Geometrical preference of ruthenium oxidation states: metal redox and isomerisation of [Ru(S₂CNEt₂)₂(PPh₃)₂]^0,+

Abstract

Reaction of Na[S₂CNEt₂] with [Ru(PPh₃)₃Cl₂] afforded cis-[RuL₂(PPh₃)₂](L = S₂CNEt₂), which upon oxidation by cerium(IV) furnished trans-[RuL₂(PPh₃)₂]⁺, isolated as its PF₆^– salt. Both complexes are obtained in nearly quantitative yields. The trivalent complex displays one-electron paramagnetism and a rhombic frozen-solution EPR spectrum which has been analysed affording axial and rhombic distortion parameters of ≈ 8000 and ≈ 1700 cm^–1 respectively. A ligand-field transition within the Kramers doublets is observed at 6450 cm^–1, the predicted value being ≈ 7000 cm^–1. Variable-temperature (253–303 K) voltammetric studies have revealed that electrode reactions proceed in a stereoretentive fashion [E½(cis), 0.23 and E½(trans), –0.09 V vs. saturated calomel electrode (SCE)] but the species cis-[RuL₂(PPh₃)₂]⁺ and trans-[RuL₂(PPh₃)₂] are unstable and spontaneously isomerise affording the stable trans-trivalent and cis-bivalent complexes respectively, the equilibrium concentration of the unstable isomers being very small. The rates and activation parameters of these isomerisation reactions presumably proceeding by the twist mechanism (ΔS^‡ highly negative) have been determined. The factors controlling geometrical preference of oxidation states viz., metal–phosphorus back-bonding and PPh3···PPh3 steric repulsion are considered in relation to the osmium analogues reported previously.