Structure and stability of carboxylate complexes. Part XI. The molecular g-values of some monomeric copper carboxylate complexes

Abstract

The e.p.r. spectra of single crystals of copper(II) glycollate, lactate, hydrogen maleate, methoxyacetate, phenoxyacetate, and ethoxyacetate have been measured. A general method for obtaining molecular g-tensors from single-crystal measurements on pure compounds is described. Each complex has a distorted octahedral structure and the metal part of its ground-state wave-function (spin–orbit coupling being neglected) is composed of the linear combination ψ=a|x²–y²〉–b|3z²–r²〉 producing a wave-function ψ≈cx²+ ey²+fz². The g-values of each complex have been used to estimate its wave-function parameters and the magnitudes of c, e, and f were apparently related to the ligand perturbation along x, y, and z, the greater the perturbation the larger the magnitude of the wave-function parameter. For donor atoms at comparable distances the wave-function parameters suggest a probable order of σ-perturbing power O(carboxylate) > O(water) > O(hydroxy). Although the methoxyacetate complex has a nearly regular compressed tetragonal structure, its ground-state wave-function lies approximately midway between what are usually called |x²–y²〉 and |3z²–r²〉. The axial symmetry of the bond lengths is not reflected in the ligand field and O(H₂O) produces a much stronger σ-perturbation than O(methoxy). An analysis of the energies of the electronic transitions ψ′→ψ(where ψ′=a|3z²–r²〉+b|x²–y²〉) of the complexes with an angular overlap model suggests an order of σ-perturbing power O(carboxylate) > O(water) > O(hydroxyl) O(ether) in good agreement with the g-value interpretation, and values of b calculated from the electronic spectra are reasonably consistent with those estimated from the e.p.r. data.