| Sumario: | The reversible reduction of [Os(CN)5(NO)]2-, the osmium analog of the nitroprusside ion, has been studied by spectroelectrochemical techniques (IR, UV-Vis, EPR). The strong shift of the nitrosyl stretching band from 1850 to 1560 cm-1 (in acetonitrile) already indicates nitrosyl ligand-centered reduction in comparison to the smaller shifts for the cyanide frequencies (2140 → 2080 cm-1). Remarkably, the g factor analysis (g|| = 2.017, g⊥ = 2.003) for electrogenerated [Os(CN)5(NO)]3- does not reflect any effect from the 5d element osmium with its high spin-orbit coupling constant; only the need to apply helium temperatures for EPR signal observation indicates the presence of a heavy transition metal.
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