The homologous compounds M2Cl4(PR3)4, with M = Mo and W, have been used in a combined theoretical and experimental study to compare the electronic structures of quadruple bonds between molybdenum atoms and between tungsten atoms. The theoretical work was carried out by the SCF-Xα-SW method on the model systems with R = H, but using in all other respects the experimentally measured bond lengths and angles for the compounds with R = CH3. Relativistic corrections were made for both the molybdenum and tungsten compounds, but were found to be significant only for the tungsten compound. The PMe3 compounds were used for measurements, made with both He I and He II excitation, of the photoelectron spectra in the gas phase. For both compounds the highest filled orbital is the M-M δ-bonding orbital and the measured ionization energies are 6.44 and 5.81 eV for the Mo and W compounds, respectively. For both compounds the next observed ionizations, at 7.70 eV (Mo) and 7.05, 7.45 eV (W), can be assigned on experimental criteria to the M-M π-bonding orbitals. The spin-orbit splitting of the W—W π peak shows features attributable to mixing of σ, π, and δ components by the spin-orbit coupling operator. These peaks are followed by ionizations assignable to M—P bonding electrons at 8.41 eV (Mo) and 8.36 eV (W). The calculations predict this order correctly for the W compound but reverse the Mo-Mo π and Mo-P ionization energies. The W-W bonding appears to be weaker than the Mo-Mo bonding, and in general the results of this study are consistent with the greater reactivity (i.e., lower chemical stability) of the W-W quadruple bond as compared to the Mo-Mo quadruple bond.
ASJC Scopus subject areas
- Colloid and Surface Chemistry