Reactions of actinide ions with ethylene oxide

Naked and oxo-ligated actinide (An) monopositive ions were reacted with ethylene oxide, cyclo-C(2)H(4)O (EtO). Along with An = U, Np, Pu and Am, ions of two lanthanide (Ln) elements, Ln = Tb and Tm, were studied for comparison. Metal and metal oxide ions, M(+), MO(+) and MO(2)(+), were generated by laser ablation and immediately reacted with EtO. Unreacted and product ions were detected by time-of-flight mass spectrometry. It was apparent that the overall reaction cross-sections decreased in the order U(+) > or = Np(+) > Pu(+) > Am(+). A primary reaction channel for each studied metal was the formation of MO(+) from M(+), in accord with the expected exothermicity of oxygen abstraction from EtO. For U, Np and Pu, the dioxides were also major products, indicating OAn(+)--O dissociation energies of at least 350 kJ mol(-1), the energy required for O-atom abstraction from EtO. For Am, Tb and Tm, the dioxides were only very minor products, reflecting the stabilities of the trivalent states and resistance to oxidation to higher valence states; the structures/bonding in these MO(2)(+) are intriguing given that the formal pentavalent bonding state is effectively unattainable. It was demonstrated that EtO, unlike more thermochemically favorable but kinetically restricted O-donors, is effective at achieving facile oxidation of actinide metal ions to the monoxide, and to the dioxide if the second O-abstraction reaction is exothermic. Several intriguing minor products were also identified, most of which incorporate metal--oxygen bonding and are attributed to the oxophilicity of the f-block elements; the contrast to the behavior of first-row d-block transition elements is striking in this regard. Particularly noteworthy was the formation of MH(4)(+) (and MOH(4)(+)), evidently via abstraction of all four H atoms from a single C(2)H(4)O molecule; the structures/bonding in these novel 'hydride' species are indeterminate and warrant further attention.