APPLICATION OF MOLECULAR MECHANICS (MMX) CALCULATIONS TO AN INVESTIGATION OF THE CONFORMATIONS OF COORDINATED TRIPHENYLPHOSPHINE IN ORGANOMETALLIC COMPOUNDS OF THE TYPES (ETA-6-C6R6)CR(CO)2PPH3 AND (ETA-5-C5H5)FE(CO)(PPH3)R'(R = H, ME-R' = H, ALKYL, ACYL)

MMX calculations are utilized to determine the low-energy structures and hence the preferred conformations arising from metal-phosphorus and phosphorus-carbon bond rotation in the organometallic compounds (eta6-C6R6)Cr(CO)2PPh3 (R = H, Me) and (eta5-CrH5)Fe(CO)(PPh3)R' (R' = H, alkyl, acyl). In general, the computed bond lengths and angles and both the ligand-M-P-C(ipso) and the M-P-C-(ipso)-C(ortho) torsional angles of the low-energy conformations are in very good agreement with X-ray crystallographic data. Conformational energy profiles are alw computed, utilizing MMX, for PPh3 rotation in the two series of compounds, and excellent agreement with the quantitative estimates of steric barriers to rotation reported in the literature is obtained.