The dianionic PdI dimers [TBA]2[(TPPMS) 2Pd2(μ-C3H5)2] (1) [TBA = tetrabutylammonium, TPPMS = PPh2(3-C6H 4SO3)-] and [TBA]2[(TPPMS) 2Pd2(μ-C3H5)(μ-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 π2 orbitals, which are localized on the terminal carbon atoms of the bridging allyl ligands. In 2, the Pd centers stabilize the single π2 orbital of the bridging allyl ligand, and it is lowered in energy. The presence of the high-energy out-of-phase combination of the π2 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 η1-allyls, whereas species with one bridging allyl and one bridging chloride ligand are unreactive. © 2012 American Chemical Society.
