Experimental Characterization of an Electron-Rich (σ²π⁴δ²δ^*2) Metal-Metal Triple Bond. Synthesis, Reactivity, and Photoelectron Spectral Studies of Trimethylphosphine Complexes of Dirhenium(II)

The reaction of (n-Bu₄N)₂Re₂X₈(X = Cl or Br) with PMe₃ gives high yields of the triply bonded complexes Re₂X₄(PMe₃)4. These compounds are oxidized by NOPF₆ to give paramagnetic [Re₂X₄(PMe₃)₄]PF₆, and the chloro derivative reacts with Ph₂PCH₂PPh₂ (dppm) and Ph₂PNHPPh₂ (dppa) to give Re₂Cl₄(PMe)₂(dppm) and Re₂Cl₄(PMe₃)₂(dppa), respectively. NMR spectroscopy (¹H and ³¹P{¹H}) shows that the latter complexes possess fairly symmetrical structures in which the PMe₃ ligands are in cis dispositions with respect to the bridging dppm and dppa ligands. The volatility of Re₂Cl₄(PMe₃)₄has permitted the measurement of its gas-phase photoelectron spectrum which accords with this compound possessing a σ²π⁴δ²δ*²configuration. The δ* ionization band is slightly narrower than the δ ionization band and occurs at about 0.9 eV lower binding energy. The π ionization gives evidence of spin-orbit splitting as expected for the heavy-atom rhenium character. An ionization assigned to removal of an electron from the valence a orbital is observed at a binding energy 1 eV higher than the ir ionization. Comparison of these ionizations with those of the corresponding W₂Cl₄(PMe₃)₄ (σ²π⁴δ²) complex is especially informative. In particular, these observations support a strong interaction between the valence a density on one metal atom and the core density on the neighboring metal atom in these complexes.