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 MO with water were studied experimentally for M = V, Nb, Ta, Pr, Pa, U, Pu, and Am. Addition of two HO can occur by adsorption to yield hydrate (HO)MO or by hydrolysis to yield hydroxide M(OH). Displacement of HO by acetone indicates hydrates for Pr, U, Pu, and Am, whereas nondisplacement indicates hydroxides for Nb, Ta, and Pa. Computed potential energy profiles agree with the experimental results and furthermore indicate that acetone unexpectedly induces dehydrolysis and displaces two HO from (HO)VO(OH) to yield (acetone)VO. Structures and energies for several M, as well as for Th and U, 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 is characteristic of a 6d-block transition metal; hydration of U and U is characteristic of 5f actinyls; and Pa is intermediate between 6d and 5f. The praseodymium oxide cation PrO is assigned as an actinyl-like lanthanyl with properties governed by 4f bonding.