Werner's theory

Werner's Theory

1. Werner's theory
Oxidation state (oxidation number)
•Anumberassignedtoanelementinchemicalcombinationwhich
representsthenumberofelectronslost(orgained,ifthenumber
isnegative),byanatomofthatelementinthecompound.
Coordination number
•Thenumberofatomsorionsimmediatelysurroundingacentral
atominacomplexorcrystal.
[A
m
(B)
n
]
+
C
-
↑
Coordination number satisfied by secondary valency
Oxidation state (oxidation number satisfied
by primary valency
↓

2. Alfred Werner received the Nobel
Prize of Chemistry in 1913 for
his coordination theory of
transition metal amine complexes.

3. Werner'stheoryofcoordinationcompounds
•In1893Wernerputforwardatheory,calledWerner'stheoryof
coordinationcompoundstoexplainthestructureandproperties
ofCo(III)andPt(IV)ammines.
•Theseamminesaregivenbelow:
•Co(III)ammines
–CoC1
3
.6NH
3
→Orangeyellow
–CoC1
3
.5NH
3
.H
2
O→Pink
–CoC1
3
.5NH
3
→Violet
–trans-CoC1
3
.4NH
3
→Green
–cis-CoC1
3
.4NH
3
→Violet
–CoC1
3
.3NH
3
→Blue-green
•Pt(IV)ammines
–PtC1
4
.6NH
3
–PtC1
4
.5NH
3
–PtC1
4
.4NH
3
–PtC1
4
.3NH
3
–PtC1
4
.2NH
3

4. PostulatesofWerner’scoordinationtheory
Typesofvalency
•Twotypesofvalencyshownbythecentralmetalatomin
coordinationcompounds.
•Incomplexcompounds,thecentralmetalatomexhibitstwo
typesofvalencyviz.primaryvalency(alsocalledprincipal
valency→correspondstooxidationstate→electronslostor
gained)andsecondaryvalency(alsocalledauxiliaryvalency
→correspondstocoordinationnumber→ligandsaroundthe
centralmetal).
•Themetalatomalwaystendstosatisfybothofitsvalencies.
Characteristicsofsecondaryvalency
•Inmodemterminology,thesecondaryvalencyofthemetallic
atominacomplexcompoundisequaltothecoordination
numberofthatmetal.
•Secondaryvalencyofametalissatisfiedeitherbyanionsorby
neutralmoleculesaloneorbyanionsandneutralmoleculesboth.

5. •Forexample:
–Anions:In4KCN.Fe(CN)
2
orK
4
[Fe(CN)
6
],thesecondary
valencyofFe-atomissixandissatisfiedbysixCN
-
ions
(anions).
–Neutral:InCoC1
3
.6NH
3
or[Co(NH
3
)
6
]C1
3
,secondaryvalency
ofCo-atomissixandissatisfiedbysixNH
3
molecules(neutral
molecules).
–Neutral+anions:InCoC1
3
.5NH
3
or[Co(NH
3
)
5
C1]C1
2
,the
secondaryvalencyofCo-atomissixandissatisfiedbyfiveNH
3
molecules(neutralmolecules)andoneCl
-
ion(anion).
•Accordingtomodernconceptofcomplexcompounds,thespecies
satisfyingthesecondaryvalencyarecalledligands.
•Whilewritingthestructureofacomplexcompound,thespecies
satisfyingthesecondaryvalencyandthemetalarewritteninside
thecoordinationsphere.[Metal(Ligands)]
•ForexampleinCoC1
3
.4NH
3
,sincefourNH
3
moleculesandtwoCl
-
ionssatisfythesecondaryvalencyofCo-atom,itsstructureis
writtenas[CoC1
2
.4NH
3
]C1or[Co(NH
3
)
4
C1
2
]C1.

