X-RAY CRYSTALLOGRAPHIC EVIDENCE OF VIBRONIC COUPLING IN PSEUDO-JAHN-TELLER COPPER(II) COMPLEXES AND MONOMERIC COBALT-DIOXYGEN COMPLEXES

The X-ray results of a series of [Cu(phen)2(CH3CO2)]Y and [Cu(bipyam)2(ONO)]Y cation-distortion isomers and of a series of monomeric cobalt-dioxygen Schiff base complexes with identical equatorial ligands and different nitrogenous axial bases are presented. The distortional behavior of the Cu(II) pseudo-Jahn-Teller complexes is explicable in terms of the warped Mexican hat energy surface distorted by tetragonal and orthorhombic strain originating from chemical differences between ligands and crystal-packing forces. The Cu-O static bond lengths allow an estimation of the amount of tetragonal strain, whereas the application of Boltzmann statistics to the temperature variation of the Cu-O bond lengths allows an estimation of the amount of orthorhombic strain. The distortional behavior of the cobalt-dioxygen complexes is also explicable in terms of vibronic coupling theory. The bending, orientational preference, and order-disorder of the dioxygen group is consistent with an energy surface having four nonequivalent minima. The rotation Of O2 about the Co-O bond axis depends on the barrier height between the minima, which in turn depends on the strength of the Co-N axial base interaction.