Stereochemistry and spin state in four-coordinate transition metal compounds.

A systematic DFT computational study of the stereochemistry associated with each spin state for first transition series four-coordinate d(n) (n = 0-10) homoleptic metal complexes is presented. The stereochemistry of [MMe4](x-) complexes in the 21 spin configurations analyzed can be predicted from the d orbital occupation in the ideal tetrahedral geometry, grouped in three families with tetrahedral, square planar, or intermediate structures that can be described in some cases as sawhorses. The effect of the following factors on the spin state and stereochemical preferences has also been studied: (a) substitution of the sigma-donor methyl ligands by pi-donor chlorides, (b) a high (+4) oxidation state of the metal, and (c) substitution of the metal atom by a second transition series one. Through those factors, low-spin tetrahedral structures can be achieved, as summarized by a magic cube.