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

Abstract

Pentaamminecobalt(III) complexes, [Co(NH 3 ) 5 X] 2+ (X = Cl - , N 3 - ), 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 # (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. 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.