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The ligand substitution reactions of complexes of the type trans-[(R)Co(Chel)S](+/0) with L, where chel = (DO)(DOH)pn = 2,2'-(1,3-diaminopropanebis(2-methyl-3-butanone)oxime), R = CH(3), L = imidazole, pyrazole, 1,2,4-triazole and 1-methylimidazole, and S = water and MeOH, and chel = (Hdmg)(2) = bis(dimethylglyoximate), R = CH(2)Cl, CH(2)Br, and CH(2)I, L = thiourea and pyridine, and S = water, were studied in detail as a function of temperature and pressure. The reported activation parameters (DeltaH, DeltaS and DeltaV) support the operation of a dissociative interchange (I(d)) mechanism. Complexes of the type trans-[RCo(Hdmg)(2)L] (R = CH(2)Cl, CH(2)Br, and CH(2)I; L = H(2)O and Py) were fully optimized at the B3LYP/LANL2DZp level, and the structural data support the mechanistic assignment based on the reported activation parameters. For the reaction of trans-[(CH(3))(2)Co((DO)(DOH)pn)] with acid, the activation parameters (DeltaH, DeltaS and DeltaV) were found to be 37 +/- 1 kJ mol(-1), -86 +/- 3 J mol(-1) K(-1) and -18.9 +/- 0.7 cm(3) mol(-1), respectively, and support a protonation mechanism.

The kinetics of axial water substitution by azoles (pyrazole and 1,2,4-triazole) in three different cobaloximes, viz.trans-[Co(Hdmg)(2)(R)H(2)O] where Hdmg = dimethylglyoximate, R = PhCH(2), Et and CF(3)CH(2), were studied as a function of azole concentration, temperature and pressure in aqueous solution. The second order rate constants for the substitution of water in trans-[Co(Hdmg)(2)(R)H(2)O] for R = Et at pH 6.0, 25 degrees C and I= 0.1 M (NaClO(4)), were found to be 1309 and 1200 M(-1) s(-1) for pyrazole (Pz) and 1,2,4-triazole (Tz), respectively, and those obtained for R = PhCH(2) were found to be 755 and 691 M(-1) s(-1), respectively. The second order rate constants in the case of R = CF(3)CH(2) were found to be 0.358 and 0.348 M(-1) s(-1) for Pz and Tz, respectively. The relative order of reactivity for the different alkyls being Et > PhCH(2) > CF(3)CH(2). The activation parameters (DeltaH([not equal]), DeltaS([not equal]) and DeltaV([not equal])) obtained for these reactions were found to be in the range of 65-87 kJ mol(-1), 24-47 J mol(-1) K(-1) and 2.5-7.7 cm(3) mol(-1), respectively. These data suggest that an I(d) substitution mechanism operates where the azoles participate in the transition state.

The kinetics of axial water substitution by cysteine in six different cobaloximes, viz.trans-RCo(Hdmg)(2)H(2)O, where Hdmg = dimethylglyoximate, R = cyclo-C(5)H(9) (c-P), CH(3)CH(2) (Et), CH(3) (Me), C(6)H(5)CH(2) (Bz), C(6)H(5) (Ph) and CF(3)CH(2), were studied as a function of cysteine concentration, temperature and pressure. It was found that cysteine substitutes the coordinated H(2)O molecule trans to the alkyl group with second order rate constants that follow the order of reactivity: c-P > Et > Bz > Me > Ph > CF(3)CH(2). Rate and activation parameters (Deltan H(++), Delta S(++) and Delta V(++)) enable the formulation of a reaction mechanism that can account for the substitution behaviour of the investigated alkylcobaloximes. In particular, a gradual mechanistic changeover from I(d) to I is observed along the series of R groups from c-P to CF(3)CH(2).