The intricate paramagnetic state of [Os(Q)2(bpy)]+, Q = 4,6-di-tert-butyl-o-iminobenzoquinone

The combination of two highly non-innocent ligands with a third-row transition metal produces the title complex ion which was crystallised as [Os(Q)2(bpy)](ClO4)·C6H6 (Q = 4,6-di-tert-butyl-o-iminobenzoquinones, bpy = 2,2′-bipyridine) and could be structurally characterised to exhibit a tris-chelate situation at the metal with trans-positioned N and cis-positioned O donor atoms. The metrical ligand parameters are in agreement with two partially reduced ligands. A 3-spin interaction (Q˙−)OsIII(Q˙−) can rationalise the observed S = 1/2 situation with ligand-centred resulting spin. Ligand-based spin is confirmed by DFT (calculated spin populations Q: 1.113; Os: −0.113; bpy: 0.001) and is also apparent from the EPR signal (g1 1.981, g2 1.955, g3 1.803, Δg 0.178, 〈g〉 1.915) which is influenced by the high spin–orbit coupling constant of the osmium centre. The susceptibility measurements reveal antiferromagnetic behaviour. A one-electron oxidation and two one-electron reductions could be monitored spectroelectrochemically (UV-VIS-NIR) and analysed by TD-DFT, in comparison with the results from the ruthenium analogue. The analysis reflects the strong orbital mixing between the metal and the two Q-ligand MOs.