The reactions of Sc+ + CH4 and ScCH2+ + H-2 have been studied with the CASSCF and MR-SDCI-CASSCF methods, The reaction of the ground state Sc+ with CH4 proceeds as Sc+(D-3, s(1)d(1)) + CH4 --> ScCH4+((3)A ") --> TS2((1)A’) --> HScCH3+((1)A’) --> TS1 --> (H-2)ScCH2+((1)A(1)) --> ScCH2+((1)A(1)) + H-2 and is calculated to be endothermic by 24.8 kcal/mol. After formation of an ion-molecule complex ScCH4+, the reaction cannot proceed on the triplet surface because of a high barrier and has to cross over to singlet and reach the singlet transition state TS2 with a barrier of 28.6 kcal/mol. The insertion product HScCH3+, 4.9 kcal/mol more stable than the reactants Sc+ + CH4 produces most favorably an ion-molecule complex (H-2)ScCH2+ with a barrier of 28 kcal/mol, which can dissociate to give H-2 and ScCH2+ ((1)A(1)) with a small barrier of 3.2 kcal/mol. At higher temperatures, production of ScH+ + CH3 and ScCH3+ + H from HScCH3+ will dominate. The reverse reaction ScCH2+((1)A(1)) + H-2 proceeds very easily and leads to HScCH3+((1)A’), ScCH4+((3)A’), and Sc+(D-1 or D-3) + CH4. Comparison of the results for Sc+ (and ScCH2+) with those for Fe+ and Co+ (and FeCH2+ and CoCH2+) shows the following. (a) Carbene complexes MCH(2)(+) of early transition metal cations, M = Sc, Ti, and V, should activate H-H/C-H bonds more easily than their late transition metal analogs and lead to hydridomethyl, HMCH(3)(+), and ion-molecule M(CH4)(+) complexes. For first-row late transition metals the hydridomethyl complex does not exist thermodynamically or kinetically. (b) The products of the reaction of early transition metal cations with methane at low temperatures should be MCH(2)(+) and H-2, as this channel (I) is less endothermic than MH(+) + CH3 (2) and MCH(3)(+) + H (3) channels. For late transition metals, though the endothermicities are similar between three channels, channel 1 requires a large H-H bond activation barrier and does not take place, whereas channels 2 and 3 should be possible at high temperatures. All these similarities and differences in the reactivity of early and late first-row transition metal cations and their carbene complexes are explained by using a molecular orbital picture.