Structural and mechanistic studies of co-ordination compounds. Part 24. Application of cyclic voltammetry to study the chelation effect on acid hydrolysis of some cis- and trans-ruthenium(II) amine complexes

Abstract

Formal reduction potentials E_f of a series of tertramine ruthenium(III) complexes of the type cis- and trans-[RuL (A)X]ⁿ⁺ have been determined by cyclic voltammetry (c.v.) in aqueous solution. In general, for a given pair of unidentate ligands A and X, the E_f values increase with increased chelation fo L: (NH₃)₄ < (en)₂(en = H₂NCH₂CH₂NH₂) < 2,3,2-tet(3,7-diazanonane-1,9-diamine, RS isomer) < cyclam (1,4,8,11-tetra-azacyclotetradecane). On the other hand, for a given tetramine L, the E_f values vary with the nature of A and X in the following manner: dihalogeno- < aquahalogeno- < diaqua-couples, and chloro- < bromo- < isothiocyanato-couples. The E_f values of cis isomers are greater than those of the trans counterparts. The kinetics of aquation of some ruthenium(II) complexes of the type trans-[RuLX₂](X = Cl or Br) have also been followed over a range of temperature by the c.v. technique.These reactions are stereoretentive with negative entropies of activation, the first-order rate constants, k₁, decrease with increased chelation, and the complexes are less labile than the corresponding cis, isomers similar features to those of the corresponding cobalt(III) and ruthenium(III) complexes. A dissociative machanism with a square-pyramidal intermediate is assumed for these reactions. The importance of solvation, nephelauxetic, and σ-trans effects has been invoked to explain the above thermodynamic and kinetic behaviour of these ruthenium complexes.