B3LYP-based density functional theory (DFT) calculations are reported that provide insight into the stereoselective formation of cis-W(CO)(4)(eta(2)-C2H4)(2) observed in the pulsed 266 nm laser photolysis of tungsten hexacarbonyl (W(CO)(6)) in the presence of C2H4 in the gas phase at room temperature (J. Phys. Chem. 1995, 99, 4558). W(CO)(4)(eta(2)-C2H4) formed through the coordination of C2H4 onto coordinatively unsaturated W(CO)(4) was found to have a pseudo-C-2 nu symmetry (distorted trigonal bipyramid with an angle of ca. 90 degrees between the two equatorial COs) with a bond dissociation enthalpy (BDE) of W-C2H4 of 125 kJ mol(-1). In the subsequent coordination of C2H4 onto the W(CO)(4)(eta(2)-C2H4), having one vacant coordinatively unsaturated site, no barrier was found in the reaction path to cis-complex formation, while there was a barrier of about 89 kJ mol(-1) to the trans-complex. The calculations show that the stereoselective formation of cis-W(CO)(4)(eta(2)-C2H4)(2) is due to kinetic rather than thermodynamic control. The trans-W(CO)(4)(eta(2)-C2H4)(2) was calculated to be more stable than cis-W(CO)(4)(eta(2)-C2H4)(2) by about 10 kJ mol(-1). The BDE of W-C2H4 in cis-W(CO)(4)(eta(2)-C2H4)(2) was estimated to be 61 kJ mol(-1).