Rates of Ligand Exchange around the Bis-Oxalato Complex [NpO2(C2O4)2]3− Measured by Using Multinuclear NMR Spectroscopy under Neutral to Semi-Alkaline Conditions

The kinetics of ligand exchange between the free oxalate ion, C2O42−, and the bis-oxalato NpV complex, [NpO2(C2O4)2]3−, in aqueous solution are reported by using 13C and 17O 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 13C and 17O 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 there is almost no dependence on temperature, which is interpreted to indicate that hydrolysis is coupled to ligand exchange under these conditions.