Rates of Ligand Exchange around the Bis-Oxalato Complex [NpO (C O ) ] Measured by Using Multinuclear NMR Spectroscopy under Neutral to Semi-Alkaline Conditions.

The kinetics of ligand exchange between the free oxalate ion, C O , and the bis-oxalato Np complex, [NpO (C O ) ] , in aqueous solution are reported by using C and O NMR spectroscopy methods. Rates of exchange were measured in the pH regime of 6.5-9.0, at which speciation is shown to be suitably simple. Because the neptunium(V) complex is paramagnetic, the rates of ligand exchange were estimated by following the width of the C and O signals assigned to the free oxalate ion in solution and by applying the Swift-Connick method for measuring rates of exchange. A set of experiments were conducted in which pH and total oxalate concentration were varied, and the linear dependence of the rate on these parameters was demonstrated. Variable-temperature NMR spectroscopy was also performed to measure activation parameters of complexation. At pH<8.0, ΔH =16.9 ±4.9 kJ mol and ΔS =-116.3 ±17.1 kJ mol  K , whereas at pH>8.0 there is almost no dependence on temperature, which is interpreted to indicate that hydrolysis is coupled to ligand exchange under these conditions.