Gas-phase reactions of hydrocarbons with An+ and AnO+ (An = Th, Pa, U, Np, Pu, Am, Cm):: The active role of 5f electrons in organoprotactinium chemistry

Activation of small (C-1-C-4) alkanes and alkenes by bare and oxo-ligated actinide cations (Th+ through Cm+) has been systematically examined using Fourier transform ion cyclotron resonance mass spectrometry. The reduced reactivities of Pu+, Am+, and Cm+ correlate with the energies for electronic excitation to a 5f(n-2)6d7s configuration, which indicates that their 5f electrons are chemically inert. The reactivity trend identified for the highly reactive early actinide ions, Th+ > Pa+ > U+ > Np+, is interpreted to indicate significant 5f electron participation in organoactinide sigma-type bond formation for Pa+. Among the seven studied AnO(+) ions, only ThO+, PaO+, and UO+ activated at least one hydrocarbon, with the reactivity of PaO+ being distinctively high. Electronic structure calculations for PaO+ show that its ground state is Pa(5f6d)O+, i.e., with one 5f and one 6d nonbonding electrons available on the metal, and all of its excited states up to 1.8 eV have a 5f orbital occupancy of >= 0.8. The high reactivity and substantial 5f character of PaO+ indicate participation of 5f electrons in hydrocarbon bond activation for oxo-ligated Pa+. The results of this work reveal that 5f electrons play a distinctive role in protactinium chemistry involving sigma-type organometallic bonding.