Spectroelectrochemical Characterization of the Two-Step Redox System {(μ-pz)[Os(CN)5]2}n-(n= 4, 5, 6; pz = Pyrazine). Similarities and Differences in Relation to the Creutz−Taube System

The pyrazine-bridged diosmium(II) complex {(μ-C 4 H 4 N 2 )[Os(CN) 5 ] 2 } 6 - has been synthesized as the hexapotassium salt and was converted to the hexakis(tetrabutylammonium) compound by ion exchange. Its stepwise oxidation to the Os 2 III Os II and Os 2 III Os III states was monitored spectroelectrochemically in acetonitrile/0.1 M Bu 4 NPF 6 in the UV−vis−NIR and IR regions and by EPR. The Os II 2 species exhibits long-wavelength solvatochromic MLCT bands at 14 800 and 18 480 cm - 1 . The pentaanionic mixed-valent ion has a comproportionation constant K c of 10 5.8 and is distinguished by several electronic absorptions in the IR region. The bands at 1972, 2480, 4000, 5000, 7170, and 11 900 cm - 1 are similar but appreciably lower in energy than those reported for the analogous complex {(μ-C 4 H 4 N 2 )[Os(NH 3 ) 5 ] 2 } 5+ . In addition to CN vibrational stretching features shifted on electron transfer there is an additional sharp band only for the 5− ion at 1582 cm - 1 which is attributed to a pyrazine ring vibration. The intensity of this band suggests an unsymmetrical situation and thus valence localization on the vibrational time scale. The EPR signals at g ⊥ = 2.0563 and g ∥ = 1.761 point toward excited states lying close to the doublet ground state and to significant metal/cyanide interaction. Compared to the Creutz−Taube ion {(μ-C 4 H 4 N 2 )[Ru(NH 3 ) 5 ] 2 } 5+ and its osmium homologue, the 5− ion described here exhibits a lower degree of metal−metal coupling.