Ab Initio Calculations of the Magnetic Coupling between a Ni(II) Ion and Two Nitroxide Radicals in cis and trans Positions

The magnetic couplings between a Ni(II) center in an octahedral environment and two nitroxides in its coordination sphere are calculated by ab initio methods. The system is well described by a Heisenberg model. Fully variational calculations (DDCI2) give results similar to those of combined variational/perturbative calculations (CASPT2), and some DFT results are obtained. The effect of some structural parameters is studied: the cis and trans positions and the angles and the distances of the nitroxides with respect to the nickel center. We show that the magnetic couplings are almost the same in the cis and in the trans complexes; thus, the only relevant geometric parameters for ferromagnetic or antiferromagnetic couplings are the orientations and the distances of the nitroxide ligands with respect to the Ni(II) center. Direct exchange between the magnetic centers is calculated: it depends neither on the angles nor on the cis or trans conformations, showing that the superexchange contribution is the only angle-dependent one.