THEORETICAL-STUDY OF ALKYLIDENE ORIENTATION AND AGOSTIC DISTORTION INOCTAHEDRAL W(PH3)2(CL)2X(CHR) COMPLEXES (X=CO, O R=H, CH3)

The orientation and the agostic distortion of the alkylidene ligand CH R (R = H, CH3) in electron-deficient tungsten complexes W(PH3)2(CI)2( X)(CHR) (X = CO, O) are studied by means of Extended Huckel calculatio ns. For X = CO, steric and electronic (p-d interaction factors favor o pposing alkylidene orientations. For R = CH3, the steric factor domina tes and the alkylidene avoids being coplanar with the phosphines. In o xo complexes, both the p-d interaction and the steric factors favor th e conformation with the oxygen lying in the alkylidene plane. The agos tic distortion is found to be easier in the complexes with X = CO beca use the pi-donating character of the oxygen in oxo complexes raises th e energy of the vacant d orbitals on the metal. The trends given by th ese calculations are in good agreement with the available experimental data.