Transition Metal Carbonyl Compounds

Dr.S. H. Burungale

1. Transition metal carbonyls and
related compounds
Dr.S.H.Burungale

2. Metal carbonyls
Themetal-carbonbondsinmetalcarbonylshavebothσ
andπcharacters.Aσbondisformedwhenthecarbonyl
carbondonatesalonepairofelectronstothevacant
orbitalofthemetal.Aπbondisformedbythedonation
ofapairofelectronsfromthefilledmetaldorbitalinto
thevacantanti-bondingπorbital(alsoknownasback
bondingofthecarbonylgroup).Theσbondstrengthens
theπbondandvice-versa.Thus,asynergiceffectis
createdduetothismetal-ligandbonding.Thissynergic
effectstrengthensthebondbetweenCOandthemetal.

3. Classification of Metal Carbonyls:
1.Classificationonthebasisofligands:
Metalcarbonylscanbeclassifiedintotwocategories:
(i)Homolepticcarbonylcomplexes:Thecomplexinwhichthemetalisbound
toonlyCOasligandsareknownashomolepticcarbonylcomplexes.for
example,Ni(CO)4,Cr(CO)6,Fe(CO)5,Co2(CO)8,Mn2(CO)10,Fe3(CO)12,
Ir4(CO)12etc.
(ii)Heterolepticcarbonylcomplexes:Thecomplexesinwhichametalis
boundtoCOaswellasotherligandssuchasPR3,PPh3,PF3,NO.RNC
etc.Forexample,Ni(CO)3PPh3,Mo(CO)3(PF3)3,Cr(CO)3(NO)2etc.

4. 2.Classification on the basis of a number of metal atoms and the
structures of Metal Carbonyls:
(i)Mononuclear Metal Carbonyls:These carbonyls contain only one
metallic atom and these carbonyls do not contain any bridging CO
ligand.For example Ni(CO)4,Cr(CO)6,Fe(CO)5 etc.
(ii)Polynuclear Metal Carbonyls:Polynuclear carbonyls contain only
one or two metal atoms and these are classified as
(a)Homonuclear Metal Carbonyls:These contain metal atoms of only
one element.For example
Fe2(CO)9,Mn2(CO)10,Co2(CO)8,Fe3(CO)12,Co4(CO)12,Rh4(CO)12,I
r4(CO)12 etc.
.

5. (b)HeteronuclearMetalCarbonyls:Thesecarbonylscontain
metalsofdifferentelements.Forexample,MnCo(CO)4
Thepolynuclearmetalcarbonylsarealsoclassifiedas:
(a)Non-bridgedMetalCarbonyls:Thesecarbonylscontain
terminalCOligandandM-Mbonds.Forexample,Co2(CO)8
(Insolution),Mn2(CO)10,Ir4(CO)12etc.
(b)BridgedMetalCarbonyls:Thesecarbonylscontainbridged
aswellasterminalCOligandsandM-Mbonds.For
example,Fe2(CO)9,Co2(CO)8(insolidstate),Fe3(CO)12etc

6. Synthesis
Thesynthesisofmetalcarbonylsissubjectofintense
organometallicresearch.SincetheworkofMondand
thenHieber,manyprocedureshavebeendevelopedfor
thepreparationofmononuclearmetalcarbonylsas
wellashomo-andheterometalliccarbonylclusters.

7. Direct reaction of metal with carbon
monoxide
NickeltetracarbonylandIronpentacarbonylcanbe
preparedaccordingtothefollowingequationsbyreaction
offinelydividedmetalwithcarbonmonoxide.
Ni + 4CO → Ni(CO)
4(1 bar, 55 °C)
Fe + 5CO → Fe(CO)
5(100 bar, 175 °C)
Nickeltetracarbonylisformedwithcarbonmonoxide.
alreadyat80°Candatmosphericpressure,finelydivided
ironreactsattemperaturesbetween150and200°Canda
carbonmonoxidepressureof50–200bar.
[
Othermetal
carbonylsarepreparedbylessdirectmethods.

