The gas-phase reactions of Fe(CH2O)(+) and Fe(CH2S)(+) with a series of aliphatic alkanes were studied by Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. Like bare Fe+, C-C insertion, particularly terminal C-C insertion, is predominant for the reactions of Fe(CH2O)(+), while C-H insertion is preferred for Fe(CH2S)(+). About 90% of the Fe(CH2O)(+) reaction products are formed by C-C insertion with small alkane loss. For Fe(CH2S)(+), after initial C-H insertion, the proposed mechanism includes hydrogen transfer to sulfur, followed by migratory insertion of methylene into the metal-alkyl bond and formation of an activated H2S-Fe+-olefin complex, which dissociates by H2S elimination. The structures of the reaction products were probed by collision-induced dissociation, ion-molecule reactions, and use of labeled compounds, yielding information about the reaction mechanism. Collision-induced dissociation and ligand displacement reactions yield the brackets D-0(Fe+-C3H6) = 37 +/- 2 kcal/mol < D-0(Fe+-CH2S) < D-0(Fe+-C6H6) = 49.6 +/- 2.3 kcal/mol and D-0(Fe+-CH2O) < D-0(Fe+-C2H4) = 34 +/- 2 kcal/mol. The optimized geometry of Fe(CH2O)(+), obtained by density functional calculations, has C-2v symmetry with a nearly undisturbed formaldehyde unit. The Fe+-CH2O bonding is found to be predominantly electrostatic with a calculated bond energy of 32.2 kcal/mol. However, the optimized Fe(CH2S)+ structure has C-s symmetry with dative bonding between Fe+ and CH2S. D-0(Fe+-CH2S) is calculated at 41.5 kcal/mol. The differences in geometry and chemical bonding between Fe(CH2O)(+) and Fe(CH2S)(+) are correlated with the different reaction pathways observed.