A pulse radiolysis study of the rate of ligand dissociation from mixed ligand cobalt(ii) complexes

Abstract

The reduction and subsequent reactions of the series of mixed ligand cobalt(III) complex cations, [Co(bipy)₃]³⁺, [Co(phen)₃]³⁺, [Co(en)₃]³⁺, [Co(en)₂(bipy)]³⁺, [Co(en)₂(phen)]³⁺, [Co(en)₂(5-NO₂-phen)]³⁺, [Co(en)₂(DPPZ)]³⁺, [Co(bipy)₂(NH₃)₂]³⁺, [Co(bipy)(NH₃)₄]³⁺, [Co(phen)₂(NH₃)₂](ClO₄)₃]³⁺ and [Co(phen)(NH₃)₄]³⁺, (where bipy is 2,2′-bipyridine, phen is 1,10-phenanthroline, en is ethylenediamine, 5-NO₂-phen is 5-nitro-1,10-phenanthroline and DPPZ is dipyrido[3,2-a:2′,3′-c]phenazine) have been studied using pulse radiolysis. The crystal structures of the mixed ligand complexes [Co(bipy)₂(NH₃)₂](ClO₄)₃, [Co(bipy)(NH₃)₄]Cl_0.5(ClO₄)_2.5, [Co(phen)₂(NH₃)₂](ClO₄)₃ and [Co(phen)(NH₃)₄](ClO₄)₃, have been determined. The mechanism of reduction of the cobalt(III) centre by e⁻_aq has been found to vary depending upon the nature of the ligands present in the complex. When an aromatic ligand is present the formation of a coordinated ligand radical is observed. This decays via intramolecular electron transfer to produce the cobalt(II) complex. The rate constants for the intramolecular electron transfer processes are of the order of 10³–10⁴ s⁻¹. The rates of ligand dissociation of bidentate ligands from the resulting cobalt(II) complexes have been determined. The relative rates of the dissociation of the first ligand from the complexes is found to be dependent upon the identity of the ligand and is related to the ligand structure. For the dissociation of the same ligand from related complexes, the electronic structure of the complex becomes a factor.