Agostic versus Terminal Ethyl Rhodium Complexes: A Combined Experimental and Theoretical Study

Ethylene insertion into Rh–H bonds in complexes bearing an anionic fac-triphosphane ligand gives hydrido complexes, β-agostic species, or noninteracting ethyl derivatives depending on the reaction conditions. Several chemical equilibria between these species have been analyzed by NMR and DFT calculations, which revealed that they are mainly controlled by the entropy. Moreover, β-agostic species were found to be lower in enthalpy than the corresponding hydride–ethylene complexes, probably due to the steric pressure exerted by the bulky fac-triphosphane ligand.