthe
(BH)unitsorsomeothergroupalsocapableof
donating2e
-s
tothepolyhedralframeworkcould
beobtained.
•ThusaBHunitcanbereplacedbyCH
+
;P
+
,S
2+,
N
+
orO
2+
.
10/21/2014 116

117. •Eg.Thecloso-B
4C
2H
6maybegeneratedfroma
closoB
6H
6
2-
bytheremovalof2BHunitsand
adding2CH
+
groupsorunits.
•Theformulaofthision7,8-B
9C
2H
12
-
anionmaybe
writtenas[(CH)
2(BH)
9H]
-
ifwecomparewith[(BH)
11H]
3-
.
The[(CH)
2(BH)
9H]
-
anioncomparedwith[(BH)
9H]
3-
with
2CH
+
unitreplacing2BHunitsandwithaprotonH
+
stitchinguppartoftheopenedface.
10/21/2014 117

118. •Herethea+q+c=4sinceitisanido
•Deprotonationof7,8-B
9C
2H
12
2-
andthisisacarboraneisoelectronicwiththe
(BH)
11
4-
•Ontheotherhandprotonationofthenido-7,
8-B
9C
2H
12
-
iongivesaneutralcompoundnido
7,8-B
9C
2H
13.
10/21/2014 118

119. •Pyrolysis of the molecule can give a nido-7, 8-B
9C
2H
13in
solution can give a closo-polyhedron-2, 3-B
9C
2H
11.
9B 9x3e-= 27e- 9B 9 x3e-= 27e- 9B 9 x3e-=
27e-
9H 9 x1e-= 9e- 11H 11 x1e-= 11e- 12H12 x1e-= 12e-
1S
2+
1 x4e-= 4e- 1S
2+
1 x4e-= 4e- 1S
2+
1 x4e-= 4e-
= 40e- = 42e- charge =1x1e
-
= 1e-
= 44e
-
9BH 9 x2e-= -18e- 9BH
T9 x2e-= -18e- 9BH
T9 x2e-= -
18e-
= 22e- = 24e- = 26e-
10 vertices = -20e-10 vertices = -20e- 10 vertices = -20e-
a + q + c = 2e- a + q + c = 4e- a + q + c = 6e-
Closo nido arachno
10/21/2014 119

120. INORGANIC RINGS
•Themostimportantringsystemoforganic
chemistryistheC
6H
6ringeitherasaseparate
entityorinpolynuclearhydrocarbonsuchas
napththalene,anthracene,phenanthrene,etc.
10/21/2014 120

121. •InInorganicChemistry,thereareatleasttwoanalogues
ofbenzenenamely:borazine(B
3N
3H
6)andphosphazenes
(P
3N
3X
3).
•WhenBCl
3isheatedwithNH
4Cl(orRNH
3Cl)in
chlorobenzene(C
6H
5Cl)inthepresenceofFe,Ni,orCo
(usedasacatalyst)atabout140C,B,B,B-
trichloroborazineisformed.
•Thisderivativeofborazineonbeingreducedwith
NaBH
4orLiBH
4inpolyethergivesborazine.
10/21/2014 121

122. •N-orBsubstitutedborazinesmaybemadeby
appropriatesubstitutiononthestartingmaterial
priortothesynthesisofthering;e.g.
10/21/2014 122

123. BORAZINE
•BorazineisanunsaturatedcompoundofBandNatoms
withtheformulaB
3N
3H
6.
•Itisisoelectronicandisostructuralwithbenzene,having
delocalizedelectronsandaromaticcharacter.
•Thephysicalpropertiesarealsosimilar.
•Howeverdespitetheresemblanceinstructure,thereislittle
chemicalresemblancebetweenborazineandbenzene.
•ThedifferenceinelectronegativityofBandNatomsis
influential.
10/21/2014 123

124. •Henceinborazinethe–electronsareconcentratedonthe
N-atomsandthereisapartialpositivechargeontheB-
atomswhichleavesthemopenforelectrophilicattackon
theN-atom.
•Consequently borazinein contrast to benzene readily
undergoes addition reactions.
•Also unlike benzene the –electrons are not derived from
all six atoms of the ring but from the 3 nitrogen atoms.
10/21/2014 124

125. Synthesis of borazines
(1)Stock’sMethod
•Inthismethoddiborane(B
2H
6)andNH
3areheatedin1:2
molarratioatlowtemperature(-120C)toobtain
diammoniateofdiborane(B
2H
6.2NH
3)whichisaddition
compound(adduct).Whentheadductisheatedat200C,
theborazineisobtained
10/21/2014 125

126. (2) By heating BCl
3with NH
4Cl (or RNH
3Cl)
10/21/2014 126

127. (3) By heating a mixture of LiBH
4and NH
4Cl
10/21/2014 127

128. STRUCTURE
•B: 1S
2
2S
2
2P
1
Neutral atom
•*B: 1S
2
2S
1
2P
x
1
P
y
1
P
z
0
SP
2
hybrid
•B
-
: 1S
2
2S
1
2P
x
1
P
y
1
P
z
1
SP
3
hybrid
•N: 1S
2
2S
2
2P
x
1
P
y
1
P
z
1
neutral atom and SP
3
hybrid
•N
+
: 1S
2
2S
2
2P
x
1
P
y
1
P
z
0
SP
2
hybrid
10/21/2014 128

