Abstract
The magnetism of a recently synthesized trans-[(OsCl4)-Cl-IV(kappa N-1-Hind)(2)] complex (5d(4)-system), where Hind = 2H-indazole, was studied experimentally and theoretically. Relativistic CASSCF/CASPT2 calculations for this and model [(OsCl6)-Cl-IV](2-) complexes were employed to understand the nature of the low-lying multiplets. It is found that despite strong metal-ligand covalency they are basically characterized by the total angular pseudo-momentum J originating from the spin-orbit coupling of the ground-state spin S = 1 with the orbital pseudo-momentum L = 1 of the Os-IV ion. The strong spin-orbit interaction also preserves the dominant J = 0 character of the non-magnetic ground state in the trans-[(OsCl4)-Cl-IV(kappa N-1-Hind)(2)] complex despite significant deviation of the ligand environment of Os-IV from octahedral symmetry. At the same time the spin-orbit admixture of all multiplets arising from the t(2g)(4) strong-field electronic configuration is indispensable for the correct description of magnetic properties of Os-IV complexes. Moreover, based on ab initio calculations, we argue that the charge-transfer states play an important role in the magnetism of the present and probably other 5d complexes, a situation never encountered for 3d and 4f compounds.