8. Reduction of metal salts and oxides
CrCl
3+ Al + 6CO → Cr(CO)
6+ AlCl
3
WCl
6+ 6CO + 2Al(C
2H
5)
3→ W(CO)
6+ 2AlCl
3+ 3C4H10
Tungsten ,molybdenum ,Manganese, andrhodium salts may
be reduced with lithium aluminium hydride.Vanadium
hexacarbonyl is prepared with sodium as a reducing agent
inchelating solvents such asdiglyme
VCl
3
+ 4Na + 6CO + 2diglyme → Na(diglyme)
2
[V(CO)
6
] +
3NaCl
[V(CO)
6
]
−
+ H
+
→ H[V(CO)
6
] →1/2H
2
+ V(CO)
6

9. Photolysis and thermolysis
PhotolysisorThermolysisofmononuclearcarbonylsgeneratesdi-
andpolymetalliccarbonylssuchasFe
2(CO)
9.
Onfurtherheating,theproductsdecomposeeventuallyintothe
metalandcarbonmonoxide.
2Fe(CO)
5→Fe
2(CO)
9+COThethermaldecompositionof
triosmiumdodecacarbonyl(Os
3(CO)
12)provideshigher-nuclear
osmiumcarbonylclusterssuchasOs
4(CO)
13,Os
6(CO)
18upto
Os
8(CO)
23.
MixedligandcarbonylsofRu,Os,RhandIrareoftengeneratedby
abstractionofCOfromsolventssuchas(DMF).

10. Salt metathesis
salts such as KCo(CO)
4with [Ru(CO)
3Cl
2]
2leads
selectively to mixed-metal carbonyls such as
RuCo
2(CO)
11
4KCo(CO)
4+ [Ru(CO)
3Cl
2]
2→ 2RuCo
2(CO)
11+
4KCl + 11CO

11. Metal carbonyl cations and
carbonylates
Thesynthesisofioniccarbonylcomplexesispossibleby
oxidationorreductionoftheneutralcomplexes.Anionic
metalcarbonylatescanbeobtainedforexamplebyreduction
ofdinuclearcomplexeswithsodium.Afamiliarexampleis
thesodiumsaltofirontetracarbonylate(Na
2Fe(CO)
4,
1.Fe(CO)
5+2Na→Na
2[Fe(CO)
4]+CO
2.2Fe(CO)
5+2Na→Na
2[Fe
2(CO)
8]+2CO

12. CO
CO
Ni
OC
CO
CO
OCFe
CO
CO
CO
CO
Cr
OC
OC CO
CO
CO
CO
MnOC
OC
CO
CO
OC
Mn
OC
CO
CO
CO
OC
CoOC
OC
OC
Co
O
C
CO
CO
CO
C
O
Os
Os Os
OC
OC
CO
CO
OC CO
CO
CO
CO
CO
CO
Ir
Ir
Ir
Ir
OC
OC
OC
OC
CO
CO
CO
CO
CO
COOC
OC Coordination number around the metal normally remains six or lesser. 17
electron species such as Mn(CO)
5, Co(CO)
4dimerizeto gain 18 electrons
V(CO)
6does not dimerize.
Structure of Metal carbonyls

13. M
C
O
M M
C
O
terminal
bridging
2
M
M
M
C
O
bridging
3
2120-1850 cm
-1

CO
1850-1700 cm
-1 1730-1620 cm
-1 Cr
OC
OC CO
CO
CO
CO Fe
Fe
Fe
OC
FeOC
CO
CO
Cp
Cp
Cp
Cp 1620cm-
1
2018, 1826
cm-1
2000cm-
1
Terminal versus bridging
carbonyls

14. Determine the total valence electrons (TVE) in the entire molecule (that is, the number of valence electrons of the
metal plus the number of electrons from each ligand and the charge); say, it is A.
Subtract this number from n×18 where nis the number of metals in the complex, that is, (n×18) –A; say, it is B.
(a)Bdivided by 2 gives the total number of M–M bonds in the complex.
(b) Adivided by ngives the number of electrons per metal. If the number of electrons is 18, it indicates that there
is no M–M bond; if it is 17 electrons, it indicates that there is 1 M–M bond; if it is 16 electrons, it indicates
that there are 2 M–M bonds and so on.
How to determine the total number of metal -
metal bonds Fe
Fe
Fe Co
Co Co
Co
Molecule TVE
(A)
(18 ×n) –A
(B)
Total M–M
bonds(B/2)
Bondspermetal Basicgeometryofmetal
atoms
Fe
3
(CO)
12
48 54–48=6 6/2=3 48/3=16;2
Co
4
(CO)
12
60 72–60=12 12/2=6 60/4=15;3
[η
5
-CpMo(CO)
2
]
2
30 36–30=6 6/2=3 30/2=15;3 Mo≡Mo
(
4
-C
4
H
4
)
2
Fe
2
(CO)
3
30 36–30=6 6/2=3 30/2=15;3 Fe≡Fe
Fe
2
(CO)
9
34 36–34=2 2/2=1 34/2=16;1 Fe–Fe

15. PHYSICALPROPERTIES
State:Majorityofthemetalliccarbonylsareliquidsor
volatilesolids.
Colour:Mostofthemononuclearcarbonylsarecolourlessto
paleyellow.V(CO)6isabluish-blacksolid.Polynuclear
carbonylsexhibitaredarkincolour.
Solubility:Metalcarbonylsaresolubleinorganicsolvents
likeglacialaceticacid,acetone,benzene,carbontetrachloride
andether.

