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

Menon, Mahua ; Bag, Nilkamal ; Chakravorty, Animesh ; Pramanik, Amitava (1995) Geometrical preference of ruthenium oxidation states: metal redox and isomerisation of [Ru(S2CNEt2)2(PPh3)2]0,+ Journal of the Chemical Society, Dalton Transactions (10). pp. 1543-1547. ISSN 1472-7773

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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.

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