Hydrolysis of Metal Dioxides Differentiates d-block from f-block Elements: Pa(V) as a 6d Transition Metal; Pr(V) as a...

Gas-phase reactions of pentavalent metal dioxide cations M^VO₂⁺ with water were studied experimentally for M = V, Nb, Ta, Pr, Pa, U, Pu, and Am. Addition of two H₂O can occur by adsorption to yield hydrate (H₂O)₂M^VO₂⁺ or by hydrolysis to yield hydroxide M^V(OH)₄⁺. Displacement of H₂O by acetone indicates hydrates for Pr^V, U^V, Pu^V, and Am^V, whereas nondisplacement indicates hydroxides for Nb^V, Ta^V, and Pa^V. Computed potential energy profiles agree with the experimental results and furthermore indicate that acetone unexpectedly induces dehydrolysis and displaces two H₂O from (H₂O)VO(OH)₂⁺ to yield (acetone)₂VO₂⁺. Structures and energies for several M^V, as well as for Th^IV and U^VI, indicate that hydrolysis is governed by the involvement of valence f versus d orbitals in bonding: linear f-element dioxides are more resistant to hydrolysis than bent d-element dioxides. Accordingly, for early actinides, hydrolysis of Th^IV is characteristic of a 6d-block transition metal; hydration of U^V and U^VI is characteristic of 5f actinyls; and Pa^V is intermediate between 6d and 5f. The praseodymium oxide cation Pr^VO₂⁺ is assigned as an actinyl-like lanthanyl with properties governed by 4f bonding.