Ligand Effects on the Mechanisms of Thermal Bond Activation in the Gas-Phase Reactions NiX+/CH4 -> Ni(CH3)(+)/HX (X = H, CH3, OH, F)

The thermal ion-molecule reactions NiX+ + CH4 -> Ni(CH3)(+) + HX (X = H, CH3, OH, F) have been studied by mass spectrometric methods, and the experimental data are complemented by density functional theory (DFT)-based computations. With regard to mechanistic aspects. a rather coherent picture emerges such that, for none of the systems studied, oxidative addition/reductive elimination pathways are involved. Rather, the energetically most favored variant corresponds to a sigma-complex-assisted metathesis (sigma-CAM). For X = H and CH3, the ligand exchange follows a 'two-stale reactivity (TSR)' scenario such that, in the course of the thermal reaction, a twofold spin inversion, i,e., triplet -> singlet -> triplet. is involved. This TSR feature bypasses the energetically high-lying transition state of the adiahatic ground-state triplet surface. In contrast. for X = F, the exothermic ligand exchange proceeds adiabatically oil the triplet ground state, and some arguments are proposed to account for the different behavior of NiX+/Ni(CH3)(+) (X = H, CH3) vs. NiF+. While the couple Ni(OH)(+)/CH4 does not undergo a thermal lipand switch. the DFT computations suggest a potential-energy surface that is mechanistically comparable to the NiF+/CH4, system. Obviously. the ligands X act as it mechanistic distributor to switch between single vs. two-state reactivity patterns.