The balance of orbital overlap and orbital energy in the activation of methane by actinide cations: insights from inductively coupled plasma tandem mass spectrometry†

The actinides present a unique challenge to chemical theory. The classical view of covalent bonding is driven by the extent of spatial overlap of valence orbitals. Modern theory has expanded assessments of covalency to include considerations of orbital energy degeneracy to assess orbital energy mixing between metal and ligand valence orbitals. Actinide–ligand (An–L) bonding has more recently been described as a balance between orbital overlap and orbital energy mixing, where 5f and L valence orbital overlap decreases while energy mixing between An 5f and L valence orbitals increases across the series. To test these existing views, we employed inductively coupled plasma tandem mass spectrometry to examine the kinetic energy dependences of reactions of actinide cations, Th⁺–Am⁺, with methane. This is the first experimental report of the energy dependences of methane activation reactions involving the cations of Pa, Np, Pu, and Am and the first experimental determination of transuranic An⁺–D, An⁺–CD₂, An⁺–CD₃, and An⁺–CD bond dissociation energies. The correlation of the measured An⁺–CD₂ bond energies with E_p(6d²) indicates that An⁺ 6d orbitals are the dominant contributors in the An⁺–CD₂ bonds. Close examination of the relative reactivities of An⁺ offers additional support that the balance of classical and modern views of molecular bonding may lie between Np⁺ and Pu⁺ and that the increased reactivity of Th⁺–Np⁺ may be attributed to the increased spatial extension of the 5f orbitals whereas covalent An⁺ bond formation may be more driven by the decreasing energies of the 5f orbitals across the actinide series.