SIGMA-STRENGTH OF LIGANDS COORDINATED TO TRANSITION-METAL IONS

The order of a strength for coordinated halide ligands (F- > Cl- > Br- > I-) derived by analysis of the energies of the d-d transitions in Cr(III) complexes by Perumareddi and Schäffer is the opposite to that expected by many experimentalists. It is pointed out that this spectroscopic order is determined by variation in the energy of an empty metal-ligand σ antibonding orbital, whereas “normal” ideas concerning a strength are concerned with variation in ground-state properties associated with the bonding orbitals. Using simple perturbation theory it is shown that a large destabilization energy of an antibonding orbital is not always matched by a proportionally large stabilization of the bonding partner. Crude calculations of the extended Huckel type support the perturbation analysis. They show specifically that, whereas the stabilization energy of the bonding eg orbitals in CrF6 3- is less than that in CrCl6 3-, the opposite is true of the destabilization of the antibonding orbitals (mainly dz 2 or dx 2-y 2). This provides a rationalization of the spectroscopic result. It is suggested that similar considerations should also apply to ab initio calculations. © 1979, American Chemical Society. All rights reserved.