6. •Thesecondaryvalencieshavedirectionalnature,sincethe
speciessatisfyingthesecondaryvalency(i.e.,ligands)are
directedtowardsthefixedpositionsinspace.
•Thenumberofspeciessatisfyingthesecondaryvalencygives
adefinitegeometrytothecomplexcompound.
•Forexample,whenthenumberofmonodentatespecies
(ligands)whichsatisfythesecondaryvalencyofthemetalina
complexcompoundis2,3,4or6,thegeometryofthe
complexcompoundwouldbelinear,trigonalplanar,
tetrahedralorsquareplanarandoctahedralrespectively.
•IneachofthefourCo(Ill)amminesviz.,CoC1
3
.6NH
3
,
CoC1
3
.5NH
3
,CoCl
3
.4NH
3
andCoC1
3
.3NH
3
,sincethe
secondaryvalencyofCo-atomis6,eachaminehasoctahedral
geometry.
•Thespeciessatisfyingthesecondaryvalencycannotbe
obtainedinthefreestate,whentheaqueoussolutionofthe
complexcompoundundergoesionisation.
[Metal ← Ligands]
L → Geometry

7. •Forexample,whentheaqueoussolutionofCoC1
3
.4NH
3
or
[Co(NH
3
)
4
C1
2
]C1undergoesionisation,fourNH
3
molecules
andtwoClionswhichsatisfythesecondaryvalencyofCo-
atomarenotobtainedinthefreestateasshownbelow:
•Theabovediscussioncanbeconcludedbysayingthatthe
speciessatisfyingthesecondaryvalencyarenotionisable
(non-ionisable)andhencecannotbeprecipitated.
•Theattachmentofthespeciessatisfyingthesecondaryvalency
withthemetalatomisshownbyasolidline
Characteristicsofprimaryvalency
•Inmodemterminology,theprimaryvalencyofthemetallic
atominacomplexcompoundisequaltotheoxidationstate
(oroxidationnumber)ofthatmetal,e.g.,theprimaryvalency
ofCo-atominallthefourCo(III)amminesisequalto+3.

8. •Primaryvalencyofametalinacomplexcompoundisalways
satisfiedbyanions,e.g.,theprimaryvalencyofCo-atomin
eachofthefouramminesisequalto+3andissatisfiedby
threeCl-ions.
•Theanionssatisfyingtheprimaryvalencyarewrittenoutside
thecoordinationspherewhiletheanionswhichsatisfyboth
thevalencies(dualcharacter)arewritteninsidethe
coordinationsphere.
•Thusthespeciessatisfyinigprimaryvalencymaybepresent
insideand/oroutsidethecoordinationsphere.
•Thespeciessatisfyingboththevalencies(i.e.,thespecies
placedinsidethecoordinationsphere)aredirectedtowards
specificdirectionsinspaceandhencetheyhavedirectional
characteristics.
•Thespecieswhichsatisfyprimaryvalencyandareplaced
outsidethecoordinationspherehavenodirectional
characteristics.
[Co
III
(Ligands)]
3+
→ [Co
III
(Ligands)]Cl
3
[Metal(Ligands)] Anions
[Metal ← Anions] Anions

9. •Thespeciessatisfyingtheprimaryvalencydonotgiveany
geometrytothecomplexcompound.
•Thespeciessatisfyingtheprimaryvalencycanbeobtainedeither
completelyorpartiallyintheirfreestate,whenthecomplex
compoundundergoesionisationinaqueoussolution.
•Examples:
•WhentheaqueoussolutionofCoC1
3
.6NH
3
or[Co(NH
3
)
6
]C1
3
undergoesionisation,allthethreeCl
-
ionswhichsatisfyprimary
valencyofCo-atomareobtainedinthefreestate.
•AlthoughtheprimaryvalencyofCo-atominCoC1
3
.5NH
3
or
[Co(NH
3
)
5
C1]C1
2
issatisfiedbythreeCl
-
ions,onlytwoCl
-
ionsareobtainedinthefreestate,whentheaqueoussolution
ofthisammineundergoesionisation.
Anions → No information for geometry

10. •Theabovediscussionshowsthatallorsomeofthespecies
satisfyingtheprimaryvalenciesareionisableandhencecanbe
precipitatedbymeansofasuitableprecipitatingagent.
•Forexample,intheaboveamminetwoCl
-
ionscanbe
precipitatedasAgC1(whiteppt.)byaddingaqueoussolution
ofAgNO
3
.
•Fromtheabovediscussionwecanconcludethatallthe
anionsplacedoutsidethecoordinationspherecanbe
obtainedinthefreestateandhencecanbeprecipitated.
•Thusalltheanionsplacedoutsidethecoordinationsphere
areionisable.
•Theattachmentofthespeciessatisfyingtheprimary
valencytothemetalisshownbybrokenline(–––).[Metal(Ligands)]
Anions
→
Ionisable

