Mixed valency of a 5d element: The osmium example

Pronounced spin–orbit coupling effects and the stabilization of higher oxidation states for the osmium analogues of mixed-valent ruthenium compounds result in more complex electronic spectra, frequent EPR silence, and valence transfer-dependent comproportionation constants for the intermediates.

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► Relative to ruthenium mixed-valent systems, the osmium analogues exhibit different donor/acceptor characteristics.
► Comproportionation constants reflect the ligand-dependent hole- vs. electron-transfer mechanism of valence exchange.
► Strong spin–orbit coupling results in frequent EPR silence due to rapid relaxation and line broadening.
► Several low-energy absorption bands are usually observed because of strong spin–orbit coupling effects.