Kinetics of Dissociation of the Cobalt(II) and Copper(II) Complexes of 2,2′,2″-tris(monomethylamino)triethylamine in Aqueous Acidic Solution

Abstract The stability constant of the complex formed between cobalt(II) and trimethyltren (Me3tren), 2,2′,2″-triaminotriethylamine monomethylated on each terminal amino group, in aqueous medium was determined (log Κ = 12.10 ± 0.01 at 25.0° C and ionic strength of 1.0 mol dm-3 (NaClO4)). The deprotonation constant of the aqua complex, [Co(Me3tren)(H2O)]2+, was also established (pKa = 9.31 ± 0.02 under the same conditions). It is concluded on the basis of these values and properties such as the UV/vis spectrum, that the complex possesses trigonal bipyramidal geometry, as does the corresponding copper(II) complex. Dissociation of both complexes, as a function of aqueous acid concentration, was monitored spectrophotometrically. For the copper(II) system there is a modest dependence on the acid concentration when [H+] < 0.4 mol dm-3 , and above this value the reaction is independent of acid concentration, with the firstorder rate constant reaching a limiting value of about 100 s-1 at 25.0°C. Dissociation of the cobalt(II) complex does not depend on the acid concentration and occurs at a rate of 34 s-1 . A mechanism accounting for these findings is proposed, and the rate and activation parameters are discussed in terms of the dissociation of other trigonal bipyramidal complexes.