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

Chemical Society of Ethiopia
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  • Non-Bridging Ligands
  • Pentaamminecobalt(Iii) Complexes
  • Azidocobalt(Iii) Complex
  • Chlorocobalt(Iii) Complex
  • Kinetics Of Reduction Of Cobalt(Iii) Complexes
  • Ruthenium(Ii) Polypyridyl Complexes


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

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