Effect of Ligand Variation on the Site of Protonation in the Metal Carbynes CpL₂Mo⋮CBu and TpL₂Mo⋮CBu [L = CO, P(OR)₃]

The site of thermodynamic protonation of the Fischer carbynes CpL₂Mo⋮CBu and TpL₂Mo⋮CBu [L = CO, P(OR)₃] with HBF₄ depends on the number of π-acid CO ligands. As the number of carbonyls increases from zero to two and the electron density at the metal center is decreased by back-bonding, the protonation site shifts from the metal to the carbyne carbon. Parallel behavior in the Tp and Cp series of complexes allows the change in protonation site to be attributed to electronic effects in the ancillary ligands.