Metal carbonyls

Homoleptic carbonyls contain only CO ligands; most transition metals form them, with simple well-defined structures.

Shapes: tetracarbonylnickel(0), Ni(CO)₄, is tetrahedral; pentacarbonyliron(0), Fe(CO)₅, is trigonal bipyramidal; hexacarbonylchromium(0), Cr(CO)₆, is octahedral.

Decacarbonyldimanganese(0), [Mn₂(CO)₁₀], is built from two Mn(CO)₅ square pyramids held together by one metal-metal bond. In octacarbonyldicobalt(0), [Co₂(CO)₈], the two cobalt atoms are bonded directly and two CO groups also bridge them.

The M–C bond has both σ and π character. The σ bond forms when the carbon's lone pair is donated into an empty metal orbital.

The π bond forms when a filled metal d orbital donates electrons back into the empty antibonding π* orbital of CO (back bonding).

The two donations reinforce each other (synergic bonding): σ donation makes the metal richer in electrons for back donation, and back donation lets it accept more σ density. This strengthens the metal-CO bond.