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^#(N^3-) = 20.3 kJ mol^-1, ΔH^#(Cl^-) = 40.6 kJ mol^-1, ΔS^#(N₃^-) = -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.

KEY WORDS: Non-bridging ligands, Pentaamminecobalt(III) complexes, Azidocobalt(III) complex, Chlorocobalt(III) complex, Kinetics of reduction of cobalt(III) complexes, Ruthenium(II) polypyridyl complexes

Bull. Chem. Soc. Ethiop. 2006, 20(2), 227-234.