Base hydrolysis of αβ-nitro-, azido- and isothiocyanato-[1,9-bis(2′-pyridyl)-2,5,8-triazanonane]- and [1,11-bis(2′-pyridyl)-2,6,10-triazaundecane]-cobalt(III) cations. Departure from first-order hydroxide dependence

Abstract

The rate law for the displacement of X^– from syn-αβ-[Co(picdien)(NO₂)]²⁺ and anti-αβ-[Co(picdien)(NCS)]²⁺[picdien = 1,9-bis(2′-pyridyl)-2,5,8-triazanonane] by hydroxide in aqueous solution has been found to depart from the usual first-order dependence on [OH^–] at [OH^–] > 0.01 mol dm^–3. Since there are three non-equivalent amine protons present, analysis of the data gives composite quantities, representing the sum of the three proton-transfer equilibrium constants, and the sum of the products of each of these equilibrium constants with the rate constant for the dissociation of the corresponding conjugate base. This is an example of proton anomaly, but while it is not possible to make an unambiguous assignment of equilibrium constant and rate constant for the formation and reaction of the most labile conjugate base, it is possible to set limits. The anti-αβ-[Co(picdien)(N₃)]²⁺ cation and all the [Co(picditn)X]²⁺[picditn = 1,11-bis(2′-pyridyl)-2,6,10-triazaundecane] complexes examined, on the other hand, maintain a first-order dependence on [OH^–] over the whole range of concentration studied.