Density functional theory (DFT, B3PW91) calculations have been carried out on the reactivity of ethene with model systems M(NR)(=CHCH3)(X)(Y) for M = Mo or W, R = methyl or phenyl, X = CH2CH3, OCH3, or OSiH3, and Y = CH2CH3, OCH3, or OSiH3, which are representative of experimental olefin metathesis catalysts, and the results are compared to those previously obtained for Re(CCH3)(=CHCH3)(X)(Y). The general pathway comprises four steps: olefin coordination, [2+2] cycloaddition, cycloreversion, and olefin de-coordination. Two key factors have been found to control the detailed shape of the energy profiles: the energy of distortion of the tetrahedral catalyst and the stability of the metallacycle intermediate, which is controlled by the M-C bond strength. The efficiency has been evaluated by calculating the turnover frequency (TOF) based on the steady-state approximation, and the most striking feature is that the unsymmetrical catalysts (X not equal to Y) are systematically more efficient for all systems (Mo, W, and Re). Overall, the Re complexes have been found to be less efficient than the Mo and W catalysts, except when Re is unsymmetrically substituted: it is then calculated to be as efficient as the best Mo and W catalysts.