This document provides an overview of metal carbonyls. It discusses how metal carbonyls are formed from transition metals and carbon monoxide, and examples like nickel tetracarbonyl and iron pentacarbonyl. The molecular orbital diagram of carbon monoxide is shown, explaining why it can participate in pi-backbonding. Infrared spectroscopy is described as a useful technique for analyzing metal carbonyls, as it can distinguish terminal from bridging carbonyl ligands based on the infrared absorption frequency. Factors like metal charge and other ligands that affect the carbonyl stretching frequency are also outlined. Finally, some applications of infrared spectra of metal carbonyls are mentioned.

1. Samrat Prithviraj Chauhan Govt. College Ajmer
2020-2021
Department Of Chemistry
Metal Carbonyls
Submitted By: Diksha Nagar
M.Sc.(Chemistry) Semester 2

2. Introduction
Why study metal
carbonyl?
Molecular Orbital
diagram of CO
Infrared Spectroscopy-
Spectra of Metal Carbonyls
Reference
CONTENT

3. Introduction
Ludwig Mond 1839-1909
Father of Metal Carbonyl Chemistry
Founder of Imperial Chemical Industry,
England
Metal carbonyls are organometallic compound formed
by the combination of CO molecules with transition
metal atoms in low oxidation state
1890-1930 textbooks Ni(CO)4 Fe(CO)5
Ni(CO)4, Fe(CO)5, Co2(CO)8, Mo(CO)6
1890 1891 1910

4. Examples
Coordination number around the metal normally remains six or lesser. 17 electron
species such as Mn(CO)5, Co(CO)4 dimerize to gain 18 electrons.V(CO)6 does not dimerize.

5. Why study metal carbonyls?
 Carbon monoxide is not ordinarily considered a very strong Lewis base and
yet it forms strong bonds to the metals in these complexes.
 The metals are usually in the zero oxidation state and sometimes in
negative oxidation state.
 Their complexes were distinctly different from the Werner complexes of
NH3 and H2O ligands. They are always 18 electron complexes.
 Simplest of organometallic compounds where M-C  bonding is well
understood. CO is one of the strongest  acceptor ligands. Back bonding
( bonding) and variation in electronic properties of CO can be monitored
very efficiently by Infrared spectroscopy
 A range of metal carbonyls are used as catalysts in Chemical Industry.
C CH2
R
H
CO,
H2
CH CH2
HC
R
O
H
HCo(CO)4
Hydroformylation
Alkene to Aldehyde

6. Molecular Orbital diagram of CO
The highest occupied molecular orbital
(HOMO) of CO is weakly antibonding
molecular orbital and is MO which is
carbon based. Secondly, the *
antibonding orbital which is the lowest
unoccupied molecular orbital (LUMO) is
also of comparatively lower energy
which makes it possible to interact with
metal t2g orbitals for  bonding. There
exists a strong back bonding of metal
electrons to the  * antibonding
orbitals of CO
Why does CO bind a metal through its less
electronegative carbon atom than its more
electronegative oxygen ? What makes it a good
 acceptor ?
AO of C
atom
AO of O
atom
MO of CO

8. Infrared Spectroscopy: Spectra of Metal Carbonyl
 Counting the electron helps to predict stability of metal carbonyls.
But it will not tell you whether a CO is bridging or terminal. For this
IR spectroscopy is very useful.
 Infrared spectroscopy is a particularly informative technique for
characterizing carbonyl complexes. It is very useful in identifying the
structures suggests the C-O bond order in M-CO complexes are
lesser than in free.
 The CO bond vibrates around 2143cm-1 in the gaseous state.
Fe
OC
OC
OC
Fe
O
C
CO
CO
CO
C
O
O
C
terminal
bridging
The range in
which the
band appears
decides
bridging or
terminal

9. Terminal versus bridging carbonyls
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
Fe
OC
CO
CO
Cp
Cp
Cp
Cp
2000 cm-1 1826 cm-1
1620 cm-1

10. Factors which affect CO stretching frequencies
M C O M C O
 CO Higher  CO Lower
1. Charge on the metal
As the electron density on a metal
centre increases, more 
backbonding to the CO ligand(s)
takes place. This weakens the C–O
bond further as more electron
density is pumped into the empty
* anti-bonding carbonyl orbital.
This increases the M–C bond
order and reduces the
C-O bond order. That is, the
resonance structure M=C=O
becomes more dominant.

11. 2. Effect of ligands
• With each negative charge
added to the metal centre, the
CO stretching frequency
decreases by approximately 100
cm–1.
• The better the  donating
capability of the other ligands on
the metal, more electron density
given to the metal, more back
bonding (electrons in the
antibonding orbital of CO) and
lower the CO stretching
frequency.
Mo
L
L CO
CO
CO
L
More back bonding =
More lowering of the C=O bond order = More
lower  CO stretching frequency

12. Uses of Infra-red(IR) Absorption Spectra of Metallic
Carbonyls
To determine the geometry of metallic
carbonyls.
To determine the bond order of ligated CO.
To differentiate between terminal and
bridging carbonyl groups.
To study reaction rates.

13. Reference
SHRIVER & ATKINS Inorganic Chemistry
Inorganic Chemistry Principles of Structure and
Reactivity By James E. Huheey, Ellen A. Keiter, Richard L.
Keiter, Okhil K. Medhi
Basic Organometallic Chemistry By B D Gupta, A J Elias

14. Thankyou