129. •Inthisstructuralformulatheformal–veand+vecharges
havebeenassignedtotheBandNatomsrespectively.
•Theseillustrationsareisoelectronicwithcarbon(inSP
2
andSP
3
hybridizations)sothattheborazinehasthesame
skeletalconfigurationasinbenzene.
•AllB-Nbonddistancesare1.44Åwhichisbetweenthe
calculatedB-Nsinglebond(1.54Å)andB-Ndouble
bond(1.36Å).
•Thevalencebondapproachdescribesthestructurein
termsoftwocanonicalformswhereasamolecularorbital
descriptioninvolvesthree(3)-orbitalsembracingallsix
(6)atomsinthehexagon.
•Thesedelocalizedorbitalsdiffersomewhatfromtheir
benzeneanaloguesbecausetheconstituent2P
zatomic
orbitalsofBandNarenotidenticalinenergy.
10/21/2014 129

130. •Physicalproperties
•Borazineisindeedacloseanalogueofbenzene.
--Similarityofthephysicalpropertiesofthealkyl
substituted
derivativesismoreremarkable.
--Forexampletheratiooftheabsoluteboilingpointofthe
substitutedborazinetosimilarlysubstitutedbenzene
derivativesis0.930.01.
•Suchsimilaritiesleadtoadescriptionofborazineasan
inorganicbenzene.
10/21/2014 130

131. Physical properties of borazineand benzene
Chemicalproperties
•Thechemicalpropertiesofborazineandbenzenearequite
different.
Borazine Benzene
Molecular weight 80.5 78.1
b.p. (C) 55.0 80.10
m.p. (C) -56.2 5.51
Critical temperature (C) 252 288.0
Liquid density at b.p. (g/cm
3
)0.81 0.81
Crystal density at m.p. (g/cm
3
)1.00 1.01
Trouton constant (J/K mole)89.5 88.2
Surface tension (Nm
-2
) 0.0311 0.0310
Dipole moment 0 0
Intermolecular distance(pm)144 142
Bond distance to H (pm)B-H (120), N-
H(102)
C-H (100)
10/21/2014 131

132. •Bothhave-cloudsofelectrondensitydelocalizationover
alltheringatoms.
•Howeverbecauseofthedifferenceinelectronegativity
betweenBandN,the-cloudinborazineisdescribedas
beinglumpywithmoreelectrondensitylocalizedonthe
nitrogenatomN.
•Thispartialdelocalizationweakensthe-bondinghenceN
retainssomeofitsbasicitywhereasBsomeacidity.
•Asaresultofthis,polarspeciessuchasHClcantherefore
attackthedoublebondbetweenNandB.
•NB:ThedifferentelectronegativityofBandNturnto
stabilizebondingtoBbyelectronegativesubstituentsand
bondingtoNbyelectropositivesubstituents.
10/21/2014 132

133. Thetendencyforborazinetoundergoadditionreactionratherthan
substitutioniswellcontrastedbytheelectrophilicsubstitutionreaction
ofbenzene(i.ehalogenationofbenzene)asindicatedbelow:
10/21/2014 133

134. •Borazineanaloguesofnaphthalene,andrelated
hydrocarbonshavebeenmadebypyrolysisofborazineor
itspassingthroughasilentelectricdischarge.
•Thereisalsothefourmemberedaswellasthe8-
memberedringslike
10/21/2014 134

135. CYCLOBORAZINES
•Unlikehydrogenationofbenzeneyieldingcyclohexanes,straight
forwardhydrogenationofborazinesdonotyieldcycloborazines,
butratheryieldpolymericmaterialsofindefinitecompositions.
•Substitutedderivativesofsaturatedcycloborazineform
readilybyadditionreactions.
•Anothermethodinclude
10/21/2014 135

136. REACTIONS OF BORAZINES
(1)Additionreactions:
(a)OnemeleculeofB
3N
3H
6addsthreemoleculesofH
2O,
ROH,RXorHXinthecoldwithoutacatalyst
--ThemorenegativegroupisgenerallyattachedtoB,since
B-atomislesselectronegativethanN-atominB-Nbond.
--ForexamplewhenHXderivativeisheatedat50-100Cit
loses3H
2moleculestoyieldB,B’,B”-trihaloborazine.
10/21/2014 136

137. --Thisadditionreactionshownbyborazineisnotshownby
benzene.
(b)Onemoleculeofborazineadds3moleculesofX
2at0C
andgivesB-trihalo-N-trihaloborazinewhichonheatingat
about60Closes3moleculesofHXandformsB-
trihaloborazinei.e.
--Substitutionoccursinborazinequitemorereadilydueto
the
considerablymorereactivenatureofborazinetobenzene.
--Thisreactioncanbecomparedwiththatshownbybenzene
wheresubstitutiontakesplace.i.e.
10/21/2014 137

138. (2)Hydrolysis
(a)BorazineisslowlyhydrolyzedbyH
2OtoproduceH
2,
B(OH)
3andNH
3.
--Thehydrolysisisfavouredbyincreaseintemperature
10/21/2014 138

139. (b)Underproperconditionsborazinereactswith3molecules
ofH
2OtoproduceB-trihydroxyborazine(OH)
3B
3N
3H
3
inwhichtheOHgroupsareattachedtoB-atoms.
(3)Hydrogenation
•Benzenecanbehydrogenatedtoproducecyclohexane,C
6H
12,
Borazineontheotherhandcanbeconvertedtocycloborazine
onlybyspecialtechniquessuchasshown:
--Directadditionyieldspolymericmaterialswithindefinite
composition.
10/21/2014 139

140. BOROXINE
•Iso-electronicwithborazineisboroxineH
3B
3O
3whichis
formulatedas[XBO]
3
•Boroxineisplanarbuthaslesseven–delocalizationthan
borazineandpossessasixmemberedring.
•Boroxinecanbepreparedbytheexplosiveoxidationof
B
2H
6orB
5H
9,orhightemperaturehydrolysisofboron.
•Althoughthiscompoundisthermodynamicallyunstableat
roomtemperaturewithrespecttoB
2H
6andboricoxide,
neverthelessitcanbecharacterized.
10/21/2014 140

