16-electron complexes

A square-planar complex of a d⁸ ion, like [Ni(CN)₄]^2-, have a valence electron count of 16 rather than 18. Identical 16- electron complexes are created through other elements in groups 9, 10 and 11, for instance Vaska's compound Ir(CO) (PPh₃)₂Cl (3). Some 16-electron complexes (particularly in the 3d series) can readily add other ligand to create a five coordinate 18-electron complex like [Ni(CN)₅]^3-. Other significant reaction is termed as oxidative addition, in which a molecule X-Y adds by cleavage of the bond to create an 18-electron complex which can be considered as d6 octahedral:

X-Y could be a simple molecule like H₂ or HCl, or an organic compound. Vaska's compound also reacts along with O₂ to create Ir(CO)(PPh₃)₂(O₂)Cl (4). In this example, O₂ remains intact on coordination, even though the bond lengthens, suggesting that 4 can be considered as a complex along with a bidentate peroxo ligand.

The opposite of oxidative addition is reductive elimination. Such type of reversible processes is significant in several catalytic cycles involving transition metal compounds.