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states (and those of the 6D even less). For instance, the dehydrogenation reaction is spin forbidden from the 6S ground state and presumably involves spin-orbit mixing. For this reason, it is very inefficient. The quartet states react more efficiently because the dehydrogenation reaction now conserves spin and is thermodynamically favorable.The overall reactivity of Cr+ is described by the potential energy surfaces shown in Figure 5. The same simple molecular orbital arguments used successfully in explaining the reactions of atomic transition metals with H2* 12345are used to generate this picture and appear to remain valid. The spin-forbidden surface crossings which are crucial to our interpretation of these experimental results have also been inferred in other transition-metal-ion reactions with methane3,4 and have been postulated for Cr+.13 In this regard, the detailed conclusions of this work are very similar to those of other recent and ongoing studies of M+ with CH4.2-4,6 Where Cr+ differs from these systems is in the estimated position of the surface crossing, about 1.8 eV above the ground-state reactants for Cr+, compared with <0.5 eV for Ti+ and V+.3,4 In this respect, it seems most surprising that Cr"1"^) exhibits this type of reactivity at all. Further, the relatively inert Cr+(6S) dehydrogenates methane, while Fe+, generally considered much more reactive, does not.6

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