Magneto-structural correlations of the antiferromagnetic interaction in bis(maleonitrile dithiolato) nickel(III) ion-radical salts - R⁺ Ni mnt₂^-

We have studied some R⁺ Ni mnt₂^- compounds where the Ni mnt₂^- units, which are S = 1 2 systems, stack as dimers in the solid state. The variations of the cations R⁺ lead to small changes in the packing in the crystal and, thus, to variations in the anti- ferromagnetic couplings which are very sensitive to these changes. We have carried out extended Hückel molecular orbital calculations with the structural parameters known from crystal structure data on this series of compounds, and tried to relate the magnetic interactions as measured by susceptibility with the splitting between the pair of highest occupied molecular orbitals. We have come across a wide variety of overlapping patterns in this series and, on the basis of MO results, we expect examples ranging from almost uniform chains to almost isolated dimers. Due to the extensive delocalisation of the unpaired electron density over the ligand framework and the plane-to-plane type overlap within a dimeric unit, many short contact distances between various atoms play important roles as super exchange pathways. Hence, any attempt to arrive at correlations between magnetic data and a few structural parameters, such as interplanar distance, bridging angle and 'bridging' distance, is doomed to failure.