141. •Aboron-phosphorusanalogueofborazinehasbeen
synthesizedratherrecently.
•TheelectronegativitiesofBandParesimilar,unlike
thoseofBandN.
•Asaresultpolarizationshouldbelessextensiveinthis
compoundthanborazine.
•TheB
3P
3ringisplanarwithequalB-Pbondlengthsand
shortenedB-Pbondssuggestingsignificantaromaticity.
10/21/2014 141

142. PHOSPHAZENES (Phosphonitriliccompounds)
•Thesearecycliccompoundsofphosphorusandnitrogen
ofgeneralformula[PNCl
2]
n.
•Thereactionproducesamixtureofringcompounds
(NPCl
2)
n,wheren=3,4,5,….andfairlyshortlinear
chains.
•Themostcommonrings(n=3and4)containsixoreight
atoms.
•Theformerareflatandthelatterexistsin‘chairandboat
conformations.
10/21/2014 142

143. •Analogousbromo-compoundsmaybepreparedinthesame
mannerexceptthatbromineshouldbeaddedtosuppress
thedecompositionofPBr
5.
•ThefluoridemustbepreparedindirectlyusingNaFin
nitrobenzenei.e.
•Theiodidesarenotknown
10/21/2014 143

144. STRUCTURE
•NitrogenatomsareSP
2
hybridizedandtwosuchhybrid
orbitalsareinvolvedin-bonding.
•SimilarlyeachphosphorusisSP
3
hybridizedandsuch
hybridsareinvolvedin-bonding.
•Thereisthenone“inplane”–bondinginvolvingthelone
pairofN(SP
2
orbital)inxyplaneandthevacantdxyor
dx
2
-y
2
oftheP-atom.
10/21/2014 144

145. •Therearetwo“outofplane”interactions:
(a)Heteromorphicinteractions:ThesinglyoccupiedPz
1
orbitalonthenitrogenoverlapswiththedxz
0
ordyz
0
orbitalofphosphorus(p–dbonding)
(b)Homomorphicinteractions:ThereareinteractionsofPz
orbitalsoftwonitrogenthroughthedyzorbitalofthe
phosphorusinbetween.
10/21/2014 145

146. TetramericPhosphonitrilesor cyclotetraphosphaza-
tetraenes
•Theseare[NPCl
2]
4and[NP(NMe
2)
2]
4
•Thetetramericringsoftheabovecompoundsaremore
flexiblethanthoseofthetrimers.
•Thereisenoughstrainandtheyexhibitnon-planar
structureswithoutseriousdeteriorationoftheP─N–
bonding.
•Thechloro-derivativesareusefulforintroducingother
substituentsattheP-atomsinthering.
10/21/2014 146

147. •Eg.When
hexachlorobis(ethylamino)tetraphosphazatetraene
is treated with excess dimethylaminein chloroform the
product that results is not only the full aminationof both
the
(PCl
2) groups and one of the PClgroup but also one of the
ethylamino(NHEt) group bridges the opposite PClgroup
with the elimination of HCl.
•This arises from an internal trans-annular attack by one of
the NHEtgroups. The product is the only known bicyclic
phosphazenecompound.
10/21/2014 147

148. •This bicyclo-structure is reminiscent of ADAMANTANE
•TheamidehoweverundergoeseliminationofNH
3with
increasingtemperaturetoformcross-linkingbetween
ringsorpossiblyraptureofringstoformlinearpolymers.
10/21/2014 148

149. •Verylittleisknownaboutthesepolymers.
•Howeverrelativelylowmolecularweightpolymer
believedtobea3-ringspecieswiththeformulation
Cl
9P
6N
7hasbeenisolated
10/21/2014 149

150. INORGANIC RUBBERY POLYMERS
•(1)Ifexcessphosphoruspentachlorideisusedinthe
preparationofphosphazene,thenitispossibletoisolate
linearpolymersasindicatedbelow:
•(2)Whenthephosphonitriliccompoundisheateditis
possibletoobtainlinearpolymers.
•NB:Thereactivechlorinearestillsusceptibleto
nucleophilicattackanddisplacement
10/21/2014 150

151. •IfR=CH
3CF
3(trifluoroethane)thentheproduct
obtainedisawaterrepellentpolymerwhichisusedto
fabricateartificialbloodvesselsandprostheticdevices.
•Althoughthehydrolyticstabilityofthephosphazene
polymersmakethemattractiveasstructuralmaterials,it
ispossibletocreatehydrolyticallysensitive
phosphazenesthatmaybeusefulmedicallyinslow
releasedrugs.
•Steroids,antibiotics,catecholamines(egdopamine&
epinephrine)havebeenlinkedtophosphazenemolecules
withtheintentionthathydrolysiswillprovidethesedrugs
inatherapeuticsteadystate.
10/21/2014 151

152. •Examples
10/21/2014 152

153. OTHER HETEROCYCLIC RINGS & CAGES
•1. S-N Ring compounds
•S-N ring compounds are prepared by ammonolysis
10/21/2014 153

154. •Thecorrespondingtetramerisalsoknowni.e.
(2)Tetrasulphurtetranitrides
•TheammonolysisofsulphurmonochlorideS
2Cl
2either
insolutionorinaninertsolventorheatedoversolid
ammoniumchlorideyieldtetrasulphurtetranitrides;S
4N
4.
•S
4N
4isabrightorangesolidinsolubleinH
2Obutsoluble
insomeorganicsolvent.
10/21/2014 154

