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

793-803A coordination theory approach to the specific site labilization in ligand photosubstitution reactions has been proposed for hexacoordinate mixed-ligand 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 σ and π interactions between the metal and ligands, and of the Jahn-Teller effects of the ground and excited species. In terms of the function, some experimental facts have been analyzed. It is likely that the alteration in the electron-pair number upon excitation accounts for the difference between the d3 type (including d1-3 and weak field d4) and strong field d6 type (including strong field d4-7) complexes in the leaving ligand character. The σ and π interactions may act differently, or even oppositely, on the leaving ligand. The impact of σ interaction on the leaving ligand has also been assessed for complexes with other electronic configurations