On the Mechanisms of Degenerate Ligand Exchange in [M(CH 3 )] + /CH 4 Couples (M=Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt) as Explored by Mass Spectrometric and Computational Studies: Oxidative Addition/Reductive Elimination versus σ‐Complex‐Assisted Metathesis

The degenerate ligand exchange in [M(CH 3 )] + /CH 4 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 CD 4 and 13 CH 4 , 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[CH 3 )] + /CH 4 , a σ‐complex‐assisted metathesis (σ‐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 ground states 3 A and 2 A. In contrast, for the [Pd(CH 3 )] + /CH 4 system, on the 1 A ground‐state potential‐energy surface both the OA/RE and σ‐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(CH 3 )] + /CH 4 complexes of the 5d elements, the σ‐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(CH 3 )] + complexes could not be generated experimentally, computational studies predict that both systems may undergo thermal reaction with CH 4 , 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.