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

Abstract

Gas-phase reactions of pentavalent metal dioxide cations MVO2+ 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)2MVO2+ or by hydrolysis to yield hydroxide MV(OH)4+. Displacement of H2O by acetone indicates hydrates for PrV, UV, PuV, and AmV, whereas nondisplacement indicates hydroxides for NbV, TaV, and PaV. 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 MV, as well as for ThIV and UVI, 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 ThIV is characteristic of a 6d-block transition metal; hydration of UV and UVI is characteristic of 5f actinyls; and PaV is intermediate between 6d and 5f. The praseodymium oxide cation PrVO2+ is assigned as an actinyl-like lanthanyl with properties governed by 4f bonding.