Metal Carbonyls: Carbon monoxide forms complexes with most of the transition metals in low oxidation states. These complexes are called metal carbonyls. In these complexes, metal atoms are in their zero, low positive, or negative oxidation states. In this article, let’s learn everything about metal carbonyls in detail. There are three points of interest when it comes to metal carbonyls:

1. Carbon monoxide is not considered a very strong Lewis base, yet it forms strong bonds to the metals in these complexes.
2. In these complexes, the metal atoms are always in their low oxidation states, mostly zero, low positive or negative oxidation states.
3. About \(99\% \) of these complexes obey the effective atomic number rule, which accounts for their stability.

In metal carbonyls, the \({\text{CO}}\) molecule bonds itself to the metal atoms through its carbon end as
\({\text{M}} \leftarrow {\text{CO}}.\) Therefore, the metal carbonyls are regarded as organometallics. The \({\text{CO}}\) ligand can bind to the transition metals in three different ways:

(i) It can act as a terminal ligand.
(ii) It can act as a bridging ligand.
(iii) It can act as a triply bridging ligand

The most common of all is the terminal carbonyls.

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Bonding in Metal Carbonyls

We know that the Lewis acidity of \({\text{CO}}\) is very small. The tendency of \({\text{CO}}\) to form a very large number of complexes in low oxidation states of metals is quite astonishing. The high stability of metal-carbon bonds in metal carbonyls is due to the multiple nature of the \({\text{M-CO}}\) bond. The metal-carbon bond in metal carbonyls has both \(\sigma \) and \(\pi \) character.

In terms of resonance, the bonding in metal carbonyls may be represented as:

However, the explanation for bonding can be given in terms of molecular orbital theory as given below:

Carbon monoxide has a triple bond with lone pair of electrons on both carbon and oxygen atoms as

\({}^ \bullet _ \bullet {\text{C}} \equiv{\text{O}}{}^ \bullet _ \bullet \)
Since these complexes contain metal atoms in zero or low positive oxidation states, there is no attractive interactions between the metal and the ligands as is possible for the positively charged metal ion. It is the main characteristic of the \({\text{CO}}\) ligand that it can stabilize low oxidation states. This is because it possesses vacant \(\pi \)- orbitals in addition to lone pairs. The formation of a sigma bond by donating a lone pair of electrons into the suitable vacant metal orbitals leads to the excessive negative charge on the metal (in zero or negative oxidation state). To counter the accumulation of negative charge on the metal, a \(\pi \)- bond is formed by the back donation of electrons from filled metal orbitals into the vacant \(\pi \)-type orbitals on the ligand. So, in metal carbonyls:

1. There is a dative overlap of the filled orbital of carbon (of \({\text{CO}}\)) and suitable empty orbital of the metal forming a dative \(\sigma \)- bond \(\left({{\text{M}} \leftarrow {\text{CO}}} \right).\) This is shown in fig.(a).
2. There is a \(\pi \)- overlap involving the donation of electrons from filled metal \({\text{d}}\)- orbitals into vacant anti-bonding \({\pi ^*}\) molecular orbitals. This results in the formation of the \(\pi \)- bond \(\left({{\text{M}} \to {\text{CO}}} \right).\) This is also called back donation or back bonding [ fig.(b)]

The bonding in metal carbonyls is shown below. In these figures, red and yellow orbitals represent filled orbitals, while green orbitals represent empty orbitals.

The formation of \(\sigma \) dative bond tends to increase the electron density on the metal atom. At the same time, the formation of the \(\pi \)- bond from metal to carbon tends to decrease the electron density on the metal. The effect of \(\sigma \) bond formation strengthens the \(\pi \) bond and vice versa. This is called the synergic effect. This effect is represented in the figure below. Thus, as a result of the synergic effect, the bond between \({\text{CO}}\) and metal is strengthened. This ability of ligand \(\left({{\text{CO}}} \right)\) to accept electron density into vacant \(\pi \) orbitals is called \(\pi \)- acidity. Therefore, \({\text{CO}}\) is called a \(\pi \)- acceptor ligand and metal carbonyls are referred to as complexes of \(\pi \)- acceptor (or \(\pi \)- acid) ligands.

This accounts for the fact that \({\text{CO}}\) is a very weak Lewis base towards non-transition metal halides like \({\text{B}}{{\text{X}}_3},{\text{Al}}{{\text{X}}_3},\) etc. but forms very strong complexes with transition metals. This is obviously because of the drift of \(\pi \) electron density from \({\text{M}} \to {\text{C}},\) which increases the \(\sigma \) donor power of \({\text{CO}}{\text{.}}\)

