Ruthenium in isomeric S4P2 co-ordination: metal redox and geometrical reorganisation

Abstract

The redox behaviour of dithiocarbonato complexes cis-[Ru^II(ROCS₂)₂(PPh₃)₂](1) has been electrochemically examined. One-electron oxidation affords cis-[Ru^III(ROCS₂)₂(PPh₃)₂]⁺, (1⁺), which rapidly isomerises to the trans form, (2⁺). Electroreduction of (2⁺) furnishes trans-[Ru(ROCS₂)₂(PPh₃)₂], (2), which isomerises at a measurable rate to (1) probably by a twist mechanism (ΔS^‡≈–110 J K^–1 mol^–1). The formal potentials of the couples (1⁺)–(1) and (2⁺)–(2) are respectively ≈ 0.7 and ≈ 0.3 V vs. saturated calomel electrode. The equilibrium constants for the reactions (1⁺)⇌(2⁺), (2)⇌(1), and (1⁺)+(2)⇌(2⁺)+(1) are respectively > 10², > 10², and ≈ 10⁶. The cis and trans S₄P₂ co-ordination spheres are thus specific for bi- and tri-valent ruthenium respectively. The species (2⁺)(low-spin d⁵) display rhombic e.s.r. spectra in frozen solution (77 K). The rhombic crystal-field component (≈ 2 000 cm^–1) is relatively small compared to the axial component (≈ 80000cm^–1). An optical transition within Kramers doublets is observed at ≈ 6000 cm^–1. One (2⁺) complex (R = PhCH₂) has been isolated in the pure state as its PF₆^– salt.