STERIC AND ELECTRONIC BALANCE IN METAL-PHOSPHINE COORDINATION: THE PHOSPHINE TWIST

Abstract

The relationship between the bite angles of cis phosphines and the electron distribution and bonding to the metal is studied by gas phase valence photoelectron spectroscopy. The complexes selected for the electronic structure comparison are cis-Mo(CO)₄(PMe₃)₂, Mo(CO)₄DMPE (DMPE = 1,2-bis dimethylphosphinotethane), Mo(CO)₄DMPM (DMPM = bis(dimethylphosphino) methane), cis-W(CO)₄(PMe₃)₂, W(CO)₄DMPE, and cis-W(CO)₄DMPM. The Mo carbonyl complexes give simple photoelectron spectra with the valence ionizations originating from the phosphine lone pairs bonding to the metals and from the metal d ⁶ configurations. The W complexes give similar spectra, but have an additional electronic spin-orbit perturbation. The ionizations from the phosphine lone pairs that donate to the metals in σ bond formation show the effects of the different bite angles of the ligands. However, the total interaction and charge distribution of the phosphines with the metals look very similar in each case. The metal-based ionizations also show very similar bonding and charge distribution in each case. The similarity of the cis-(PMe₃)₂ and DMPE spectra is interesting in light of the ～15° difference in P-M-P angles. The metal-based ionizations of the DMPM complexes are slightly different from those of the other complexes, primarily because of through-space interactions with the methylene carbon in the phosphine backbone. The similarity in electronic interactions with the metal in these complexes is traced to a twist of the phosphine coordination to the metal which adjusts for the steric or bite angle constraints of the ligands with a minimum effect on the bonding to the metal. This results in slightly bent metal-phosphorus bonds.