USE OF PHOSPHINES AS VERSATILE MOLECULAR PROBES FOR STUDYING THE COORDINATION STATES OF MOLYBDENUM IONS IN MO SIO2 CATALYSTS BY EPR/

Adsorption of trimethyl-, triethyl- and tri(t-butyl)phosphine which ha ve differing Tolman angles was used for the EPR characterization of th e coordination sphere and spatial accessibility of Mo5+ ions present o n the surface of reduced supported Mo/SiO2 catalysts, Due to the prese nce of P-31 nuclei a well resolved 1:2:1 ligand superhyperfine structu re (ca. 3 mT) was observed even in the first derivative spectra showin g that two phosphine molecules are directly bonded to exposed tetracoo rdinated Mo-4c(5+) species to form together with the remaining ligands a distorted octahedron with one distinct molybdenyl bond, This method detects tetracoordinated Mo species even when their EPR spectra are o bscured by the simultaneous presence of the EPR signal of dominant hex acoordinated Mo-6c(5+) ions, The ligand arrangement in the tetracoordi nated Mo-4c(5+) can be best explained in terms of a D-2d pseudosymmetr y giving rise to the resultant C-s structure for this surface complex.