On the mechanisms of degenerate ligand exchange in [M(CH3)](+)/CH4 couples (M = Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt) as explored by mass spectrometric and computational studies: Oxidative addition/reductive elimination versus sigma-complex-assisted metathesis

The degenerate ligand exchange in [M(CH3)](+)/CH4 couples occurs in the gas phase at room temperature for M=Ni, Ru, Rh, Pd, and Pt, whereas the complexes containing Fe and Co are unreactive. Details of hydrogen-atom scrambling versus direct ligand switch have been uncovered by labeling experiments with CD4 and (CH4)-C-13, respectively. The reactivity scale ranges from unreactive (M=Fe, Co) or inefficient (M = Ni, Pd) to moderately (M=Ru) and rather reactive (M=Rh, Pt). Quite extensive, but not complete, H/D exchange between the hydrogen atoms of the incoming and outgoing methyl groups is observed for M=Pt, whereas for M=Ni and Pd a predominantly direct ligand switch prevails. DFT calculations performed at the B3LYP level of theory account well for the thermal nonreactivity of the Fe and Co couples. For [Ni[CH3)](+)/CH4, a sigma-complex-assisted metathesis (sigma-CAM) is operative such that, in a two-state reactivity (TSR) scenario, two spin flips between the (3)A ground and (1)A excited states take place at the entrance and exit channels of the encounter complexes. For M=Ru and Rh, only oxidative addition/reductive elimination (OA/RE) is favored energetically, and the reaction is confined to the electronic around states (3)A and (2)A. In contrast, for the [Pd(CH3)(+)/CH4 system, on the (1)A ground-state potential-energy surface both the OA/RE and sigma-CAM variants are energetically comparable, and the small reaction efficiency for the ligand switch is reflected in transition states located energetically close to the reactants. For the [M(CH3)](+)/CH4 complexes of the 5d elements, the sigma-CAM mechanism does not play a role. For M=Pt, the energetically most favored path proceeds in a spin-conserving manner on the (1)A potential-energy surface, which accounts for the extensive single and double hydrogen-atom exchange preceding ligand exchange. Although for M=Os and Ir the [M(CH3)](+) complexes could not be generated experimentally, computational studies predict that both systems may undergo thermal reaction with CH4, and an OA/RE mechanism will commence on the respective high-spin ground states; however, the bond-activation and ligand-exchange steps will occur on the excited low-spin surfaces in a TSR scenario.