Kinetics and mechanism of hydrolysis of cis-Benzimidazole chloro bis-(ethylenediamie)cobalt(III) and cis-Benzimidazole bromo bis(ethylenediamine)cobalt(III) cations

Abstract

The kinetics of hydrolysis of cis-[CoX(bzmH)(en)₂]²⁺ (X = CI or Br; bzmH = benzimidazole; en = ethylenediamine) cations have been investigated in perchlorate medium of l = 0.3 mol dm^-3 and at 20-60°C. In the range pH 6.8-8.5 and 20-40°C the rate law for aquation takes the form -dln[Co^I1I]/dt = (k₁ + k₂K_NH[H⁺]^-l)/(1 + K_NH[H⁺]^-l) where k₁, and k₂, are the aquation rate constants of [CoX(bzmH)(en)₂]²⁺ and [CoX(bzm)(en)₂]⁺ respectively and K_NH is the acid-dissociation constant of the co-ordinated benzimidazole. The pK_NH of benzimidazole in [CoCI(bzmH)(en)₂]²⁺ is 8.6 at 25°C and l = 0.3 mol dm^-3 from pH-titralion and spectrophotometric measurements. Co-ordinated benzimidazole is 1O⁴ times stronger as an acid than free benzimidazole. The labilizing action of benzimidazole on the Co-X bond is stronger than that of imidazole in the k₁ path. This effect is, however, reversed in the k₂ path [i.e. k₂(im) > k₂(bzm)]. The imido-base [CoX(bzm)(en)₂]⁺ labilizes the Co-X bond ca. 700 times stronger than its conjugate-acid analogue.