155. •Thestructurehasbeenfoundtohavetwononbonding
S-atomsatadistanceofabout2.58Å.
•Thisisconsiderablyshorterthanthesumofthevander
waalradiiwhichis3.60Å.
•Althoughthisnonbondingdistanceislongerthanthe
normalS─Sbondwhichisabout2.06Å,some
interactionsmustoccurbetweenthetransS-atoms.
•AlltheS─Nbonddistancesareequalandabout1.62Å
indicatingextensivedelocalizationratherthan
alternatingdiscretesingleanddoublebonds.
10/21/2014 155

156. •Fluorination of S
4N
4
•It produces tetrathiozytetrafluoridei.e.
•Reduction of S
4N
4
10/21/2014 156

157. •All possible S
x(NH)
8-xisomers except N─N bonds are
known.
10/21/2014 157

158. •Oxidative chlorination of S
4N
4
•NB: It is unexplained that chlorination produces the
ClSNtrimerwhile fluorination retains the tetramer unit
10/21/2014 158

159. •SULPHURNURYL CHLORIDE ( NSOCl)
3
•(NSOCl)
3may also be prepared from sulphamicacid.
10/21/2014 159

160. HOMOCYCLIC INORGANIC RING
COMPOUNDS
•Severalelementsformhomocyclicrings.
•Thethermodynamicallystableformatroom
temperatureconsistsofS
8rings.
•Theoxidationofseveralnon-metalsinstrongly
acidicsystemsproducespoly-atomiccationic
speciesofthegeneraltypeY
n
+m
10/21/2014 160

161. •ThebestcharacterizedofthesearetheS
4
+2
,Se
4
+2
,Te
4
+2
.
•Thestructuresofthesecompoundshavebeenshowntobe
planarandhasbeenshownthatallthesespeciesaresquare
planar.
•ThestructuresarestabilizedbytheHückelsextetof-
electrons.
•Se
8
+2
isknowntobebicyclic.
•Thetransannularbondis2.84Åwhichislongerthanthoseof
thering2.32.2Åbutareconsiderablylessthanthesumof
thevanderwaalsradiiofabout3.80Å.
10/21/2014 161

162. •Otherionsofthistype,presumablycyclic,thoughless
thoroughlystudiedaretheS
8
2+
,S
16
2+
,Sb
4
2+
,Sb
8
2+
,Sb
n
n+
,
Te
6
2+
etc.
•Buthavebeensuggestedasproductsofmildoxidation
ofsomenon-metals.
•Theoxidationofredphosphoruswithhypo-halidesin
alkalinesolutionproducestheanionofaninteresting
phosphorusacid.
•Thisacidhasbeenshowntobecyclic.
10/21/2014 162

163. •CYCLIC OXOCARBON ANION [(CO)
n]
-2 (-4)
•TheoxocarbonateionC
5O
5
2-
isthefirstmembertobe
synthesized.
•Itwasisolatedin1825byGmelinandthusshareswithbenzene
thehonourofbeingthefirstaromaticcompounddiscovered.
•Itwasthefirstinorganicsubstancediscoveredthatisaromatic.
•Itisabacterialmetabolicproductandwaspossiblythefirst
organiccompoundsynthesized.
10/21/2014 163

164. •Alloftheseoxocarbonanionsarearomaticaccordingto
simpleMOcalculations.
•Thearomaticstabilizationoftheanionisapparently
responsibleforthefactthatsquaricH
2C
4O
4isabout
strongassulphuricacid.
•NB:OxalicacidcontainingCinacomparableoxidation
statebutnotaromatic.
•K
a1≈K
a2forsquaricandsulphuricacid.
•TheK
a2ofoxalicacidis3ordersofmagnitudesmaller.
10/21/2014 164

165. NON-METAL CAGED COMPOUNDS
•Thesimplestcagedtypemoleculeisfoundinwhite
phosphoruswhichisaP
4molecule.
•Thismoleculeismorestableatroomtemperatureandisa
tetrahedronofphosphorusatoms.
•Suchastructurerequiresbondangleof60
•Inasmuchasthelowestinterorbitalangleavailable
usingonlysandP-orbitalis90
•ThesmallerbondangleinP
4mustbeaccomplishedeither
throughtheintroductionofconsiderabled-characteror
throughtheuseofbentbonds.
10/21/2014 165

166. •Theformerinvolvingd-orbitalsrequiresconsiderable
promotionenergyandisthereforeunlikely.
•Thelaterinvolvingbent-bondsresultinthelossinbonding
energyofsome96KJmol
-1
duetostrainbutisthoughtto
beenergeticallyfavoured.
•Inanyeventthemoleculeisdestabilizedandquite
reactive.
•P
4cagesreactreadilywithO
2toformamixtureofoxides
andcanalsobeconvertedintoamorestableallotropes.
10/21/2014 166

167. OXIDES OF PHOSPHORUS
•Thesehavecagestructureoftetrahedralsymmetry
•ThemolecularformulaeP
4O
6orP
4O
10usuallyreferred
toasthetrioxideandpentoxiderespectively.
•Bothareanhydrides.
10/21/2014 167

168. •Only one phosphorus sulphideP
4S
10is isoelectronic and
isostructuralwith the phosphorus oxide.
•This is obtained by mixing P
4and S
8in appropriate
stoichiometric quantities.
•Other sulphidesare obtained by the reactions below:
10/21/2014 168

169. INORGANIC CHAINS
SILICATES
•ThesearemetalderivativesofsilicicacidH
4SiO
4.
•Theyarepreparedbyfusingmetaloxidesormetal
carbonateswithsand(SiO
2).
•AllthesilicateshavetheSiO
4
4-
anionformedbySP
3
hybridizationofSi-atoms,butthevarioussilicatesdiffer
fromoneanotherinthemannerinwhichtheSiO
4
4-
anionsarelinkedtogether.
10/21/2014 169

