The neutral title compounds with Q = 3,5-di-tert-butyl-o-quinone or 4,6-di- tert-butyl-N-phenyl-o-iminobenzoquinone (Qx) were studied by UV-vis-NIR spectroelectrochemistry and by EPR spectroscopy in the case of the odd- electron monocation and monoanion intermediates. Supported by DFT and TD-DFT calculations, the results indicate stepwise electron removal from predominantly ligand-based delocalised MOs on oxidation whereas the stepwise electron uptake on reduction involves unoccupied MOs with considerably metal–ligand mixed character. In both cases, the strong near-infrared absorption of the neutral precursors diminishes. In comparison to the ruthenium series, the osmium analogues exhibit larger transition energies from enhanced MO splitting and a different EPR response due to the higher spin–orbit coupling. The main difference between the quinone (1nnn, 2nnn) and corresponding monoiminoquinone systems (3nnn, 4nnn) is the shift of about 0.6 V to lower potentials for the monoimino analogues. While the absorption features do not differ markedly, the EPR data reflect a higher degree of covalent bonding for the complexes with monoimino ligands.
