Superexchange and some molecular orbital calculations of the exchange parameter for copper(II) carboxylate dimers

Abstract

For the copper(II) carboxylate dimer Cu₂(RCO₂)₄, the exchange parameter J[½(E_singlet–E_triplet)] is calculated using a simplified INDO molecular orbital procedure with configuration interaction. The primary calculation utilizes ten atomic orbitals, namely the metal-ion 3d_x²–y² orbitals and an sp² hybrid orbital from each oxygen atom of the four carboxylate ligands. Using limited and complete configuration interaction procedures, values of –80 and –130 cm^–1 are calculated for the exchange parameter. Some INDO calculations with two rather than four RCO^–₂ ligands indicate that overlap between the two oxygen orbitals of each carboxylate ligand, rather than the copper d_x²–y²–d_x²–y²δ-type overlap, is of primary importance for determining the magnitude of the exchange parameter. This O–O overlap becomes effective because the oxygen lone-pair electrons are calculated to delocalize appreciably into the singly-occupied Cu(II) orbitals.

For the lowest-energy S= 0 and S= 1 states of the dimer with two carboxylate ligands, the weights for all valence-bond structures are calculated from the complete configuration-interaction wave-functions. The valence-bond structure with a Cu—Cu δ-bond in the S= 0 state is found to have a small weight. The importance of the O–O overlap for antiferromagnetism is deduced from valence–bond resonance considerations.