16. Toxicity:
Duetolowmeltingpointsandpoorthermalstability,theyshowtoxicityrelatedto
thecorrespondingmetalandcarbonmonoxide.Exposuretothesecompoundscan
causedamagetolungs,liver,brainandkidneys.Nickeltetracarbonylexhibits
strongestinhalationtoxicity.Thesecompoundsarecarcinogenicoverlong-term
exposure.
MagneticProperty:Allthemetalcarbonylsotherthanvanadium
hexacarbonylarediamagnetic.Themetalswithevenatomicnumberform
mononuclearcarbonyls.Thus,alltheelectronsinthemetalatomsarepaired.In
caseofdinuclearmetalcarbonylsformedbymetalswithoddatomicnumber,the
unpairedelectronsareutilizedfortheformationofmetal-metalbonds.

17. Thermal Stability:
Most of the metal carbonyls melt or decompose at low temperatures.
Solid carbonyls sublime in vacuum but they undergo some degree of
degradation.
Thermodynamic Stability:
Metal carbonyls are thermodynamically unstable. They undergo aerial
oxidation with different rates. Co2(CO)8 and Fe2(CO)9 are oxidized
by air at room temperature while chromium and molybdenum
hexacarbonyls are oxidized in air when heated

18. REACTIONS OF METAL CARBONYLS
1.SubstitutionReactions
Manysubstitutionreactionsoccurbetweenmetalcarbonylsandother
potentialligands.
Forexample,
Cr(CO)
6+ 2 py → Cr(CO)
4(py)
2+ 2 CO
Ni(CO)
4+ 4 PF
3→ Ni(PF
3)
4+ 4 CO
Mo(CO)
6+ 3 py → Mo(CO)
3(py)
3+ 3 CO

19. Substitutionreactionsofmetalcarbonylsfrequentlyindicatedifferencesinbonding
characteristicsofligands.InthecaseofMn(CO)
5Br,radiochemicaltracerstudies
haveshownthatonlyfourCOgroupsundergoexchangewith
14
CO.
Mn(CO)
5Br+4
14
COMn(
14
CO)
4(CO)Br+4CO
ThefourCOmoleculesthatundergoexchangereactionsarethoseintheplane,
whicharealltranstoeachother.ThisindicatesthattheCOtranstoBrisheldmore
tightlybecauseBrdoesnotcompeteforπbondingelectrondensitydonatedfrom
Mn.InthecaseoftheotherfourCOgroups,competitionbetweenthegroups,
whichareallgoodacceptors,causesthegroupstobelabilized

20. 2.ReactionswithHalogens
Reactionsofmetalcarbonylswithhalogensleadtothe
formationofcarbonylhalidecomplexesbysubstitution
reactionsorbreakingmetal-metalbonds.
Thereaction
[Mn(CO)
5]
2+Br
2→2Mn(CO)
5Br
involvestheruptureoftheMn–Mnbond,andoneBris
addedtoeachMn.

21. Inthereaction
Fe(CO)
5+ I
2→ Fe(CO)
4I
2+ CO
oneCOisreplacedontheironbytwoiodineatomssothat
thecoordinationnumberoftheironisincreasedto6.The
formulasforthesecarbonylhalidesobeytheEANrule.
ThereactionofCOwithsomemetalhalidesresultsinthe
formationofmetalcarbonylhalidesdirectly,asillustratedin
thefollowingexamples:
PtCl
2+ 2 CO → Pt(CO)
2Cl
2
2 PdCl
2+ 2 CO → [Pd(CO)Cl
2]
2

