INVESTIGATION OF THE APPLICABILITY OF MOLECULAR MECHANICS (MMX) CALCULATIONS TO A CONFORMATIONAL INVESTIGATION OF ORGANOMETALLIC COMPOUNDS OF THE TYPES (ETA-5-C5H5)FE(CO)2CH2R, (ETA-5-C5H5)FE(CO)2C( = O)R, AND (ETA-5-C5H5)FE(CO)(PPH3)R (R = H, ALKYL, ACYL)

The potential utility and applicability of a new molecular modeling program, MMX, to organo-transition-metal chemistry is assessed by comparing calculated ligand rotation conformational energy profiles with relevant experimental data. To this end, both the identity of the lowest energy conformation and the enthalpy differences between the various conformations arising from alkyl or acyl ligand rotation in the flexible organometallic complexes (eta5-C5H5)Fe(CO)2CH2R (R = H, Me, Ph, C6H3(M-Me)2, Me3Si) (eta5-C5H5)Fe(CO)2C(O)R'(R' = Me, Ph, C6H3(m-Me)2), and (eta5-C5H5)Fe(CO)(PPh3)R" (R" = H, Me, Et, CH2Ph, CH2SiMe3, C(O)Me) are determined utilizing variable-temperature IR and NMR spectroscopy. For most of the compounds studied, the computed conformational energy profiles identify correctly the low-energy conformation, and provide surprisingly accurate estimates of the relative energies of the less stable conformations. The effects of electronic and entropic factors may complicate interpretations in a small number of cases, but the overall results seemingly validate the use of MMX to consider subtle steric effects on organometallic structural chemistry.