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 (E1/2) 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 E1/2 values of cis- and trans-[RuLX2]+,0 couples increase in the following order of X: N3 - < Cl- < Br- < NCS- < NO2 - 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 E1/2 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 E1/2 value over the range -0.80 to +0.83 V vs. Ag/Ag+(0.1 mol dm-3).