The factors controlling both the binding of small molecules to several tungsten complexes and agostic bonding in the W(CO)(3)(PCy3)(2) complex have been examined through B3LYP hybrid density functional theory and ab initio MP2 calculations with and without basis set superposition error (BSSE) corrections. This approach attempts to isolate insofar as possible the separate effects of intrinsic bonding interactions, electron induction by ligands, and steric hindrance and strain. An important conclusion from this study is that for bimolecular reactions, BSSE corrections must be included for quantitative predictions. There is a reasonably good correlation between the BSSE-corrected B3LYP and MP2 results for bond dissociation enthalpies (BDEs) of very small molecules (H-2, N-2, and CO), but generally B3LYP BDEs tend to be smaller than the corresponding MP2 values. In the few cases where a comparison with experimental data can be appropriately made, it appears that the BSSE-corrected MP2 BDEs are more reliable. Using N-2 as a probe molecule, the strength of the agostic bond in W(CO)(3)(PCy3)(2) has been examined by calculating the BDE of N-2 in a series of tungsten complexes with increasing electron inducing effect without agostic bonding, then extrapolating the expected trend to the case of agostically bonded W(CO)(3)(PCy3)(2). Comparison of the extrapolated value to the calculated BDE of W(CO)(3)(PCy3)(2)(N-2) yields an estimated strength of the agostic bond of from 7 to 9 kcal mol(-1). Approximately 5 kcal mol(-1) of the interaction is assigned to the net agostic interaction associated with moving from a nonagostic local minimum configuration of the PCy3 ligands to the agostically bonded global minimum.