Non-bridging ligand effects on the kinetics of reduction of chloro- and azido-pentaamminecobalt(III) by some polypyridyl complexes of ruthenium(II)

Abstract

Pentaamminecobalt(III) complexes, [Co(NH3)5X] 2+ (X = Cl-, N3-), are reduced by [Ru(bipy)3]2+ and [Ru(terpy)(bipy)Cl]+ in aqueous media at a constant ionic strength of 0.5 M (HCl/LiCl). At 308 K, the second order rate constants (M-1 s-1) are as follows: 17.9 for the reduction of the azidocobalt(III) complex by [Ru(bipy) 3]2+, and 1.41 and 2.63 for the [Ru(terpy)(bipy)Cl] + reduction of the azido- and chlorocobalt(III) complexes, respectively. Activation enthalpies (ΔH‡) and entropies (ΔS‡) were determined from temperature dependence measurements with the following results: ΔH‡ = 72.1 kJ mol-1 and ΔS‡ = 13.3 J mol-1 K-1 for the [Ru(bipy) 3]2+ reduction of the azidocobalt(III) complex, while for the reduction of the cobalt(III) complexes by [Ru(terpy)(bipy)Cl]+, ΔH‡(N3-) = 20.3 kJ mol-1, ΔH(Cl-) = 40.6 kJ mol-1, ΔS‡(N 3-) = -177 J mol-1 K-1, and ΔS‡(Cl-) = -106 J mol-1 K-1, The relative rates of electron transfer in the different reactions and the influence of π-acceptor ligands on the ruthenium(II) reduction of the cobalt(III) complexes are discussed. © 2006 Chemical Society of Ethiopia.