Organometallics Lecture ppt 28.10.2022.pdf

Nonthing

1. 1
Chemistry of Organometallic Compounds

2. 2
•Organometalliccompounds:directmetal-carbonbonds.
•Anorganometalliccompoundisbroadlydefinedasonethatinvolvesoneormore
carbonatomsofanorganicgroupormoleculeandatransition,lanthanide,actinide,or
maingroupmetalatom.
•In1848,E.C.Franklandsynthesizeddimethylzinc,andthensubsequentlydiscovered
Zn(C
2H
5)
2,Hg(CH
3)
2,Sn(C
2H
5)
4andB(CH
3)
3.
•Amongtheoldestandwell-usedorganometalliccompoundsincludetheGrignard
reagentsRMgX,alkyllithiums,RLi.Theyhavefoundextensiveuseinthelaboratory-
scalesyntheticchemistry.
•Sincethe1960s,exploratorysyntheticresearchinorganometalliccompoundshasbeen
dominatedbystudiesofd-blockelements.
Main group: (AlMe
3)
2
What are organometallic compounds?

3. 3
•Hapticity“”wasoriginallydevelopedtoindicatehowmanycarbonsofa-systemwere
coordinatedtoametalcenter.
•Itisnowusedtodescribethebondingmodeofaligandtoametalcenter.
•An
5
-cyclopentadienylligand,forexample,hasallfivecarbonsoftheringbondingtothe
transitionmetalcenter.
•
x
valuesforcarbonligandswherethexvalueisoddusuallyindicateanioniccarbonligands
(e.g.,
5
-Cp,
1
-CH
3,
1
-allylor
3
-allyl,
1
-CH=CH
2).Thenumberofelectronsdonatedbythe
ligandisusuallyequaltox+1.
•Whenxvalueiseven,
x
usuallyindicateneutralcarbon-systemligands(e.g.,
6
-C
6H
6,
2
-
CH
2=CH
2,
4
-butadiene,
4
-cyclooctadiene).Thenumberofelectronsdonatedbytheligandin
theeven(neutral)caseisusuallyjustequaltox.
Hapticity
Cationic 2e
-
donor :
Neutral 2e
-
donors :
Anionic 2e
-
donors :
Anionic 4e
-
donors :
Anionic 6e
-
donor :

4. 4
•Hapticityisthecoordinationofaligandtoametalcenterviaanuninterruptedand
contiguousseriesofatoms.
•ThehapticityofaligandisdescribedwiththeGreekletterη('eta').Forexample,
η
2
describesaligandthatcoordinatesthrough2contiguousatoms.
•Denticityreferstothenumberofdonorgroupsinasingleligandthatbindtoacentral
atominacoordinationcomplex.Inmanycases,onlyoneatomintheligandbindstothe
metal,sothedenticityequalsone,andtheligandissaidtobemonodentateor
unidentate).Ligandswithmorethanonebondedatomarecalledpolydentateor
multidentate.
•ThedenticityofaligandisdescribedwiththeGreekletterκ('kappa').Forexample,κ
6
-
EDTAdescribesanEDTAligandthatcoordinatesthrough6non-contiguousatoms.
•Denticityisdifferentfromhapticitybecausehapticityrefersexclusivelytoligandswhere
thecoordinatingatomsarecontiguous.
Hapticity vs Denticity

5. 5
18-Electron Rule and Stability of Organometallic Complexes (1920 by Sidgwick)
•18-ElectronRule:Inmononuclear,diamagneticcomplexes,thetotalnumberofelectronsnever
exceeds18(noblegasconfiguration).Thetotalnumberofelectronsisequaltothesumofd-
electronsplusthosecontributedbytheligands.
•Stabilityofmanyorganometalliccomplexes,particularlythoseinvolvingthefirstrowtransition
metalscanbepredictedbyusing18-electronrule.
•Thisrulecanbestatedas,whenametalachievesanouterelectronconfigurationofns
2
(n-
1)d
10
np
6
,therewillbe18electronsinthevalenceorbitalsandaclosedstableconfigurationwill
result.
•Formanysecondandthirdrowtransitionmetals16electroncomplexesarealsofound.
•The18-electronruleisquiteadvantageouslypredictthestabilityandexistenceoforganometallic
compounds.
Examples:
Electroncountingtodeterminethenumberofvalenceelectronsaboutametalcenterinagiven
transitionmetalcomplex:
•Determinetheoxidationstateofthetransitionmetalcenter(s).Todothisonemust:
i.noteanyoverallchargeonthemetalcomplex
ii.knowthechargesoftheligandsboundtothemetalcenter
iii.knowthenumberofelectronsbeingdonatedtothemetalcenterfromeachligand
•Adduptheelectroncountsforthemetalcenterandligands.