170. CLASSIFICATION
•Oneclassificationbasedonthearrangementofthesilicon
tetrahedronwithinthestructuredividethesilicatesintosix
classes.
Orthosilicates(Nesosilicates)
•TheycontaindiscreteSiO
4
4-
tetrahedralanions.The
oxygenatomofeachSiO
4
4-
isalsocoordinatedtothe
metaliontoimpartelectricalneutralitytothestructure.
10/21/2014 170

171. •Orthosilicatesarenotcommoninmineralsalthoughthey
arepresentinmineralssuchasolivine,(Mg,Fe)SiO
4.
•Othermineralsthathavethestructureofsilicatesare:
Phenacite;Be
2SiO
4,Willemite;Zn
2SiO
4,Olivite;
Mg
2SiO
4,Zircon;ZrSiO
4,Garnets;M
3
2+
M
2
3+
(SiO
4)
3
(whereM
2+
=Ca
2+
,Fe
2+
,Mg
2+
etc.andM
3+
=Al
3+
,Cr
3+
,
Fe
3+
etc.).
•InGarnetsSiO
4tetrahedraarearrangedaboutM
2+
and
M
3+
sotheybecome8-and6-coordinatedrespectively.
•Theyarequitehardandcrystalline,sotheyareusedas
abrasivesandcutforgemstones,e.g.Caperugbyand
carbuncle.
•OrthosilicatesarealsothemajorcomponentofPortland
cement
10/21/2014 171

172. Pyrosilicates(disilicateanion)(Sorosilicates)
•TheycontainthediscreteSi
2O
7
6-
anionwhichisformedby
joiningtwoSiO
4
4-
tetrahedralunitsthroughoneoxygen
atom.
•ThebestexamplearetheHermimorphite,abasichydrated
silicateofzinc,Zn
3(Si
2O
7).Zn(OH)
2.H
2Oandthorteveitite,
Sc
2(Si
2O
7).
Eg. Thorteveitite, Sc
2(Si
2O
7); Hermimorphite,
Zn
3(Si
2O
7).Zn(OH)
2.H
2O
10/21/2014 172

173. Cyclicorringsilicates(metasilicates,cyclosilicates)
•ThesecompriseoftheSiO
4residuesbridgingthroughtwoopen
oxygenatoms.
•Somanyringsizescouldbeproduced,butmanyarefound
naturally.
•TheycontaincyclicorringanionslikeSi
3O
9
6-
orSi
6O
18
12-
.
•Eg.Benitoite;BaTiSi
3O
9,Wollastonite;Ca
3Si
3O
9,Catapleit;
Na
2ZrSi
3O
9,Beryl;Ba
3Al
2Si
6O
18,Dioptase;Cu
6Si
6O
18.
•Nocyclotrimetasilicates(eg.Benitoite;BaTiSi
3O
9)havebeen
usedassemi-preciousstone.
10/21/2014 173

174. •ThecyclohexametasilicateionSi
6O
18
12-
occursasberyl;
Ba
3Al
2Si
6O
18inemeralds.
•Tourmarineisanexampleofamixedborosilicatecontaining
thisBO
3
3-
anionswhichoccursinbasicsaltseg.
[Al(OH)
4(BO
3)
3Si
6O
18]
7-
.
•Thisionoccurinsapphire(blue)andtopaz(paleyellow).
•ThecrystalstructuresshowthattheringsofSi
6O
18
12-
occupy
sheetswhichareboundtoothersbymetalionsinbetween
them.
Chainsilicates
PyroxeneandAmphibole(Inosilicates)
•Theycontaintheanionswhichareformedbysharingoftwo
oxygenatomsbyeachtetrahedra.
•Theanionsmaybeofthetypes(a)(SiO
3)
n
2n-
-pyroxes,(b)
(Si
4O
11)
n
6n-
-amphibole
10/21/2014 174

175. •Thechainsinthesilicatescontaining(SiO
3)
n
2n-
anions
linkedthroughoxygenatomslieparalleltoeachotherand
cationsliebetweenthechainsandbindthemtogether.
•SuchsilicatesarepresentedbythePYROXENEmineral
andseveralsyntheticsilicates.
10/21/2014 175

176. •Eg.SyntheticLi
2SiO
3,Na
2SiO
3,
•Pyroxene;spodumene;LiAl(SiO
3)
2,Jadeit;
NaAl(SiO
3)
2,
•Enstatite;MgSiO
3,Diopside;CaMg(SiO
3)
3.
•Syntheticsodiummetasilicatepossessthischain
structurewithSi─Obondsmuchshorterthanexpected
forsinglebonds(0.174nm).
•Thisisthoughttoresultthroughp-dbonding
betweenOandSiatoms,andoccursinbothkindsof
Si─Obondsinpyroxenesthoughmoresointhe
terminalSi─Obonds.TheSi─O─Sianglealsoreflects
some-bonding.
10/21/2014 176

177. •ThesilicatescontainingSi
4O
11
6n-
anionshavedouble
chainsinwhichthesimplecationsareheldtogetherby
sharedoxygenatoms.
•Suchsilicatesarerepresentedbytheamphibolesminerals
whichincludeasbestosminerals.
•Theamphibolesaremorecomplicatedthanthepyroxenes
andcontainrepeatingunitsaswellasmetalsand
hydroxideions.
•Structurally,amphibolesaresimilartopyroxenesthough
theycontainsomeOHgroupswhichareattachedtothe
cations.
10/21/2014 177

