RELATIVE BONDING CAPABILITIES OF MOLECULES TO METALS AS MEASURED BY GAS-PHASE PHOTOELECTRON-SPECTROSCOPY

The gas-phase photoelectron spectroscopy of compounds of the general f orm CpM(CO)(2)L(Cp = eta(5)-C5H5; M = Cr, Mn, or Fe; L = ligand of int erest) provides a direct experimental measure of the relative bonding capabilities of ligands to metals. The metal-based ionization energies of the formal d(6) metal centers of these compounds are sensitive to the electron donation/ acceptance of the ligands, and the splitting pa tterns of these ionizations are sensitive to the relative pi-donation/ acceptance properties of the ligands. This is the only experimental te chnique that can separate the ligand pi-bonding effects from the sigma -bonding and charge potential effects. A general formula is described by which the rr-bonding capabilities of ligands can be compared. (C) 1 997 Elsevier Science B.V.