11. Howtoshowtheattachmentbetweenthecentralmetal
andthespeciessatisfyingboththevalencies
•Theattachmentbetweenthemetalandthespecieswhich
satisfyboththevalenciesisshownbyacombinedsolid-
brokenline .
•ForexampleinCoC1
3
.5NH
3
or[Co(NH
3
)
5
C1]C1
2
,since
oneCl
-
ionsatisfiesprimaryaswellassecondaryvalency
ofCo-atom,theattachmentofthisC1
-
iontoCo-atomis
shownas
ToexplainthestructureofCo(III)amminesonthebasisof
Werner'scoordinationtheory
•WiththehelpofthepostulatesofWerner'scoordination
theory,thestructureofCo(III)amminesviz.CoCl
3
.6NH
3
,
CoC1
3
.5NH
3
,CoC1
3
.4NH
3
andCoC1
3
.3NH
3
canwellbe
explained.Inalltheseammines,theprimaryvalencyofcobalt
isequalto3whilethesecondaryvalencyisequalto6.

12. •Primaryvalencyof3issatisfiedbythreeCl
-
ionsand
hencetheseionsareattachedwithCo
3+
ionbydottedline
(---).
•SecondaryvalencyofsixissatisfiedeitherbysixNH
3
moleculesaloneorbybothNH
3
moleculesandCl
-
ions.
•Thespeciessatisfyingthesecondaryvalencyareattached
withCo
3+
ionbysolidline
1.CoC1
3
.6NH
3
•TheprimaryvalencyofCo-atomwhichisequalto+3is
satisfiedbythreeCl
-
ionsandhencetheattachmentof
theseionswithCo
3+
ionisshownbybrokenlines
[followingfigure(a)].
•Whenthisammineisacteduponbyanaqueoussolution
ofAgNO
3
,allthethreeC1
-
ionspresentinthisammine
areprecipitatedasAgC1(whiteppt).

13. •TheprecipitationofallthethreeCl
-
ionsindicatesthat
theseionsareoutsidethecoordinationsphereanditis
onlysixNH
3
moleculeswhich,alongwithCoatom,are
presentinsidecoordinationsphere.
•ThusthestructureofCoC1
3
.6NH
3
canbewrittenas
[Co.6NH
3
]C1
3
or[Co(NH
3
)
6
]C1
3
.
•ThisstructureshowsthatthesecondaryvalencyofCo
atomwhichisequaltosixissatisfiedbysixNH
3
moleculesplacedinsidethecoordinationsphereand
hencetheattachmentofNH
3
moleculestoCo-atomis
shownbysolidlines[seefollowingfigure(a)].

15. 2.CoC1
3
.5NH
3
•TheprimaryvalencyofCo-atominthisammineisequal
to3andissatisfiedbythreeCl
-
ions.
•TheattachmentoftheseionswithCo
3+
ionis,therefore,
shownbybrokenlines[followingfigure(b)].
•Thetreatmentofthisamminewiththeaqueoussolution
ofAgNO
3
precipitatesonlytwoCl
-
ionsasAgC1(white
ppt.).
•TheprecipitationofonlytwoC1
-
ionsindicatesthattwoCl
-
ions
areoutsidethecoordinationsphereandtheremainingoneCl
-
ion
andfiveNH
3
moleculesarepresentinsidethecoordinationsphere.
•Thusonthebasisoftheseobservations,thestructureof
CoC1
3
.5NH
3
canbewrittenas[CoC1.5NH
3
]C1
2
or
[Co(NH
3
)
5
C1]C1
2
.

16. •ThisstructureshowsthatthesecondaryvalencyofCo-
atomwhichisequaltosixissatisfiedbyoneCl
-
ionand
fiveNH
3
moleculesplacedinsidethecoordinationsphere.
•TheattachmentofoneCl
-
ionandfiveNH
3
moleculesto
Co-atomis,therefore,shownbysolidlines.
•Theabovediscussionshowsthat,onthebasisofWerner's
theory,thestructureofCoC1
3
.5NH
3
canbeshownas
depictedinfollowingfigure(b).
•ThisfigureshowsthatoutofthethreeCl
-
ions,oneCl
-
ionisattachedwithCo-atombyacombinedsolid-broken
line,sincethisionsatisfiesbothprimaryaswell
assecondaryvalencyofCo-atom.

