Kinetics and mechanism of hydrolysis of cis-benzimidazolechlorobis(ethylenediamine)cobalt(III) and cis-benzimidazolebromobis(ethylenediamine)cobalt(III) cations
Abstract
The kinetics of hydrolysis of cis-[CoX(bzmH)(en2)]2+(X = Cl or Br; bzmH = benzimidazole; en = ethylenediamine) cations have been investigated in perchlorate medium of I= 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[CoIII]/dt=(k1, +k2KNH[H+]–1)/(1 +KNH[H+]–1) where k1 and k2 are the aquation rate constants of [CoX(bzmH)(en)2]2+ and [CoX(bzm)(en)2]+ respectively and KNH is the acid-dissociation constant of the co-ordinated benzimidazole. The pKNH of benzimidazole in [CoCl(bzmH)(en)2]2+ is 8.6 at 25 °C and I= 0.3 mol dm–3 from pH-titration and spectrophotometric measurements. Co-ordinated benzimidazole is 104 times stronger as an acid than free benzimidazole. The labilrzing 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.