Rates of Ligand Exchange around the Bis-Oxalato Complex [NpO₂ (C₂ O₄ )₂ ]^3- Measured by Using Multinuclear NMR...

The kinetics of ligand exchange between the free oxalate ion, C₂ O₄ ^2- , and the bis-oxalato Np^V complex, [NpO₂ (C₂ O₄ )₂ ]^3- , in aqueous solution are reported by using ¹³ C and ¹⁷ 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 ¹³ C and ¹⁷ 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^-1 and ΔS^≠ =-116.3 ±17.1 kJ mol^-1  K^-1 , 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.