Electronic structure of monodentate-coordinated diphosphine complexes. Photoelectron spectra of pentacarbonylmethylene bis(dimethyl phosphine)molybdenum and pentacarbonylethylene bis(dimethyl phosphine)molybdenum

Photoelectron spectroscopy is used to study the electronic structure of molybdenum carbonyl complexes that contain diphosphine ligands bound to the metal through only one of the two phosphorus atoms. This represents the first examination of the relative bonding capabilities of diphosphine ligands in the absence of chelating geometries, which is important for understanding many chelate effects. Photoelectron spectra are reported for Mo(CO)₅DMPE and Mo(CO)₅DMPM and compared to the spectra of Mo(CO)₅PMe₃and the corresponding free phosphine and diphosphine ligands (PMe₃is trimethylphosphine, DMPE is 1, 2-bis-(dimethylphosphino)ethane, and DMPM is bis(dimethylphosphino)methane). The energy splittings between the d²⁶⁴metal-based ionizations of these complexes indicate that the β-back-bonding ability is the same for each of these phosphine ligands and is relatively small, about 25% that of carbon monoxide. The metal-based ionizations shift only slightly to lower binding energy from the PMe₃to the DMPE to the DMPM complex (total shift = 0.10 eV) due to a slightly increasing negative charge potential at the metal along this series. This would normally be interpreted as slightly increasing σ-donor strength in the order PMe₃< DMPE < DMPM. However, the difference between the ionization energy of the coordinated lone pair (CLP) of the phosphine and the ionization energy of the lone pair of the free ligand indicates an opposite trend in σ-donor strength with PMe₃(1.28 eV) > DMPE (1.27 eV) > DMPM (1.23 eV). The shift of the uncoordinated phosphine lone-pair ionization (ULP) of the monocoordinated diphosphine complexes, which is affected primarily by charge potential effects, reveals that the important factor is a transfer of negative charge from the uncoordinated end of the phosphine through the alkyl linkage to the coordinated phosphine. This transfer is more important for the DMPM ligand because of the shorter alkyl chain between the phosphorus atoms. Aside from these subtle details of charge distribution, the primary conclusion is that the diphosphine ligands, DMPE and DMPM, have σ-donor and w-acceptor strengths extremely similar to those of PMe₃.