17. Note→Somechlorideionsmustdodoubledutyandhelpsatisfy
thebothofprimaryvalenceandsecondaryvalence.

18. 3.CoC1
3
.4NH
3
•TheprimaryvalencyofCo-atomwhichisequalto3is
satisfiedbythreeCl
-
ionsandhencetheattachmentof
theseionstoCo-atomisshownbybrokenlines[see
followingfigure(c)].
•Thetreatmentofthisamminewiththeaqueoussolution
ofAgNO
3
precipitatesonlyoneCl
-
ionasAgC1(white
ppt.).
•TheprecipitationofonlyoneCl
-
ionindicatesthatoneCl
-
ionisoutsidethecoordinationsphereandtheremaining
twoCl
-
ionsandfourNH
3
moleculesarepresentinside
thecoordinationsphere.

19. •ThusthestructureofCoC1
3
.4NH
3
canberepresentedas
[CoC1
2
.4NH
3
]Clor[Co(NH
3
)
4
C1
2
]C1whichshowsthat
thesecondaryvalencyofCo-atomwhichisequaltosixis
satisfiedbytwoCl
-
ionsandfourNH
3
moleculesplaced
insidethecoordinationsphereandhencetheattachment
ofthesesixspecieswithCo-atomhasbeenshownby
solidlines.
•Thus,onthebasisofWerner'stheory,thestructureof
CoC1
3
.4NH
3
canbeshownasgiveninfollowingfigure
(c)inwhichtheattachmentoftwoCl
-
ionswithCo-atom
hasbeenshownbysolid-brokenlines,sincetheseions
satisfybothtypesofvalencyofCo-atom.

21. 4.CoC1
3
.3NH
3
•TheprimaryvalencyofCo-atomwhichisequalto3is
satisfiedbythreeC1
-
ionsandhencetheattachmentof
theseionswithCo-atomisrepresentedbybrokenlines
[seefollowingfigure(d)].
•Whenthisammineistreatedwiththeaqueoussolutionof
AgNO
3
,noprecipitateisobtained.
•Thenon-formationofanyprecipitateshowsthatnoCl
-
ionis
outsidethecoordinationsphere,i.e.,allthethreeC1
-
ionsand
threeNH
3
,alongwithCo-atom,arepresentinsidethe
coordinationsphere.
•Thusthestructureofthisamminecanbewrittenas
[CoC1
3
.3NH
3
]
0
or[Co(NH
3
)
3
C1
3
]
0
.
•ThisstructureshowsthatthesecondaryvalencyofCo-atom
whichisequaltosixissatisfiedbythreeCl
-
ionsandthree
NH
3
moleculesplacedinsidetheco-ordinationsphere.

22. •ThusthelinkingofthesesixspecieswithCo-atomis
representedbysolidlines.
•Thisdiscussionindicatesthat,onthebasisofWerner's
theory,thestructureofCoC1
3
.3NH
3
canbeshownas
giveninfollowingfigure(d)inwhichtheattachmentof
threeCl
-
ionswithCo-atomhasbeenrepresentedby
solid-brokenlines,sincetheseionssatisfybothtypesof
valencyofCo-atom.

24. ToexplainthestructureofPt(IV)amminesonthebasis
ofWerner'scoordinationtheory
•WehavealreadysaidthePt(IV)givesfiveammines
whicharePtC1
4
.6NH
3
,PtC1
4
.5NH
3
,PtCl
4
.4NH
3
,
PtC1
4
.3NH
3
andPtCl
4
.2NH
3
.
•Ineachoftheseammines,primaryvalencyofPtisequal
toitsoxidationstate(=+4)whilethesecondaryvalencyis
equalto6.
•Thuseachamminehasoctahedralgeometry.
•Thestructureoftheseamminescanbewrittenasshown
infollowingtable.
Cryoscopicmeasurements
Cryoscopicmeasurementsgivesthenumberofionsfurnishedbythe
dissociationofonemoleculeofthecompound.

26. LimitationsofWerner’sTheory
•Likeallthemajortheories,Werner’sTheorywasnot
freefromlimitations.
•Thecommonlimitationsofthetheoryare:
–Itcouldnotexplaintheinabilityofallelementsto
formcoordinationcompounds.
–TheWernerstheorycouldnotexplainthe
directionalpropertiesofbondsinvarious
coordinationcompounds.
–Itdoesnotexplainthecolour,themagneticand
opticalpropertiesshownbycoordination
compounds.

27. The End