Ab Initio Calculations of the Geometries and Bonding Energies of Alkane and Fluoroalkane Complexes with Tungsten Pentacarbonyl
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Ab initio calculations of the bonding energies of alkanes and fluoromethanes to W(CO)5 have been performed in several basis sets and at a variety of different levels of electron correlation. The Møller - Plesset second-order perturbation (MP2) optimized geometries of the complexes show that a variety of coordinate modes are availabel to alkanes and that the fluoromethanes are coordinated through fluorine. The lowest energy geometry has an η2 agostic bond but two transition states, a second η2 structure and an η3 structure, are close in energy. Although the barriers for exchange of H's at one C are low, the barrier for the exchange of C sites is significantly higher. The MP2 bonding energies have been recalculated with diffuse functions on the metal and with polarization functions on the ligands. Basis set superposition errors (BSSE) have been calculated with every basis set. Even before the BSSE corrections, the MP2 bonding energies are in agreement with the experimental trend within each class of complexes (alkanes and fluoromethanes). These results verify that bonding energies increase with increasing alkane size and that CH3F has the largest bonding energy 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 set. The bonding energy for W(CO)6CH4 has been calculated at different electron correlation levels such as Møller - Plesse third-order (MP3) and fourth-order (MP4) perturbation and quadratic configuration interaction with singles and doubles (QCISD). Excellent agreement with the experimental data was obtained when the MP2 bonding enegies in the largest basis set were corrected for BSSE, zero-point energy (ZPE), temperature, and the difference between the MP2 and QCISD energy.
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