Superexchange and spin-orbit coupling in chlorine-bridged binuclear cobalt(II) complexes

Quantum chemical ab initio calculations are performed for the magnetic exch ange coupling in binuclear chlorine-bridged Co(II) complexes of the form L3 CoCl3Co3. In order to simplify the calculations, the terminal ligands are r eplaced with He-type model ligands L. The calculations are carried out at t he restricted open-shell Hartree-Fock (ROHF), complete active space SCF (CA SSCF), and valence configuration interaction (VCI) levels, with inclusion o f spin-orbit coupling and external magnetic fields. The 12-fold degenerate T-4(1g) ground state of the Co2+ cation in a perfect octahedral ligand fiel d is split by the trigonal distortion in the complex and by spin-orbit coup ling. Both effects have the same order of magnitude, 200-500 cm(-1). The gr ound state of either cation is a Kramers doublet, E-1/2, separated by about 300 cm(-1) from the lowest excited states. The coupling of the two E-1/2 g round states through the chlorine bridges is antiferromagnetic; the binucle ar complex has a nondegenerate A(2)" ground state, followed by a nondegener ate first excited A(1)' state 15 cm(-1) above the ground state and a 2-fold degenerate second excited E" state at 57 cm(-1). The next states are about 300 cm(-1) higher in energy. From these energy levels the magnetic suscept ibility chi is calculated by means of a Boltzmann average. II shows a subst antial anisotropy: chi(parallel to) is rather large because of a first-orde r Zeeman splitting of the E" state in a magnetic field parallel to the mole cular axis, while chi(perpendicular to) is small since it is only caused by second-order Zeeman effects. The calculated temperature dependence of chi agrees fairly well with experimental data; however, a phenomenological Heis enberg-Dirac-van Vleck Hamiltonian cannot be used to describe the measured susceptibility data.