The addition of ethene to Mo(NH)(CHR)(OR')(2) (R = H, Me; R' = CH3, CF3) has been studied with both ab initio molecular orbital and density functional theory calculations. Geometry optimizations were carried out with the HF/3-21G, HF/HW3, and B3LYP/HW3 methods. The energies were further evaluated with the MP2/HW3 and B3LYP/HWF (HWF basis set is equivalent to the 6-311G** basis set) methods. Ethene significantly favors attacking on the CNO face. The attack on the COO face by ethene is disfavored by 12.3 and 18.8 kcal/mol for R' = CH3 and CF3, respectively. The transition structure for the CNO face addition is in a distorted trigonal bipyramidal geometry, with the NH and one of the OR' groups axial. The calculated activation energy is low for R' = CH3, and it is significantly lower for R' = CF3. In agreement with the experiment, the syn alkylidene is calculated to be more stable than the anti rotamer (R = CH3). This is apparently due to the stabilizing agostic interaction involving the anti-a of the syn rotamer on the COO face. However, the transition structures derived from the syn and anti rotamers have similar stabilities, due to the disappearance of the agostic interaction. Thus, the anti alkylidene is effectively a more reactive catalyst than the syn alkylidene. The molybdacyclobutane product significantly favors a square pyramidal geometry when R' = CH3, but has a slight preference for a trigonal bipyramidal geometry when R' = CF3.