Ionization energies of metal chelates. Acetylacetonates, trifluoroacetylacetonates, and hexafluoroacetylacetonates of trivalent metals of the first transition series

Abstract

For several β-diketonate complexes of metals of the first transition series the electron impact ionization efficiency curves obtained using unfiltered electron beams were fitted to empirical post-threshold ionization laws. In the energy region extending to 2–3 V above threshold, fits of high quality were obtained for a second power ionization law (i.e. ion current was proportional to the square of the electron energy in excess of the threshold energy) for complexes of Al(III), Sc(III), Mn(III), Fe(III), and Co(III) (d0, d0, d4, d5, and d6, respectively). In the cases of Ti(III), V(III), and Cr(III) complexes (d1, d2, and d3, respectively) the experimental points lay above the "best fit" curves in the immediate post-threshold region. Electron energies were initially calibrated by fitting experimental data points for xenon to its published ionization functions. In later experiments, Al(tfa)3 or Al(hfa)3 was used for calibration purposes. When the electron impact results were compared with published photoelectron spectra of the complexes (where available) it became apparent that, in the threshold region, the strongest ionization processes involved electron removal from ligand dominated π orbitals. In the cases of Ti(III), V(III), and Cr(III) complexes, weaker ionization processes involving electron removal from orbitals of predominantly metal 3d character (correlating with t2g in octahedral symmetry) could also be detected at lower electron energies.