6. 6Rh
Ph3P Cl
Ph
3P PPh3
Rh is s
1
d
8
= 9e
since Cl is -1, Rh is +1 (the complex is neutral)
4 ligands x 2e each = 8e9e - 1e + 8e = 16e
therefore coordinately unsaturated
Calculation -18-Electron Rule

7. 7
Calculation -18-Electron Rule

8. 8
Other Examples for practice: [V(CO)
6]
-
,[Fe(H
2O)
6]
2+
, [Co(CN)
6]
3-
,[Cu(NH
3)
4]
+
Mn
2(CO)
10, Co
2(CO)
8
Cr(CO)
6 Cr
6CO
Total
Ni(CO)
4 Ni
4CO
Total
Fe(CO)
5 Fe
5CO
Total
Cr(NO)
4 Fe(Cp)
2 Cr(C
6H
6)
2
Fe(CO)
4PPh
3 Fe(CO)
2(NO)
2 [Mn(CO)
5]
-
•There are multiple examples of transition metal compounds with less (group 3, 4 and 10) or more than
18 electrons in the metal shell
18-Electron Rule –For Practice

9. 9
Transition metal carbonyl complexes (metal carbonyls) are among the most well studied
organometallic compounds. Almost all transition metals form carbonyls.
Ten of the CO ligandsare terminal and two span an
Fe---Fe edge, resulting in C
2vpoint group symmetry.
Mn
2(CO)
10has no bridging CO ligands: it
can be described (CO)
5Mn-Mn(CO)
5.
The major isomer contains two bridging CO ligandslinking the Co atoms
and six terminal CO ligands, three on each Co atom. It can be described
by the formula (CO)
3Co(μ-CO)
2Co(CO)
3and has C
2vsymmetry.
Metal Carbonyl Compounds
Pentacarbonylhydridomanganesehas
octahedral symmetry, point group C
4v. Ferroceneis an organometallic
compound Fe(C
5H
5)
2.
Practice for 18-Electron Rule –All the above complexes

10. 10
BondingofCO
•TheMOconfigurationofCOmoleculeis:
KK1
nb
2
2
2
1
4
3
nb
2
•Carbonylligandisconsideredasaweak2-electron
-donorandverystrong-acceptor.
•Twotypesofinteractionsareinvolvedinthe
complexationofcarbonylwithtransitionmetalion.
•Electrondonationofthelonepaironcarbon-
orbitaltovacantmetaldorbital.
•Thiselectrondonationmakesthemetalmore
electronrich-compensateforthisincreasedelectron
density,afilledmetald-orbitalmayinteractwiththe
empty*orbitalonthecarbonylligand.
•WhatstabilizesCOcomplexesisM→Cπ–bonding
orsynergisticbonding
•Thelowertheformalchargeonthemetalionthe
morewillingitistodonateelectronstotheπ–orbitals
oftheCO.
•Thus,metalionswithhigherformalcharges,e.g.
Fe(II)formCOcomplexeswithmuchgreaterdifficulty
thandozero-valentmetalions.
Bonding in CO

11. 11
•Zeise’sSalt, potassium trichloro(ethene)platinate(II), K[Pt(C
2H
4)Cl
3].H
2O
synthesized in 1827 reported as the first olefin complex, confirmed to have
H
2C=CH
2as a ligandin 1868.
•Zeiseisolated stable yellow crystals when he refluxed an alcoholic solution of
potassium tetrachloroplatinate.
•This compound is commercially available as a hydrate. The hydrate is
commonly prepared from K
2[PtCl
4] andethylenein the presence of
acatalyticamount ofSnCl
2. The water of hydration can be removedin vacuo.
K
2[PtCl
4] + C
2H
5OH K[Pt(C
2H
4)Cl
3] + Cl
-
+ H
2O
Structure and Bonding (Study in Details from Reference Books)
•The alkeneC=C bond is approximately perpendicular to the PtCl
3plane and
occupies the fourth coordination site of the square planar complex with the
carbon-carbon axis perpendicular to the platinum-ligandplane.In Zeise'ssalt
and related compounds, the alkenerotates about the metal-alkenebond with a
modestactivation energy.
•The bond between the ethylene molecule and the Pt metal ion may be
considered as a dative –bond to a suitable hybrid orbital on the Pt atom (dsp
2
)
and a synergic –bond.
Zeise’sSalt
Important -Structure and Bonding in Zeise’ssalt (Study in Details from Reference Books)

