Relative bonding capabilities of molecules to metals as measured by gas-phase photoelectron spectroscopy

The gas-phase photoelectron spectroscopy of compounds of the general form CpM(CO)₂L (Cp = η⁵-C₅H₅; M = Cr, Mn, or Fe; L = ligand of interest) provides a direct experimental measure of the relative bonding capabilities of ligands to metals. The metal-based ionization energies of the formal d⁶ metal centers of these compounds are sensitive to the electron donation/acceptance of the ligands, and the splitting patterns of these ionizations are sensitive to the relative π-donation/acceptance properties of the ligands. This is the only experimental technique that can separate the ligand π-bonding effects from the σ-bonding and charge potential effects. A general formula is described by which the π-bonding capabilities of ligands can be compared.