It has been suggested that the addition of carbon to Mo and W may improve their catalytic properties and even grant these metal carbides behaviors similar to those of late transition metals such as Pd and Pt. First-principles studies on the C-C bond activation of ethane by 4d transition metal (TM) atoms and TMC molecules have been carried out to develop a conceptual model underlying the changes. We find that the addition of carbon to TM atoms leads to large variations in the activation barrier depending on the metal, and that MoC indeed reveals a pronounced reduction in the C-C bond activation energy. A critical examination of molecular orbitals shows that the changes in reactivity are not linked to a dramatic increase in the filling of 4d states as implied by the analogy with Pd. The reactivity is governed by the location and filling of the Ss and 4d orbitals, with the different orbitals controlling different facets of reactivity. The Ss state controls the initial binding of ethane, with a strong anticorrelation between the ethane binding energy and the Ss occupation, while the location of the 4d(z)(2) orbital controls the reaction barrier that controls the activation energy for cleaving the C-C bond.