Electron donor and acceptor properties of trimethylphosphine, triphenylphosphine, and related ligands with transition metals by photoelectron spectroscopy.

Abstract

Gas-phase photoelectron spectroscopy is used to evaluate the a-donation and 1t-acceptance abilities of a series of Group 15 ligands. The free ligands PMe(3-n)Ph(n) (n=0-3), EMe₃ (E=As) and EPh₃ (E=As, Sb, Bi) are examined (Me=CH₃, Ph=C₆H₅) and the trends in lone pair ionizations are discussed. Organometallic compounds investigated include CpMn(CO)2L (L=PPh₃, AsPh₃), Cp'Mn(CO)₂L (L=PPh₃, PMe₃, AsPh₃, SbPh₃), and Mo(CO)₅L (L=PMe₃, PPh₃). Contrary to initial predictions based on electronegativities and solution pKₐ's, the trend in ionization energies for the series PMe(3-n)Ph(n) is PPh₃ < PMePh₂ < PMe₂Ph < PMe₃. The trend in first ionizations is governed by a filled-filled interaction between the lone pair and phenyl rings. The metal ionization energies of organometallic compounds containing PMe₃ and PPh₃ reflect this trend indicating that the above order is also the order of a-donation. These metal ionizations also indicate that all members of the series, PMe(3-n)Ph(n), have the same amount of π-acceptance. Just as in the phenyl containing phosphine ligands, the ligands EPh₃ exhibit filled-filled interactions between the lone pair and phenyl orbitals, but not uniformly. This filled-filled interaction decreases with the heavier elements. The order of lone pair ionizations for EPh₃ is PPh₃ < AsPh₃, SbPh₃, BiPh₃. The destabilization expected upon descent of the periodic table is offset by the effects of decreasing relaxation energy and decreasing filled-filled interactions. Metal ionizations do not strictly follow the trends of the free ligands EPh₃. These metal ionization energies coincide for organometallic compounds where E is phosphorus or arsenic. Metal ionization energies for E=Sb are very slightly stabilized compared to E=P, As. The ligands EPh₃ display the same amount of σ and π interactions in the organometallic compounds examined. It is observed that the identity of R is more important in determining σ/π interactions than the element directly bound to the metal. This work serves to bring together previous literature and current experimental results to understand the structure and bonding in the ligands ER₃ (E=P, As, Sb, Bi and R=Me, Ph).