Edwin Garcia, Juhyoun Kwak, and Allen J. Bard, "Electrochemistry in Liquid Sulfur Dioxide. 8. Oxidation of Iron, Ruthenium, and Osmium Bipyridine Complexes at Ultramicroelectrodes at Very Positive Potentials," Inorg. Chem., 27 (24), 4377 - 4382 (1988.11.__.)
The anodic potential range of liquid SO2 for use in electrochemical studies was extended to about +4.7 V vs. SCE by employing a new purification method and using tetra-n-butylammonium hexafluoroarsenate [(TBA)AsF6] as supporting electrolyte. The anodic solvent limit occurs at potentials +0.7 V more positive potentive than in previous studies and is attributed to supporting electrolyte oxidation. The electrochemistry of M(bpy)32+ complexes (M = Fe, Ru, and Os) at -70 oC was studied by cyclic voltammetry at an ultramicroelectrode (25 mm diameter, Pt) with scan rates up to 10 kV/s. Fe and Ru oxidation waves to the 3+ form (metal center oxidation), and 4+, 5+, and 6+ species (bpy-centered oxidations) are observed. While the 4+ forms were stable, the 5+ and 6+ species decomposed, presumably by reaction with solvent or supporting electrolyte. For M = Os, waves for oxidation of stable, metal-center 3+ and 4+ species observed. A third wave, to an unstable 5+ species, represented by OsIV(byp*+)(bpy)2, is also seen. Standard potentials for these waves and rate constants for the decomposition of the 5+ and 6+ species are estimated. The order of stabilities of the highly oxidized forms is Fe(5+) ~ Ru(5+) > Ox(6+) > Ru(6+) > Fe(6+).