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The stability constant of the complex formed between cobalt(II) and trimethyltren (Me 3 tren), 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  (NaClO 4 )). The deprotonation constant of the aqua complex, [Co(Me 3 tren)(H 2 O)] 2+ , was also established (pK a  = 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 + ] &lt; 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.

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