Kinetics and mechanism of hydrolysis of cis-chlorobis(ethylenediamine)-(imidazole)cobalt(III) and cis-bromobis(ethylenediamine)(imidazole)cobalt(III) cations

Abstract

The kinetics of hydrolysis of cis-[CoX(imH)(en)₂]₂+(imH = imidazole; en = ethylenediamine; X = Cl or Br) cations have been investigated in perchlorate medium of I= 0.3 mol dm^-3. In the range pH 0.5-5.7 the rate law for aquation takes the form -dln[Co^III]/dt=k₁+k₂K_NH[H⁺]^-1 where k₁ and k₂ are the aquation rate constants of [CoX(en)₂(imH)]²⁺ and [CoX(im)(en)₂]⁺ respectively and KNH is the acid dissociation constant of the co-ordinated imidazole. At 50 ° C the values of k₁,k₂KNH, ΔH^‡, and ΔS^‡ for the k₁ path are (1.21 ± 0.02)× 10^-5 s^-1, (4.95 ± 0.11)× 10^-11 mol dm^-3 s^-1, 92.3 ± 1.2 kJ mol^-1, -54 ± 3 J K^-1 mol^-1 for the chloro-, and (5.52 ± 0.10)× 10^-5 s^-1, (33.4 ± 0.7)× 10^-11 mol dm^-3 s^-1, 94.5 ± 0.3 kJ mol^-1, and -34 ± 1 J K^-1 mol^-1 for the bromo-complex respectively. Values of k₂ obtained from the base-hydrolysis studies are (1.28 ± 0.17)× 10^-2 and (2.46 ± 0.22)× 10^-2 s^-1 at 31.8 °C for the chloro- and bromo-complexes respectively, and the imido-complex [CoCl(im)(en)₂]⁺ also undergoes second-order base hydrolysis with a rate constant of 5.1 ± 1.0 dm³ mol^-1 s^-1 at the same temperature. The labilizing action of imidazole and its conjugate base on the Co-X bond appears to be comparable to that of pyridine and hydroxide respectively. Co-ordinated imidazole is 105 times stronger as an acid than free imidazole. The sulphate- and mercury(II)-catalysed aquations of both the substrates have also been studied. The value of k_ip/k₁, where k_ip and k₁ are the rate constants of aquation of [CoX(imH)(en)₂]²⁺, [SO₄]^2-and [CoX(imH)(en)₂]²⁺ species respectively, is 2.3 ± 0.2 at 60 °C for X = Cl and 4.6 ± 0.1 at 50 °C for X = Br. The mercury(II)-catalysed aquation follows second-order kinetics, -dln[Co^III]/dt=kHg[Hg²⁺] : at 30.5 °C the rate constant (k_Hg]) and activation enthalpy and entropy are (3.48 ± 0.03)× 10^-2 dm³ mol^-1 s^-2, 68.4 ± 0.7 kJ mol^-1. and -48 ± 2 J K^-1 mol^-1for the chtoro- and 12.4 ±0.5 dm3 mol^-1s^-1, 53.4 ± 0.4 kJ mol^-1, and -48 ± J K^-1 mol^-1 for the bromo-complex respectively.