Proton nuclear magnetic resonance study of ligand exchange on the pentakis(N-methylacetamide)dioxouranium(VI) ion

Abstract

Pentakis(N-methylacetamide)dioxouranium(VI) ion, [UO₂(nma)₅]²⁺, is shown to be the predominant species in solutions of [UO₂]²⁺ and nma in CD₂Cl₂ or CD₃CN. The virtual constancy of the rate of nma exchange on [UO₂(nma)₅]²⁺ over a 56.9-fold range of free-nma concentration in CD₃CN is interpreted in terms of a dissociative mechanism. A typical set of kinetic data for this process is k_ex(273 K)= 336 ± 16 s^–1, ΔH^‡= 54.4 ± 0.6 kJ mol^–1, and ΔS^‡= 3.3 ± 2.3 J K^–1 mol^–1 for a solution in which [UO₂(nma)₅²⁺], [nma], and [CD₃CN] are 0.162 4, 0.661 3, and 15.92 mol dm^–3 respectively. In CD₂Cl₂ higher values are observed for ΔH^‡ and ΔS^‡, but both data sets lie on an isokinetic plot of ΔH^‡ against ΔS^‡ for [UO₂L₅]²⁺ systems for which the rate of exchange of L is independent of [L]. The significance of this isokinetic relationship is discussed.