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[CoIII]/dt=k1+k2KNH[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 kip/k1, where kip 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[CoIII]/dt=kHg[Hg2+] : at 30.5 °C the rate constant (kHg) and activation enthalpy and entropy are (3.48 ± 0.03)× 10–2 dm3 mol–1 s–1, 68.4 ± 0.7 kJ mol–1. and –48 ± 2 J K–1 mol–1 for the chtoro- and 12.4 ±0.5 dm3 mol–1 s–1, 53.4 ± 0.4 kJ mol–1, and –48 ± J K–1 mol–1 for the bromo-complex respectively.