Magnetic Exchange in Transition Metal Complexes. VII. Spin Coupling in μ4-Oxohexa-μ-halotetrakis [(Triphenylphosphine Oxide or Pyridine) Copper(II)]: Evidence for Antisymmetric Exchange

Abstract

Magnetic susceptibility measurements are reported for a group of tetrameric Cu(II) complexes Cu4OX6L4 with X = Cl, L = (C6H5)3PO and X = Br, L = (C6H5)3PO or C5H5N. All samples show an effective magnetic moment μeff per copper ion which exhibits a maximum in the 40–60°K temperature range and which, in general, cannot be quantitatively accounted for by simple Heisenberg theory. The results contrast with those observed for the closely related anionic cluster [Cu4OCl10]4− which exhibits a monotonic variation of μeff with temperature and is compatible with simple Heisenberg theory. It is suggested that the difference may be due to a different relative arrangement of 3d Cu(II) energy levels in the anionic cluster, leaving an orbital singlet state lowest in the [Cu4OCl10]4− environment but an orbitally degenerate level lowest in the other complexes. To test the hypothesis the copper-pair exchange Hamiltonian is derived for the orbitally degenerate case and shown, for the tetramer environment, to contain a large antisymmetric component. Using this orbitally degenerate spin-pair Hamiltonian, the statistical problem for a tetramer of interacting spins is solved to give an effective magnetic moment per copper ion as a function of temperature. The resulting theory gives excellent agreement with the experimental results for the Cu4OX6L4 complexes. In addition, the required values for the exchange parameters within the theory are physically realistic and may be qualitatively understood in terms of orbital symmetry relationships.