178. •Eg.Tremolite;Ca
2Mg
5[(OH)
2(Si
4O
11)
2],Crocodolite;
Na
2Fe
3
2+
Fe
2
3+
[(Si
4O
11)
2]
2(OH)
2
Asbestos
•Asbestoswasthetermoriginallyusedtodescribefibrous
amphiboles,butitnowincorporatesmanytwo
dimensionalpolymersencounteredamongthe
aluminosilicates.
•Thuschrysotile,onceregardedasanamphibole
(OH)
6MgSi
4O
11.H
2O,isactually(OH)
4Mg
3Si
2O
5.H
2O,the
Si
2O
5anionhavingalayerstructure.
•ReplacingtheMgbyAlgivesthealuminosilicate
(OH)
4Al
2Si
2O
5encounteredinkaolin.
10/21/2014 178

179. •PolysiloxaneswithtrifunctionalSiatomformladder
polymersandcyclicoligomers(whichinvolvejoiningthe
ladderends).
•ThesilicateSi
6O
15
6-
isanexampleofthistwo6-membered
siloxaneringsbridgedbythreeoxygenatoms,these
providingtheringsoftheladder.
•TheSi─Obondlengthslieintherange0.160–0.167nm,
theterminalonesbeingshorter.
Cleavage/fibrousnature
•TheS─Obondsinthechainarestronganddirectional.
•Adjacentchainsareheldtogetherbythemetalions
present.
•Thustheamphibolesaswellasthepyroxenescleave
paralleltothechainsformingfibers.
10/21/2014 179

180. Two-dimensionalsheet(layer)structure(Phyllosilicates)
•Thesecontaintwo-dimensionalsheetpolymerssharing
threeapicesofSiO
4tetrahedra.
•Twoarrangementshavebeenfoundnaturallyandare
formulatedasSiO
1+3/2ie(Si
2O
5)
n
2n-
.
•Themetalionspresentholdthelayerstogetherbyweak
electrostaticforces.
•Asaresultthemineralscontainingaresoftandcleave
easily.
•Suchsheetlikeanionsarefoundinmicasanddifferent
typesofclayminerals.
•Eg.Talc;Mg
2(SiO
5)
2Mg(OH)
2,Kaolin;Al
2(OH)
4(SiO
5)
2
10/21/2014 180

181. Three dimensional or framework silicates (Tectosilicates)-
Aluminosilicates
•WhenallthefouroxygenatomsofaSiO
4tetrahedronare
sharedwithadjacenttetrahedraandtheprocessisrepeated
aninfinite3-dimensionalstructureresults.
•Sincealltheoxygenatomsarethebridgeatoms,thesilicate
isneutral.
10/21/2014 181

182. •IncaseSiisnotreplacedbyanyothermetalatomthe
silicateisneutralandwillhavetheneutralformula(SiO
2)
n.
•Suchastructureisfoundinquartz,tridymiteand
crystoballite.
•HoweverifsomeotherSi
4+
ionarereplacedbyAl
3+
ionsin
thetetrahedralpositionintheSiO
2structureinorderto
maintainelectricalneutrality,someothermonovalentand
divalentcationsmustbeintroduced.
•SuchreplacementofSi
4+
cationsbyAl
3+
andother
monovalentordivalentcationsgiveriseto
aluminosilicates.
•Thealuminosilicatesaresubdividedintothefollowing
minerals:(1)Feldspars(2)Zeolites(3)Ultramarines.
10/21/2014 182

183. FELSPAR
•TheseareofgeneralformulaM(Al,Si)
4O
8andarethe
mostimportantrockformingmineralcomprisingsome
2/3
rds
ofigneousrocksuchasgranitewhichisamixture
ofquartz,felsparandmicas.
•ThoseinwhichtheMislargerionsuchasK
+
orBa
2+
normallycrystallizeinamonoclinicsystem,Eg:
orthoclase;K(AlSi
3O
8),Celsian;Ba(Al
2Si
2O
8).
•ThetriclinicorplagioclasefelsparcontainasmallerM
suchasNa
+
and/orCa
2+
,egAlbite;Na(AlSi
3O
8),and
Anorthite;Ca(Al
2Si
2O
8).
10/21/2014 183

184. ZEOLITES(porotectosilicates)
•Thesearealuminosilicateswithframeworkstructures
enclosingcavitiesoccupiedbylargeionsandwatermolecules,
bothofwhichhaveconsiderablefreemovements,permitting
ion-exchange,reversiblehydrationandabsorptionofgases.
•Theframeworkconsistsofanopenarrangementofcorner-sharing
tetrahdrawhereSiO
4unitsarepartiallyreplacedbyAlO
4tetrahedra
whichrequiresufficientcationstoachieveelectroneutrality.
•ThecavitiesareoccupiedbyH
2Omoleculesandtheidealized
formulaisM
x/n
n+
[(AlO
2)
x(SiO
2)
y]
x-
.zH
2O.
•Eg: Natrolite; Na
2(Al
2Si
2O
8).H
2O, Heulandite;
Ca(Al
2Si
7O
18).6H
2O,Chabazite;Ca(Al
2Si
4O
12).6H
2O.
10/21/2014 184

185. •Thezeoliteareusedas/for(i)Molecularsieves(ii)catalysts
(iii)catalyticsupportforplatinumgroupandothermetals(iv)
cation-exchangerse.g.forwatersoftening(v)separating
straightchainhydrocarbonsfrombranchedhydrocarbons.
ULTRAMARINES
•Theseareagroupofrelatedcompoundswhichcontainno
water,butcontainanions(i.e.radicalanions)suchasCl
-
,SO
4
2-
,
S
2
2-
,S
3
-
.
•TheyareformulatedasNa
8Al
6Si
6O
24.XwhereXis2Cl
-
,SO
4
2-
,
S
2
2-
,S
3
-
.
•Theyarecloselyrelatedtothezeolitesexceptthattheydo
notcontainwatermoleculesbutratherCl
-
,SO
4
2-
,S
2
2-
,S
3
-
anions.
10/21/2014 185

