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 (CH 3 CO 2 )]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