Density functional calculations have been carried out to investigate the mechanisms of ethene hydrogenation reactions catalyzed by four different titanocene complexes, Cp2Ti(CO)(2), Cp2Ti(CH3)(2), Cp2Ti(C6H5)(2), and Cp2Ti(p-C6H4CH3)(2) (Cp: cyclopentadienyl group = eta(5)-C5H5). The molecular geometries of the ground and transition states in these reactions have also been evaluated. The hybrid density functional method B3PW91 showed the best agreement with the experimental geometries of Cp2Ti(CO)(2). B3PW91 computations of activation parameters for the thermal decomposition of Cp2TiMe2 also showed good agreement with previous experimental data on a similar complex, (eta(5)-C5Me5)(2)TiMe2. Ethene hydrogenation by Cp2Ti(CO)(2) proceeds in the following order: first and second bond dissociations of Ti-C(CO) bonds followed by the formation of Cp2TiH2 from (Cp2Ti + H-2) and then ethene hydrogenation by Cp2TiH2. B3PW91 computations indicated that continuous heating of the system is necessary until the activation barrier of ethene hydrogenation by Cp2TiH2 is overcome. This is because the first three reactions occur nonspontaneously (at room temperature, Delta G = 18.3, 12.8, and 7.4 kcal/mol, respectively). This qualitative finding is supported by the corresponding experimental temperature (= 65 degrees C). For the Cp2TiR2 catalysts (where R = methyl, phenyl, and tolyl groups), ethene hydrogenation is found to be of first and second sigma bond metathesis for the Ti-C(R) bond and H-H bond to form Cp2TiH2, followed by ethene hydrogenation by Cp2TiH2. Another ethene hydrogenation begins with the first sigma bond metathesis for the Ti-C(R) bond and H-H bond, which is followed by the reductive elimination of RH to form Cp2Ti, the formation of Cp2TiH2, and then ethene hydrogenation by Cp2TiH2. In both hydrogenation reactions for the Cp2TiR2 catalysts, first sigma bond metathesis reactions are found to be rate-determining and the Delta G(double dagger)s are calculated to be very close in value (Delta G(double dagger) = 31.5 kcal/mol, with R = CH3; 32.0 kcal/mol, with R = C6H5; and 32.6 kcal/mol, with R = C6H4CH3, at room temperature). Contrary to the case of Cp2Ti(CO)(2), all reactions by Cp2TiR2 (except for the formation of Cp2TiH2 are spontaneous, or Delta Gs < 0. The Delta Gs in the case of Cp2TiR2 an found to be sufficient to overcome the activation Gibbs free energies for the subsequent reactions. Only the activation barrier for first sigma bond metathesis by Cp2TiR2 has to be overcome by a proper temperature control. Since the Delta S for first sigma bond metathesis is negative, the hydrogenation by Cp2TiR2 takes place below room temperature. These results are supported by the corresponding experimental temperature (= 0 degrees C). Alternative hydrogenation pathways through Ti-C(CO) bond dissociation of Cp2Ti(CO)(H-2) or through alpha-H abstraction of Cp2Ti(CH3)(2) have also been discussed.