Ab initio quantum chemical calculations, including the effects of electron correlation, are performed on the methane C-H insertion reaction for the second-row transition-metal singly charged cations. Comparisons are made to experimental studies of alkane activation by naked transition-metal cations. The accuracy of the calculations is estimated through a comparison to a few highly accurate calculations, yielding typical bond energy errors of 3-8 kcal/mol in the present calculations. Applying these estimated errors to the calculated relative energies leads to good agreement with the experimental results for alkane activation, in most cases. An exception is ethane activation by the molybdenum cation, which is experimentally observed to be exothermic. In contradiction to this observation, a C-H insertion barrier of about 15 kcal/mol is predicted on the basis of the present calculations. Comparison between the naked cations and the naked neutral metal atoms for the methane activation reaction shows that although the cations have a lower C-H insertion barrier for the metals to the left and in the middle of the periodic table, for the metals to the right the insertion threshold is higher for the cations than for the neutrals. Also, for almost all second-row metals, the insertion product is less stable for the cations than for the neutrals. These differences between the neutrals and the cations can be understood in terms of differences in atomic spectra.