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

Abstract

Several factors have been found to affect the half-wave potentials (E_½) 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 E_½ 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 E_½ values of these Ru^III/Ru^II couples. It thus appears that a suitable combination of the above factors may ‘tune’ a Ru^III/Ru^II couple to possess any desired E_½ value over the range –0.80 to +0.83 V vs. Ag/Ag⁺(0.1 mol dm^–3).