Ab initio calculations of the geometries and bonding energies of alkane and fluoroalkane complexes with tungsten pentacarbonyl

Abstract

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 Moller-Plesset second-order perturbation (MP2) optimized geometries of the complexes show that a variety of coordination modes are available to alkanes and that the fluoromethanes are coordinated through fluorine. The lowest 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 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 sets. The bonding energy for W(CO)(6)CH4 has been calculated at different electron correlation levels such as Moller-Plesset 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 energies in the largest basis set were corrected for BSSE, zero-point energy (ZPE), temperature, and the difference between the MP2 and QCISD energy.