A theoretical approach to the leaving ligand problem of photosubstitution reactions of hexacoordinate transition metal mixed-ligand complexes

A coordination theory approach to the specific site labilization in ligand photosubstitution reactions has been proposed for hexacoordinate mixed-liga nd transition metal complexes. On the basis of molecular orbital theory and the ligand even-distribution assumption, the relative stability of a metal -ligand bond in an excited species is expressed as a function of molecular orbital energy gap resulting from sigma and pi interactions between the met al and ligands, and of the Jahn-Teller effects of the ground and excited sp ecies. In terms of the function, some experimental facts have been analyzed . It is likely that the alteration in the electron-pair number upon excitat ion accounts for the difference between the d(3) type (including d(1-3) and weak field d(4)) and strong field d(6) type (including strong field d(4-7) ) complexes in the leaving ligand character. The a and pi interactions may act differently, or even oppositely, on the leaving ligand. The impact of s igma interaction on the leaving ligand has also been assessed for complexes with other electronic configurations.