9. Metal Carbonyls

Metal Carbonyls

Compounds of metals with CO as a ligand are called metal carbonyls.

They are of two types.

(a) Monomeric : Those metal carbonyls which contain only one metal atom per molecule are called monomeric carbonyls. For examples :[Ni(CO)₄] (sp³, tetrahedral)  ;  [Fe(CO)₅ ] (dsp³, trigonal bipyramidal)

[Cr(CO)_6] (d² sp³, octahedral) ; [V(CO)₆] (d² sp³, octahedral, only carbonyl which is paramagnetic having one unpaired electron. This is least stable among all the four carbonyls)

(b) Polymeric : Those metal carbonyl which contain two or more than two metal atom per molecule and they have metal-metal bonds are called polymeric carbonyl. For example :  Mn₂ (CO)₁₀ , Co₂(CO)₈, etc.

The metal–carbon bond in metal carbonyls possess both s and p character. The M—C s bond is formed by the donation of lone pair of electrons on the carbonyl carbon  (CO is a weak base) into a vacant orbital of the metal. The M — C p bond is formed by the donation of a pair of electron from a filled d orbital of metal into the vacant antibonding p* orbital of carbon monoxide. Thus carbon monoxide acts as s donor (OC ¬ M) and a p acceptor (OC ¬ M), with the two interactions creating a synergic effect which strengthens the  bond between CO and the metal as shown in figure.

Synergic bonding