AB-INITIO CALCULATIONS OF THE GEOMETRIES AND BONDING ENERGIES OF ALKANE AND FLUOROALKANE COMPLEXES WITH TUNGSTEN PENTACARBONYL

Ab initio calculations of the bonding energies of alkanes and fluorome thanes to W(CO)(5) have been performed in several basis sets and at a variety of different levels of electron correlation. The Moller-Plesse t second-order perturbation (MP2) optimized geometries of the complexe s show that a variety of coordination modes are available to alkanes a nd that the fluoromethanes are coordinated through fluorine. The lowes t energy geometry has an eta(2) agostic bond but two transition states , a second eta(2) structure and an eta(3) structure, are close in ener gy. Although the barriers for exchange of H's at one C are low, the ba rrier for the exchange of C sites is significantly higher. The MP2 bon ding energies have been recalculated with diffuse functions on the met al and with polarization functions on the ligands. Basis set superposi tion errors (BSSE) have been calculated with every basis set. Even bef ore the BSSE corrections, the MP2 bonding energies are in agreement wi th the experimental trend within each class of complexes (alkanes and fluoromethanes). These results verify that bonding energies increase w ith increasing alkane size and that CH3F has the largest bonding energ y among fluoroalkane complexes. BSSE corrections play a major role in obtaining good agreement between two classes of complexes because the correction is significantly larger for alkanes in these basis sets. Th e bonding energy for W(CO)(6)CH4 has been calculated at different elec tron correlation levels such as Moller-Plesset third-order (MP3) and f ourth-order (MP4) perturbation and quadratic configuration interaction with singles and doubles (QCISD). Excellent agreement with the experi mental data was obtained when the MP2 bonding energies in the largest basis set were corrected for BSSE, zero-point energy (ZPE), temperatur e, and the difference between the MP2 and QCISD energy.