186. •Thecationproducesawiderangeofcolourandareused
forpigments.
•E.g. Ultramarine; Na
8[(AlSiO
4)]S
2, Sodalite;
Na
8[(AlSiO
4)
6]Cl
2,Nosean;Na
8[(AlSiO
4)
6]SO
4.
TheSi
4─Si
6ringlayerstructure
•ThisinvolvesalternateSi
4O
4andSi
8O
8ringsbutisrare.
•Apophihyllite(K,Na)Ca
4Si
8O
20(F,OH).8H
2Ohassucha
structurewithterminaloxygenatomsofoneSi
4O
4ring
directedtotheoppositesideofthelayertoitsSi
4O
4
neighbour.
•Cationsholdsthelayerstogether.
10/21/2014 186

187. Si
6ring
•Theseringcomprisesixsiliconandsixoxygenatoms,
andaterminaloxygenatomsallononeside.
•Consequentlyitistheoreticallypossibletoformadouble
sheetbybridgingattheseterminalpositionsthereby
obtainingalaminatedformofsilicon.
•Replacinghalfofthesiliconatomsinsuchastructureby
isoelectronicAl
-
ionleadstocompoundsofempirical
formulaMAl
2Si
2O
8(M=Ca,Ba).
•Thestructurecomprisesthesedoublelayerbound
togetherbysixcoordinatecations.
10/21/2014 187

188. Hydroxysilicates
•ThemostimportantlayersilicatesareofteninterleavedbyMg
orAlcationsheldthroughhydroxideions.
•ThesecanbeformulatedasMg
3(OH)
4Si
2O
5(chrysotile)and
Mg
3(OH)
2Si
4O
10(talc),togetherwiththealuminium
compounds.
•PartialreplacementofatomsbyAl
-
ionsgiveschargedlayer
whichareneutralizedbylayerofalkalioralkalineearthmetal
ions,asinmicas.
•Theneutralizinglayercanalsobehydratedionsorpositively
chargedMgorAlhydroxides.Thevariablesinthisstructure
arrayleadtopropertydifferencesintheselaminated
compounds.
10/21/2014 188

189. (i)ThesinglesilicatelayerSi
2O
5
2-
interleavedwith
Mgoralhydroxideresiduesisthestructuralunitpresent
inchinaclayandkaolinminerals.
ThisisnormallyformulatedasAl
2(OH)
4Si
2O
5(Kaolinite).
(ii)Thoseinvolvingtwosilicateleavesheldtogetherby
magnesiumoraluminumhydroxidesarelikekaolin,electrically
neutral.Theythereforereadilycleave.
TalcMg
3(OH)
2Si
4O
10iswidelyused,thereforeasalubricant
(Frenchchalk)andasafiller.
Meershaumisahydratedmagnesiumsilicateresemblingclay.
Aftersoakingintallowwaxitcanbemadeintopipes,andtakes
onappealingredpolish.
10/21/2014 189

190. (iii)Thealuminumanalogue,pyrophyllite,isliketalcand
bothcanhaveuptoaquarteroftheSiatomsreplacedby
Al
-
ions.
Aninterleavinglayerofcationsneutralizethecharge
withK
+
ionsinthemicas,phlogopite,
KMg
3(OH)
2Si
3AlO
10 and muscovite,
KAl
2(OH)
2Si
3AlO
10.
SincetheK
+
ionsoccupylargeholeswith12-fold
coordination,theK
+
O
-
electrostaticbondisnecessarily
weak
Somicascleavereadilyalongtheselayers.
10/21/2014 190

191. FurthersubstitutionwithAl
-
producesbrittlemicas,since
morehighlychargedcationisnecessarytoneutralizethe
extracharge,sotheelectrostaticbondingisstrongerand
themicaisharder.
Thesearewidelyusedintheelectricalindustry.
(iv)Interleavingwithlayersofhydratedcationsgives
hydratedmicaswithmuchsmallercationchargedensity
whichcleaveveryeasily.
10/21/2014 191

192. FurtherreplacementofbothcationandSiatomsintalc,and
interleavingwithhydratedMg
2+
ionsproducesvermiculite,
(Mg,Fe,Al)
3(Al,Si)
4O
10(OH)
2.4H
2O -
[Mg
2.36Fe
III
0.48Al
0.16)(Si
2.72Al
1.28)O
10(OH)
2]
-0.64
[Mg
0.32(H
2O)
4.32]
+0.64
.
Thisdehydratesreadilytoatalc-likestructure,andisused
widelyasasoilconditionerandporousfiller.
Relative Hardness of Typical Hydroxysilicates
Hydroxysilicate Formula Hardness (Moh’s scale)
Talc Mg
3(OH)
2Si
4O
10 1 -2
Mica KMg
3(OH)
2Si
3AlO
10 2 -3
Brittle CaMg
3(OH)
2Si
2Al
2O
10 31/2 -5
10/21/2014 192

193. Monimorillonite,[Mg
1/3Al
12/3Si
4AlO
10(OH)
2]
-1/3
results
frompyrophyllitebyreplacingonesixthoftheAl
3+
ions
byMg
2+
ions.
Likemanyclaysminerals,thiscanbereadilyhydratedand
exhibitscation–exchangeproperties.
ItisanimportantconstituentofFullersearth,found
widelyinSouthernEngland.
(v)Withkaolin,talcandpyrophyllitethelayersare
unchargedandsoonlyweaklyboundtogether,whilethe
micasareheldbycationlayersorhydratedcations(eg
vermiculite).
Interleavingwithchargedhydroxidelayersgivesthe
chloriteminerals.
10/21/2014 193

