High-resolution electron spectroscopy and σ/π structures of M (pyridine) and M+ (pyridine) (M=Li, Ca, and Sc) complexes

Metal-pyridine (metal=Li, Ca, and Sc) complexes are produced in laser-vaporization molecular beams and studied by pulsed-field-ionization zero-electron-kinetic-energy (ZEKE) spectroscopy and theoretical calculations. Both σ and π structures are considered for the three complexes by theory, and preferred structures are determined by the combination of the ZEKE spectra and calculations. The Li and Ca complexes prefer a σ bonding mode, whereas the Sc complex favors a π mode. Adiabatic ionization energies and metal-ligand vibrational frequencies are determined from the ZEKE spectra. Metal-ligand bond dissociation energies of the neutral complexes are obtained from a thermodynamic cycle. The ionization energies follow the trend of Li-pyridine (32 460 cm-1) <Ca-pyridine (39 043 cm-1) <Sc-pyridine (42 816 cm-1), whereas the bond energies are in the order of Ca-pyridine (27.0 kJ mol-1) <Li-pyridine (49.1 kJ mol-1) <Sc-pyridine (110.6 kJ mol-1). The different bonding modes between the main group metals and transition element are discussed in terms of Sc 3d orbital involvement. The bond energy differences between the Li and Ca metals are explained by the number of valence s electrons and the size of the metal atoms.