Why is hafnium so unreactive?

The reaction of atomic hafnium cations with CH4 and CD4 is studied using a guided ion beam tandem mass spectrometer. In contrast to most third-row transition metal ions, the dehydrogenation reaction to form HfCH2+ + H2 is endothermic. At higher energies, other products, HfCH+, HfCH3+, and HfH+, the latter being the predominant species, are observed. Implicit in the behavior of the cross sections for HfH+, HfCH2+, and HfCH3+ is a HHf+CH3 intermediate. Modeling of the endothermic cross sections provides for 0 K bond dissociation energies (in eV) of D0(Hf+CH) = 5.10 ± 0.15, D0(Hf+CH2) = 4.37 ± 0.07, D0(Hf+CH3) = 2.12 ± 0.26, and D0(Hf+H) = 1.97 ± 0.11. These experimental bond energies are in good agreement with density functional calculations at the B3LYP/HW+/6-311 ++ G(3df,3p) level of theory. Theoretical calculations reveal the mechanism of the reaction and illustrate the geometric and electronic structures of the individual products and intermediates. Unlike its first and second-row congeners, which have quartet ground states and must change spin to dehydrogenate methane, Hf+ retains its ground state doublet configuration throughout the dehydrogenation reaction, demonstrating that spin-restrictions are not responsible for the relatively low reactivity of Hf+. Instead, this can be attributed to the unfavorable doubly occupied 6s orbital in the 2D ground state of Hf+.