Towards understanding the magnetism of Os(IV) complexes: an insight.

The magnetism of a recently synthesized -[OsCl(κN-Hind)] complex (5d-system), where Hind = 2-indazole, was studied experimentally and theoretically. Relativistic CASSCF/CASPT2 calculations for this and model [OsCl] 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 originating from the spin-orbit coupling of the ground-state spin = 1 with the orbital pseudo-momentum = 1 of the Os ion. The strong spin-orbit interaction also preserves the dominant = 0 character of the non-magnetic ground state in the -[OsCl(κN-Hind)] complex despite significant deviation of the ligand environment of Os from octahedral symmetry. At the same time the spin-orbit admixture of all multiplets arising from the t strong-field electronic configuration is indispensable for the correct description of magnetic properties of Os complexes. Moreover, based on 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.