194. Thusthemicalayers(composition[Mg
3(AlSi
3O
10)(OH)
2]
-
to
[Mg
2Al(Al
2Si
2O
10)(OH)
2]
-
areheldbyions.
Thusinphlogopite,KMg
3(OH)
2Si
3AlO
10),replacingtheK
+
ion
bytheMg
2Al(OH)
6
+
iongivesachloritemineral.
Silica,SiO
2
•Thishasastructurecomprisingofsiliconatoms
tetrahedrallysurroundedbyfouroxygenatoms.
•Polymerizationoccursinavarietyofways,and
transmissionbetweenthevariouscrystallineformsoccur
butwithmuchdifficulty.
•Thehightemperatureformhavethemoreopenstructures,
whilethehighpressureonesaremorecompactaswould
beexpectedfromtheLeChatelier’sprinciple.
10/21/2014 194

195. •Whilesignificantstructuraldifferencesoccurbetween
thesefourformsofsilica,thosebetweenα-andβ-formare
smallandgenerallyonlyinvolvetherotationofafew
Si─Obonds.
10/21/2014 195

196. •Thisissupportedbythesameopticalactivitybeing
presentinbothα-andβ-quartz.
•β-quartzhasamoreregularstructurethan–quartzbut
bothinvolvefused6-memberedand12-memberedrings.
•Thestructurethereforeresemblethesilicatesheet
polymers.
•Impureformsofquartzareusedinsemi-precious
jewellery(e.g.amethyst,whichprobablyowesitspurple
colourtomanganese.
10/21/2014 196

197. •Quartzsandarewidelyusedinthebuildingtradeand
asanabrasive,whilequartzisemployedinpottery
andasahigh-temperatureliningtosurfacesetc.
•Ithasbeenwidelyusedinshort-waveradio
apparatus,duetothinquartzplatespossessingpiezo-
electricproperties.
•Themoreopenstructuresof–christobaliteare
analogoustowurtziteandzincblendeformsofzinc
sulphide.
10/21/2014 197

198. Nanoparticles
•Nanoparticlesareparticlesbetween1and100
nanometersinsize.
•Innanotechnology,aparticleisdefinedasasmallobject
thatbehavesasawholeunitwithrespecttoitstransport
andproperties.
•Particlesarefurtherclassifiedaccordingtodiameter.
•Ultrafineparticlesarethesameasnanoparticlesand
rangebetween1and100nanometersinsize.
•Fineparticlesaresizedbetween100and2,500
nanometers.
•Coarseparticlescoverarangebetween2,500and10,000
nanometers.
10/21/2014 198

199. •Nanoclustershaveatleastonedimension
between1and10nanometersandanarrowsize
distribution.
•Nanopowdersontheotherhandare
agglomeratesofultrafineparticles,nanoparticles,
ornanoclusters.
•Nanoparticlesizedcrystalsarecalled
nanocrystals
•Nanoparticleresearchiscurrentlyanareaof
intensescientificinterestduetoawidevarietyof
potentialapplicationsinbiomedical,opticaland
electronicfields.
10/21/2014 199

200. Synthesis of Nanoparticles
Top-Down Synthesis Processes
•Electron beam lithography
•Reactive-ion etching
•wet chemical etching,
•Focused ion or laser Etching.
•Dry etching.
•Reactive ion etching (RIE).
•Focused ion beam (FIB)
Bottom-up Approach
(1) Wet-chemical methods.
•Molecular beam epitaxy (MBE),
•Sputtering,
•liquid metal ion sources,
(2) vapour-phase methods.
10/21/2014 200

201. Nanoparticle Applications and the
Environment
•Researchersareusingphotocatalyticcopper
tungstenoxidenanoparticlestobreakdownoil
intobiodegradablecompounds.
•Researchersareusinggoldnanoparticles
embeddedinaporousmanganeseoxideasaroom
temperaturecatalysttobreakdownvolatileorganic
pollutantsinair.
•Ironnanoparticlesarebeingusedtocleanup
carbontetrachloridepollutioningroundwater.
•Ironoxidenanoparticlesarebeingusedtoclean
arsenicfromwaterwells.
10/21/2014 201

202. Nanoparticle Applications in Medicine
•Forbiologicaldetectionofdiseasecausingorganismsand
diagnosis
•Detectionofproteins
•Isolationandpurificationofbiologicalmoleculesandcells
inresearch
•Probing of DNA structure
•Genetic and tissue engineering
•Destruction of tumourswith drugs or heat
•In MRI studies
•In pharmacokinetic studies.
10/21/2014 202

203. Nanoparticle Applications in Energy and
Electronics
•Nanotetrapodsstuddedwithnanoparticlesof
carbonarebeingusedtodeveloplowcost
electrodesforfuelcellsbyResearchers.
•Goldnanoparticlescombinedwithorganic
moleculescreatesatransistorknownasa
NOMFET(NanoparticleOrganicMemory
Field-EffectTransistor).
•Acatalystusingplatinum-cobaltnanoparticlesis
beingdevelopedforfuelcellsthatproducestwelve
timesmorecatalyticactivitythanpureplatinum.
10/21/2014 203

204. •Researchershavedemonstratedthatsunlight,concentrated
onnanoparticles,canproducesteamwithhighenergy
efficiency.
•Aleadfreesolderreliableenoughforspacemissionsand
otherhighstressenvironmentsusingcoppernanoparticles
•Siliconnanoparticlescoatinganodesoflithium-ion
batteriescanincreasebatterypowerandreducerecharge
time.
•Semiconductornanoparticlesarebeingappliedinalow
temperatureprintingprocessthatenablesthemanufacture
oflowcostsolarcells.
•Alayerofcloselyspacedpalladiumnanoparticlesisbeing
usedinahydrogensensor.
10/21/2014 204