Neutral Metal Atoms Acting as a Leaving Group in Gas-Phase SN2 Reactions: M(CH3)+ + NH3 → CH3NH3+ + M (M = Zn, Cd, Hg)

Under ambient conditions, mass-selected "bare" M(CH3)(+) ions (M = Zn, Cd, Hg) react with ammonia with cleavage of the metal-carbon bond and concomitant C-N coupling, yielding N-protonated methylamine, CH3NH3+; the neutral atomic metals M serve as a novel type of leaving group. In contrast, all three cationic metal amides, M(NH2)(+), do not afford C-N bond formation in quasi-thermal ion/molecule reactions with methane. With regard to the mechanism, computational studies reveal that CH3NH3+ is generated in a clean gas-phase S(N)2 reaction with inversion of configuration at the carbon center; nucleophilic substitution with configurational retention is prohibited by a kinetic barrier. Further, N-H bond activation in the couples M(CH3)(+)/NH3 and C H bond activation in M(NH2)(+)/CH4 are kinetically inhibited by transition structures which are higher in energy in comparison to the entrance channels.