The open H2Si−CH2Fe+ (6A‘)/cyclic (H2Si−CH2)Fe+ (4A‘‘) and the methyl structure FeHSi−CH3+ (6A‘‘, 4A‘‘) isomers are predicted here to be the most stable FeSiCH4+ isomers. These are therefore the most likely candidates for the two distinguishable species observed in the gas-phase mass-spectrometric experiments. This conclusion is strongly supported by the computed barriers (42-50 kcal/mol) separating the relevant pairs of the FeSiCH4+ isomers, the magnitude of which is consistent with the experimental estimate. The reaction between Fe+ in the ground 6D and excited 4F states and silaethylene have been studied using multiconfigurational (CASSCF) wave functions and multiconfigurational wave functions augmented by second-order perturbation theory (CASPT2), with basis sets of valence triple-ζ plus polarization quality. The open H2Si−CH2Fe+ (6A‘)/cyclic (H2Si−CH2)Fe+(4A‘‘) and the methyl structure FeHSi−CH3+ (6A‘‘, 4A‘‘) isomers are found to be the most stable FeSiCH4+ isomers. Therefore, these are the most likely candidates for the two distinguishable FeSiCH4+ species observed in the gas-phase mass-spectrometric experiments by Jacobson and co-workers. This conclusion is further supported by the computed barriers of 42−50 kcal/mol separating the relevant pairs of the FeSiCH4+ isomers, the magnitude of which is consistent with the experimental estimate. On the quartet surface the net isomerization barrier (relative to separated reactants) is zero, whereas there is a net 7.2 kcal/mol barrier on the higher energy sextet surface.