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1 Polish Journal of Chemistry

1 2009

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

Das S., Patnaik A. K.

Polish Journal of Chemistry

2009

Vol. 83, nr 3

455-466

Base hydrolysis of some (aminomonocarboxylato)(tetraethylenepentamine)Cobalt(III) complexes were studied in aqueous medium. The pseudo-first order rate constant fitted the relationship. kobs = kOH[OH-]T at 0.005 [OH-] mol dm-3 0.10 at 293.15 K-313.15 K, (I = 0.1 mol dm-3), where kOH denotes the second order base hydrolysis rate constant. The alfa beta R isomer of the alfa-alaninato complex reacted approximately 8 times faster than its alfa beta S analogue at 298.15 K. Also the reactivity difference kOH(pyridine- 2-carboxy lato)/kOH(beta-alaninato) asymptotic to 3 for the (alfa 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 Dcb mechanism is valid for all these substrates. The ionic strength dependence of kOH in the range 0.01 Less-than or equal to I, mol dm-3 Less-than or equal to 0.1 for glycinato and beta-alaninato complex also confirmed the relationship log kOH = log kOH 0 + S1[I1/2/(1 + I1/2) + CI], where C is an adjustible parameter S1 = 2bZAZB, with b = 0.509 at 298.15 K, ZA and ZB denote charges of the reactant species. The acid catalyzed aquation indicated that alfa beta R-[(tetren)CoO2C-(CH3)NH3]3+ aquates via spontaneous (ko) and acid catalyzed (kaq) path. However, these reactions are extremely slow (ko = (1.74š0.17)×10-5 s-1, kaq = (2.72 + 0.05)×10-5 dm3 mol-1 s-1 at 333.15 K and I = 1.0 mol dm-3). Key words: kinetics mechanism, base hydrolysis, stopped flow, (aminomono carboxylato)(tetraethylenepentamine)cobalt(III)