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

The relationship between the bite angles of cis phosphines and the ele ctron distribution and bonding to the metal is studied by ps phase val ence photoelectron spectroscopy. The complexes selected for the electr onic structure comparison are cis-MO(CO)4(PMe3)2, MO(CO)4DMPE (DMPE = 1,2-bis dimethylphosphinotethane), MO(CO)4DMPM (DMPM = bis(dimethylpho sphino) methane), cis-W(CO)4(PMe3)2, W(CO)4DMPE, and Cis-W(CO)4DMPM. T he Mo carbonyl complexes give simple photoelectron spectra with the va lence ionizations originating from the phosphine lone pairs bonding to the metals and from the metal d6 configurations. The W complexes give similar spectra, but have an additional electronic spin-orbit perturb ation. The ionizations from the phosphine lone pairs that donate to th e metals in sigma bond formation show the effects of the different bit e angles of the ligands. However, the total interaction and charge dis tribution of the phosphines with the metals look very similar in each case. The metal-based ionizations also show very similar bonding and c harge distribution in each case. The similarity of the cis-(PMe3)2 and DMPE spectra is interesting in light of the almost-equal-to 15-degree s difference in P-M-P angles. The metal-based ionizations of the DMPM complexes are slightly different from those of the other complexes, pr imarily because of through-space interactions with the methylene carbo n in the phosphine backbone. The similarity in electronic interactions with the metal in these complexes is traced to a twist of the phosphi ne coordination to the metal which adjusts for the steric or bite angl e constraints of the ligands with a minimum effect on the bonding to t he metal. This results in slightly bent metal-phosphorus bonds.