Photodetachment of multiply charged anions:: The electronic structure of gaseous square-planar transition metal complexes PtX42- (X = Cl, Br)

Two square-planar 5d transition metal complexes, PtX42- (X = Cl, Br), were observed in the gas phase using ion-trap mass spectrometry and an electrospray ionization source. Photodetachment photoelectron spectroscopy was used to probe their electronic structure, and their photoelectron spectra were measured at three photon energies, 193, 266, and 355 nm. The spectra for the two complexes were found to be similar, each with 10 well-resolved electronic features. These observed detachment features were qualitatively interpreted using the currently available theoretical calculations. The unprecedented resolution of the spectra afforded in the gas phase allowed us to definitely determine the ground state molecular energy levels for these classical square-planar metal complexes. We found that in PtCl42- all the d-orbitals are above the ligand-derived orbitals, whereas in PtBr42- all the d-orbitals are stabilized while the ligand-orbitals are destabilized, leading to an overlap between the d- and ligand-orbitals. Furthermore, the binding energies of the second excess electron in PtX42- were found surprisingly to be negative by -0.25 and -0.04 eV for X = Cl and Br, respectively. The negative electron-binding energies indicate that the two gaseous dianions are in fact electronically unstable against electron loss owing to the strong intramolecular Coulomb repulsion between the two excess charges. Photon-energy-dependent studies clearly revealed the dianion nature of these species and allowed the repulsive Coulomb barriers to be estimated.