Kinetics and Mechanism of Base Hydrolysis of Some (Aminomonocarboxylato)(tetraethylenepentamine) cobalt(III) Complexes in Aqueous Medium

Base hydrolysis of some (aminomonocarboxylato)(tetraethylenepentamine)cobalt(III) complexes were studied in aqueous medium. The pseudo-first order rate constant fitted the relationship. k(obs) = k(OH)[OH-](T) at 0.005 <= [OH-] mol dm(-3) <= 0.10 at 293.15 K-313.15 K, (I = 01 mol dm(-3)), where k(OH) denotes the second order base hydrolysis rate constant. The alpha beta R isomer of the alpha-alaninato complex reacted approximately 8 times faster than its alpha beta S analogue at 298.15 K. Also the reactivity difference k(OH)(pyridine-2-carboxylato)/k(OH)(beta-alaninato) approximate to 3 for the (alpha beta S) isomer is indicative of the relative electron displacement property of the pyridine ring and the aliphatic chain attached to the bound carboxylate function which influences partly the Co-O bond strength. The D-cb mechanism is valid for all these substrates. The ionic strength dependence of k(OH) in the range 0.01 <= I, mol dm(-3) <= 0.1 for glycinato and beta-alaninato complex also confirmed the relationship log k(OH) = log k(OH)(0) + S-1[I-1/2/(1+1(1/2))+CI] where C is an adjustible parameter S-1 = 2bZ(A)Z(B), with b = 0.509 at 298.15 K, Z(A) and Z(B) denote charges of the reactant species. The acid catalyzed aquation indicated that alpha beta R-[(tetren)CoO2C-(CH3)NH3](3+) aquates via spontaneous (k(o)) and acid catalyzed (k(aq)) path. However, these reactions are extremely slow (k(o) = (1.74 +/- 0.17)x10(-5) s(-1), k(aq) = (2.72 + 0.05)x10(-5) dm(3) Mol(-1) s(-1) at 333.15 K and I = 1.0 mol dm(-3)).