Electronic Structure of Transition-Metal Tetracoordinated Complexes. 1. Theoretical ab Initio and UV-Photoelectron Spectroscopy Study of Palladium(II) and Platinum(II) Square-Planar Acetylacetonate Complexes

The electronic structures of Pd(acac)2 and Pt(acac)2 have been studied by using the pseudopotential valence-only ab initio method (including corrections for the major relativistic effects in the case of the heavier metal complex) and combined He I and He II photoelectron spectroscopy. A remarkable metal-ligand covalency involving orbitals of σ and π symmetry emerges from the analysis of the theoretical results. In particular, interactions with orbitals of a symmetry involve empty s and dxymetal orbitals and results in important ligand-to-metal charge transfer. These effects are both of a major importance in the case of the Pt complex. Detailed assignments of the UV photoelectron spectra are proposed on the basis of results of a simple perturbative treatment to correct the Koopmans eigenvalues for the reorganization effects in the ion states. Ionization energy values associated to production of selected ion states have been also evaluated by a ΔSCF treatment. The relative intensity changes of low-ionization-energy photoelectron bands observed on passing from the He I to the He II excited spectra have proven to be crucial guidance for the final assignments.