Question

What factors influence the CO stretching frequencies in metal carbonyl derivatives and why?

Answer 1

Answer:

The main factor which governs the bonding between transition-metal and CO molecule in metal-carbonyl is pi-back bonding (Synergistic effect). which is as follows;

In metal-carbonyls, presence of a electron rich sp orbitals (where p character is mainly due to the C atom and s-character is due to O-atom) takes part in the bonding with metal by the donation of electron density of sp orbitals (through C lone-pair from the carbon atom mainly).

Now due to the gain of electron density on metal center, metal becomes more electron rich (more negative) and try to stabilize itself by back donation of its nd electrons to anti-bonding orbitals of CO molecule, which posses the perfect symmetry for the overlap.

This donation of CO molecule to metal and then metal to back to CO molecules strengthen the bonding between the M-CO centers.

This bonding between the metal-CO systems is affected by the several factors; such as

1. If metal is positive charge, this results in the poor back donation of electrons from metal to CO bond. Which directly affects the M-CO bond-strength and hence the stretching frequency of CO.  

2. Electron donating ligands attached on the metal center increase the back-bonding process, while electron withdrawing ligands weakens the back-bonding.

3. Depending upon the number of coordination that CO molecule forms with metal (mono or bridge), the C---O bond stretching also decrease with strengthen of M-CO bonds.

e.g V(CO)₆^- = 1858 cm^-1; Cr(CO)₆ = 2000 cm^-1; Mn(CO)₆⁺ = 2095 cm^-1 and

free CO = 2143 cm^-1

Above values clearly represents the statements made in the above text.

Note: The greater the ability of a metal to donate electrons to anti-bonding orbitals of CO, the lower the energy of the C-O stretching vibration.

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