12. 12
•Ferroceneis anorganometalliccompound with the formula Fe(C
5H
5)
2. It is themetallocene, consisting of two
cyclopentadienyl rings bound on opposite sides of a centralmetalatom. Such organometallic compounds are
also known assandwich compounds.
•Ferrocene was first prepared unintentionally. In 1951,Peter PausonandT. J. Kealyreported the reaction of
cyclopentadienyl magnesium bromide andferric chloridewith the goal of oxidatively coupling the diene to
prepare fulvalene. Instead, they obtained a light orange powder of "remarkable stability".
•A second group was also unknowingly discovered ferrocene in 1952. Miller, Tebboth and Tremaine were trying
to synthesize amines from hydrocarbons such as cyclopentadiene and ammonia in a modification of theHaber
process. The stability of the new organoiron compound was accorded to the aromatic character of the negatively
charged cyclopentadienyls, but they were not the ones to recognize theη
5
(penta-hapto) sandwich structure.
•Robert Burns WoodwardandGeoffrey Wilkinsondeduced the structure based on its reactivity.
•IndependentlyErnst Otto Fischeralso came to the conclusion of the sandwich structure and started to
synthesize other metallocenessuch asnickeloceneandcobaltocene.
Ferrocene

13. 13
Pausonand Kealysynthesisedferroceneusingiron(III) chloride and aGrignard reagent, cyclopentadienyl
magnesium bromide. Iron(III) chloride is suspended in anhydrous diethyletherand added to the Grignard reagent,
which is prepared by reactingcyclopentadienewith magnesium andbromoethanein anhydrous benzene.
More efficient preparative methodis using either commercially availablesodium cyclopentadienideorfreshly
crackedcyclopentadienedeprotonatedwithpotassium hydroxideand reacted with anhydrous iron(II) chloride
in ethereal solvents/THF.
2NaC
5
H
5
+ FeCl
2
→ Fe(C
5
H
5
)
2
+ 2NaCl
FeCl
2
·4H
2
O + 2C
5
H
6
+ 2KOH → Fe(C
5
H
5
)
2
+ 2KCl+ 6H
2
O
The other early synthesis of ferrocenewas by Milleret al.,who reacted metallic iron directly with gas-phase
cyclopentadieneat elevated temperature.
Ferrocene -Synthesis
Important -Structure and Bonding in Ferrocene (Study in Details from Reference Books)

14. 14
Bridging versus terminal carbonyls in [Fe
2(CO)
9]OC
FeOC
OC
C
C
Fe
CO
CO
CO
C
O
O
O

15. 15
IR spectra and metal-carbon bonds
Theυ
COstretchingfrequencyofthecoordinatedCOisveryinformative
Recallthatthestrongerabondgets,thehigheritsstretchingfrequency
M=C=O(C=Oisadoublebond)canonicalstructure
Lowertheυ
COstretchingfrequencyascomparedtotheM-C≡Ostructure(triplebond)
Note:υ
COforfreeCOis2041cm
-1
)
[Ti(CO)
6]
2-
[V(CO)
6]
-
[Cr(CO)
6][Mn(CO)
6]
+
[Fe(CO)
6]
2+
υ
CO 1748 1858 1984 2094 2204cm
-1
Bridging versus terminal carbonyls
BridgingCOgroupscanberegardedashavingadoublebondC=Ogroup,as
comparedtoaterminalC≡O,whichismorelikeatriplebond:
theC=OgroupinabridgingcarbonylismoreliketheC=Oinaketone,whichtypically
hasυ
C=O=1750cm
-1
terminalcarbonylbridgingcarbonyl
(~1850-2125cm
-1
)(~1700-1860cm
-1
)
BridgingCObetween1700and2200cm
-1

16. 16
References
https://freevideolectures.com/course/3371/introduction-to-organometallic-chemistry
https://web.iitd.ac.in/~elias/links/Basic%20Inorganic%20chemistry%20part%202%20Org
anometallic%20chemistry.pptx
http://ccc.chem.pitt.edu/wipf/courses/1140_05_files/Organometallic%20Chemistry.ppt_fil
es/Organometallic%20Chemistry.ppt.ppt
https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book%3A_Basic_Principles
_of_Organic_Chemistry_(Roberts_and_Caserio)/31%3A_Transition_Metal_Organic_Co
mpounds/31.01%3A_Metallocenes
https://www.rroij.com/open-access/systematic-approach-for-the-bonding-in-ferrocene-
.php?aid=33929
https://chemizi.blogspot.com/2019/08/what-is-zeises-salt-discuss-structure.html
https://journals.iucr.org/b/issues/1969/09/00/a06881/a06881.pdf

17. Reference Books