Mechanism for C-H bond activation in ethylene in the gas phase vs. in solution - vinylic or agostic? Revisiting the case of protonated Cp*Rh(C2H4)2

When Cp*Rh(C2H4)(2)H+ (2) is exposed to C2H4 in the gas phase, inside the cell of an FT-ICR mass spectrometer, the most notable feature is the lack of any bimolecular reactivity. Collisional activation of 2 leads to ethylene loss and formation of Cp*Rh(C2H4-mu-H)(+) (3). In contrast to the reactivity of 2 in solution, ethylene dimerisation is negligible in the gas phase. Coordinatively unsaturated 3, rather than 2, is the major species in which reactivity is observed to occur. Compound 3 reacts with ethylene in three parallel processes: (a) Slow addition of ethylene to give 2; (b) rapid, intermolecular hydrogen atom exchange (monitored in separate reactions with free C2D4 to give 3-d(1-5)); (c) ligand substitution of ethylene in 3. DFT calculations reproduce these observations, showing low barriers for hydrogen scrambling, high barrier to ligand loss in 2, and even higher barriers to elimination of either H-2 or ethane. Mechanistic models for the elimination and scrambling processes are discussed.