We have discussed the bonding in metal carbonyls where the \({\text{CO}}\) group acts as a terminal ligand. Now let’s briefly discuss the bonding in metal carbonyls in which \({\text{CO}}\) acts as a bridging ligand in dinuclear complexes (Binuclear metal carbonyls). The bridging \({\text{CO}}\) groups are symmetrical and have equal \({\text{M-C}}\) distances.
The bonding in bridging metal carbonyl groups may be regarded as a \(2\) electron \(3\) centered overlap. It may be noted that the bridge occurs in conjunction with a metal-metal bond. The carbon monoxide bridges in the absence of metal-metal bonds are unstable. Thus, the metal-metal bond is essential for the stability of the bridges. The stability of \({\text{CO}}\) bridges depends upon the size of the metal atom. If the metal atoms are larger in size, the bridged structure becomes unstable relative to the unbridged structures. Therefore, the relative stability of non-bridged structures increases as the size of the metal atom increases. For example, the smaller \({\text{Fe}}\) atom in \({\text{F}}{{\text{e}}_2}{\left({{\text{CO}}} \right)_9}\) has three bridging \({\text{CO}}\) groups, whereas bigger osmium in \({\text{O}}{{\text{s}}_2}{\left({{\text{CO}}} \right)_9}\) has one bridging \({\text{CO}}\) group.
The infrared spectral studies also help to show the presence of bridging \({\text{CO}}\) groups. The \({\text{CO}}\) stretching frequency of the bridging group is lower than the terminal \({\text{CO}}\) group. It has been observed in general, the \({\text{CO}}\) frequencies for the terminal \({\text{CO}}\) groups are in the range \(1850 – 2150\,{\text{c}}{{\text{m}}^{ – 1}},\) and the frequencies for bridging \({\text{CO}}\) groups are in the range \(1750 – 1850\,{\text{c}}{{\text{m}}^{ – 1}}.\) Triply bridging \({\text{CO}}\) groups have lower \({\text{CO}}\) frequencies in the range \(1620\) to \(1730\,{\text{c}}{{\text{m}}^{ – 1}}\) in neutral molecules. The structures of polynuclear metal carbonyls are much more complex. They contain metal-metal bonds and terminal and bridging carbonyl groups.

Evidence in Support of Bonding

1. The formation of back bonding from metal to \({\text{CO}}\) molecule results in a decrease in electron density on metal. The dipole moment studies support this. It has been observed that the dipole moment of the \({\text{M-C}}\) bond is only very low, about \(0.5\,{\text{D}}\) suggesting a close approach to electronegativity.
2. The back bonding from metal to \({\text{CO}}\) is expected to increase \({\text{M-C}}\) bond strength with a corresponding weakening of \({\text{C}} \equiv {\text{O}}.\) This is due to the fact that electrons from back bonding fill the anti-bonding \({\text{MOs}}\) of \({\text{CO}}.\) As a result, the bonding ability of \({\text{CO}}\) will decrease. Therefore, as the \({\text{M-C}}\) bond becomes stronger, the \({\text{C}} \equiv {\text{O}}\) bond becomes weaker. Therefore, the multiple bonding should be evidenced by the shorter \({\text{M-C}}\) bond as compared to \({\text{M-C}}\) single bond and longer \({\text{C-O}}\) bonds as compared to normal \({\text{C}} \equiv {\text{O}}\) triple bonds. This has been confirmed experimentally.
3. Infrared spectroscopy has given valuable support for bonding in metal carbonyls. These studies provide information regarding the bond order of \({\text{M-C}}\) and \({\text{C}} \equiv {\text{O}}\) bonds. The decrease in \({\text{C-O}}\) bond order is estimated by studying the \({\text{CO}}\) stretching frequency in infrared spectroscopy.

Summary

Carbon monoxide forms complexes with most of the transition metals in low oxidation states. These complexes are called metal carbonyls. In metal carbonyls, there is a dative overlap of the filled orbital of carbon (of \({\text{CO}}\)) and a suitable empty orbital of the metal, forming a dative \(\sigma \)- bond \(\left({{\text{M}} \leftarrow {\text{CO}}} \right).\) Simultaneously there is a \(\pi \)- overlap involving the donation of electrons from filled metal \({\text{d}}\)- orbitals into vacant anti-bonding \({\pi ^*}\) molecular orbitals. This results in the formation of the \(\pi \)- bond \(\left({{\text{M}} \to {\text{CO}}} \right).\) This is also called back donation or back bonding. The structures of binuclear and polynuclear metal carbonyls are much more complex.

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FAQs on Metal Carbonyls

Q.1. Why are metal carbonyls regarded as organometallics?
Ans: In metal carbonyls, the \({\text{CO}}\) molecule bonds itself to the metal atoms through its carbon end as \({\text{M}} \leftarrow {\text{CO}}.\) Therefore, the metal carbonyls are regarded as organometallics.

Q.2. In how many ways \({\text{CO}}\) ligand can bind to the transition metals in metal carbonyls?
Ans: The \({\text{CO}}\) ligand can bind to the transition metals in three different ways:
(i) It can act as a terminal ligand.
(ii) It can act as a bridging ligand.
(iii) It can act as a triply bridging ligand
The most common of all is the terminal carbonyls.

Q.3. What is the synergic effect in metal carbonyls?
Ans: In metal carbonyls, the formation of \(\sigma \) dative bond tends to increase the electron density on the metal atom. At the same time, the formation of the \(\pi \)- bond from metal to carbon tends to decrease the electron density on the metal. The effect of \(\sigma \) bond formation strengthens the \(\pi \) bond and vice versa. This is called the synergic effect.

Q.4. Why is \({\text{CO}}\) known as \(\pi \)- acceptor ligand?
Ans: The ability of ligand \(\left({{\text{CO}}} \right)\) to accept electron density into vacant \(\pi \) orbitals is called \(\pi \)- acidity. Therefore, \({\text{CO}}\) is also called a \(\pi \)- acceptor ligand and metal carbonyls are referred to as complexes of \(\pi \)- acceptor (or \(\pi \)- acid) ligands.

Q.5. What are the oxidation states of metal in metal carbonyls?
Ans: In metal carbonyls, metal atoms are in their zero, low positive or negative oxidation states.

Chemistry related articles

Structures of metal carbonyls	Uses of metal carbonyls
Properties of metal carbonyls	Stability of coordination compounds

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