Overlap considerations relevant to the geometry of copper(II) complexes

Abstract

The binding of axial ligands in tetragonal copper(II) complexes is discussed in terms of overlap considerations. Diatomic σ-overlap integrals between the 3d-orbitals of copper(II) and the np-orbitals of N, O, F, S, Cl, and Br are shown to decrease sharply with increasing internuclear distance. The corresponding integrals between the metal 4s-orbital and ligand 2p-orbitals similarly decrease monotonically in the range considered, but less sharply. Overlap integrals involving the metal 4p-orbitals with a given ligand atom are not appreciably different at typical equatorial and axial distances. We suggest that distant axial ligands in tetragonal copper(II) systems are in fact quite strongly bound to the metal, and that the driving force behind the tetragonal distortion may be attributed to the stabilization of the filled d_z² orbital rather than to the Jahn–Teller effect. π-Overlap integrals between the metal 3d-orbitals and ligand np-orbitals fall off much more sharply with increasing internuclear distance than σ-overlaps, and π-interaction with axial ligands is probably negligible in most cases. The circumstances in which an equatorial elongation might be favoured are discussed. Overlap considerations can also explain the tetragonal distortion found in Cr^II systems.