22. 3.ReactionswithNO
Thenitricoxidemoleculehasoneunpairedelectronresidinginanantibonding
π*molecularorbital.Whenthatelectronisremoved,thebondorderincreases
from2.5to3,soincoordinatingtometals,NOusuallybehavesasthoughit
donatesthreeelectrons.Theresultisformallythesameasifoneelectronwere
losttothemetal,
NO → NO + e
followedbycoordinationofNO
+
,whichisisoelectronicwithCOandCN.
BecauseNO
+
isthenitrosylion,theproductscontainingnitricoxideand
carbonmonoxidearecalledcarbonylnitrosyls.Thefollowingreactionsare
typicalofthoseproducingthistypeofcompound:
Co
2(CO)
8+2NO→2Co(CO)
3NO+2CO
Fe
2(CO)
9+4NO→2Fe(CO)
2(NO)
2+5CO
[Mn(CO)
5]
2+2NO→2Mn(CO)
4NO+2CO
It is interesting to note that the products of these reactions obey the 18-electron rule

24. The electrons are partially transferred from a d-orbital of the metal to anti-
bondingmolecular orbitals of CO (and its analogues). ... Manyligandsother than
CO are strong "backbonders". Nitric oxide is an even stronger π-acceptor than is
CO and ν
NOis a diagnostic tool in metal–nitrosyl chemistry.

25. 4.Disproportionation
Anumberofmetalcarbonylsundergodisproportionationreactionsinthe
presenceofothercoordinatingligands.Forexample,inthepresenceofamines,
Fe(CO)
5reactsasfollows:
2Fe(CO)
5+6 Amine → [Fe(Amine)
6]
2+
[Fe(CO)
4]
2-
+ 6 CO
Thisreactiontakesplacebecauseoftheeaseofformationofthecarbonylate
ionsandthefavorablecoordinationoftheFe
2+
produced.Thereactionof
Co
2(CO)
8withNH
3issimilar.
Co
2(CO)
8+ 6 NH
3→ [Co(NH
3)
6][Co(CO)
4]
2
Formally,ineachofthesecasesthedisproportionationproducesapositive
metalionandametalioninanegativeoxidationstate.Thecarbonylligands
willbeboundtothesoftermetalspecies,theanion;thenitrogendonorligands
(hardLewisbases)willbeboundtothehardermetalspecies,thecation.

26. Thesedisproportionationreactionsarequiteusefulinthepreparationofa
varietyofcarbonylatecomplexes.
Forexample,the[Ni
2(CO)
6]
2ioncanbepreparedbythereaction
3 Ni(CO)
4+ 3 phen → [Ni(phen)
3][Ni
2(CO)
6] + 6 CO
Therangeofcoordinatingagentsthatwillcausedisproportionationisrather
wideandincludescompoundssuchasisocyanides,RNC:
Co
2(CO)
8+ 5 RNC → [Co(CNR)
5][Co(CO)
4] + 4 CO

27. Metal carbonyl hydride or Carbonylate
Anions
SeveralcarbonylateanionssuchasCo(CO)
4
–
,Mn(CO)
5
–
,V(CO)
6
–
,and
[Fe(CO)
4]
2–
obeytheEANrule.
Onetypeofsynthesisoftheseionsisthatofreactingthemetalcarbonyl
withareagentthatloseselectronsreadily,astrongreducingagent.Active
metalsarestrongreducingagents,sothereactionsofmetalcarbonylswith
alkalimetalsshouldproducecarbonylateions.
ThereactionofCo
2(CO)
8withNacarriedoutinliquidammoniaat75°Cis
onesuchreaction.
Co
2(CO)
8+ 2 Na → 2 Na[Co(CO)
4]
Similarly,
Mn
2(CO)
10+ 2 Li → 2 Li[Mn(CO)
5]

28. AlthoughCo(CO)
4andMn(CO)
5donotobeythe18-electronrule,the
anionsCo(CO)
4
–
andMn(CO)
5
–
do
Asecondtypeofreactionleadingtotheformationofcarbonylateanionsis
thereactionofmetalcarbonylswithstrongbases.
Forexample,
Fe(CO)
5+ 3 NaOH → Na[HFe(CO)
4] + Na
2CO
3+ H
2O
Cr(CO)
6+ 3 KOH → K[HCr(CO)
5] + K
2CO
3+ H
2O
With Fe
2(CO)
9, the reaction is
Fe
2(CO)
9 + 4 OH
–
→ Fe
2(CO)
8
2–
+ CO
3
2–
+ 2H
2O

29. Properties
Theneutralmetalcarbonylhydridesareoftenvolatileandcan
bequiteacidic.Thehydrogenatomisdirectlyboundedtothe
metal.Themetal-hydrogenbondlengthisforcobalt114pm,
themetal-carbonbondlengthisforaxialligands176and
182fortheequatorialligands.

30. Applications
Metalcarbonylhydrideareused
ascatalystsinthehydroformylation
ofolefins.