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

摘要

A 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 #sigma# and #pi# 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 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 #sigma# and #pi# interactions may act differnetly, or even oppositely, on the leaving ligand. The impact of #sigma# interaction on the leaving ligand has also been assessed for complexes with other electronic configurations.