Hydrolysis of Metal Dioxides Differentiates d-block from f-block Elements: Pa(V) as a 6d Transition Metal; Pr(V) as a 4f "Lanthanyl".

Gas-phase reactions of pentavalent metal dioxide cations M ^V O ₂ ⁺ with water were studied experimentally for M = V, Nb, Ta, Pr, Pa, U, Pu, and Am. Addition of two H ₂ O can occur by adsorption to yield hydrate (H ₂ O) ₂ M ^V O ₂ ⁺ or by hydrolysis to yield hydroxide M ^V (OH) ₄ ⁺ . Displacement of H ₂ O 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 H ₂ O from (H ₂ O)VO(OH) ₂ ⁺ to yield (acetone) ₂ VO ₂ ⁺ . 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 ^V O ₂ ⁺ is assigned as an actinyl-like lanthanyl with properties governed by 4f bonding.