Photoelectron Spectroscopy of Palladium(I) Dimers with Bridging Allyl Ligands

The dianionic Pd-I dimers [TBA](2)[(TPPMS)(2)Pd-2(mu-C3H5)(2)] (1) [TBA = tetrabutylammonium, TPPMS = PPh2(3-C6H4SO3)(-)] and [TBA](2)[(TPPMS)(2)Pd-2(mu-C3H5)(mu-Cl)] (2), containing two bridging allyl ligands and one bridging allyl ligand and one bridging chloride ligand, respectively, were synthesized. The electronic structures of these complexes were investigated by combining electrospray mass spectrometry with gas phase photodetachment photoelectron spectroscopy. The major difference between the photoelectron spectra of the anions of 1 and 2 is the presence of a low-energy detachment band with an adiabatic electron detachment energy of 2.44(6) eV in 1, which is not present in 2. The latter has a much higher adiabatic electron detachment energy of 3.24(6) eV. Density functional theory calculations suggest that this band is present in 1 due to electron detachment from the out-of-phase combination of the pi(2) orbitals, which are localized on the terminal carbon atoms of the bridging allyl ligands. In 2, the Pd centers stabilize the single pi(2) orbital of the bridging allyl ligand, and it is lowered in energy. The presence of the high-energy out-of-phase combination of the pi(2) allyl orbitals makes 1 a better nucleophile, which explains why species with two bridging allyl ligands react with CO2 in an analogous fashion to momoneric Pd eta(1)-allyls, whereas species with one bridging allyl and one bridging chloride ligand are unreactive.