Abstract
The temperature-jump method has been used to determine the kinetics of the reaction of
U022+ ion with monochloroacetate, sulfate, acetate, and thiocyanate and of V02+ ion
with thiocyanate. The U022+ ion forms higher order complexes with the first three of
these ligands, whereas for uranyl and vanadyl only the thiocyanate monocomplex is im- portant. Bimolecular rate constants at 20° and the given ionic strength for formation
of the 1-1 uranyl complex are: S042-, 180 M~l sec-1 at μ = 0.15 M; ClCH2COO-, 110 M-1 sec-1 at μ = 0.15 M; SCN~, 290 M~l sec-1 at μ = 1.2 M; CH3COO-, 1050 M~l sec-1 at μ = 0.15 M. For the reaction between vanadyl and thiocyanate the rate con- stant is 160 M-1 sec-1 at 20° and μ — 0.2 M. Substitution labilizes the water molecules still bound to the oxo ion, so that further substitution is speeded up. The effect, however, is not as pronounced as for divalent transition metal ions. Complexation rates for uranyl ion are most nearly typical of 3+ ions. No clear-cut correlation between the formation rate constant and ligand basicity could be demonstrated for either cation. The principal effect the oxo ion has on the rate of complexation is exerted by the rather high positive charge density on the metal, especially for uranyl. The site of nucleophilic attack on vandal may play a role in Kinetics of substitution.