The neutral title compds. with Q = 3,5-di-tert-butyl-o-quinone or 4,6-di-tert-butyl-N-phenyl-o-iminobenzoquinone (Qx) were studied by UV-visible-NIR spectroelectrochem. and by EPR spectroscopy in the case of the odd-electron monocation and monoanion intermediates. Supported by DFT and TD-DFT calcns., the results indicate stepwise electron removal from predominantly ligand-based delocalized MOs on oxidn. whereas the stepwise electron uptake on redn. involves unoccupied MOs with considerably metal-ligand mixed character. In both cases, the strong near-IR absorption of the neutral precursors diminishes. In comparison to the ruthenium series, the osmium analogs 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 (1n, 2n) and corresponding monoiminoquinone systems (3n, 4n) is the shift of $\sim$0.6 V to lower potentials for the monoimino analogs. While the absorption features do not differ markedly, the EPR data reflect a higher degree of covalent bonding for the complexes with monoimino ligands. on SciFinder(R)
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