Structural and mechanistic studies of co-ordination compounds. Part 34. Electrochemical behaviour of some octahedral ruthenium(III)/ ruthenium(II) couples containing tetra-amine or -thioether ligands

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Abstract
Several factors have been found to affect the half-wave potentials (E½) of some cis- and trans-[RuL(A)X]n+,(n– 1)+ couples, where L represents either four unidentate, two bidentate, or one quadridentate amines or thioethers, and A and X are unidentate π-acid ligands. A variation in the π-accepting capability of these L, A, or X ligands appears to be the most dominating factor. Thus, for analogous cis-[RuLCl2]+,0 couples, a change from L = L7(1,4,8,11-tetra-azacyclotetradecane) to L = L16(1,4,8,11-tetrathiacyclotetradecane) results in an anodic shift of ca. 1.0 V in both aqueous and CH3CN solutions. For some common acid ligands, the E½ values of cis- and trans-[RuLX2]+,0 couples increase in the following order of X: N3 – < Cl– < Br– < NCS– < N02 – over a span of ca. 0.7 V. Other factors, such as the presence of α-di-imine functions in the chelate rings, steric, ligand-chelation, ring-size effects and geometrical configuration of the complexes, and solvents also affect the E½ values of these RuIII/RuII couples. It thus appears that a suitable combination of the above factors may ‘tune’ a RuIII/RuII couple to possess any desired E½ value over the range –0.80 to +0.83 V vs. Ag/Ag+(0.1 mol dm–3).

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