Two concerted pathways leading to retention and inversion of stereochemistry at the migrating silicon center were found for the isomerization of formylmethylsilane to siloxyethene by ab initio molecular orbital calculations. The activation energy for the retention pathway has been calculated at the MP2/6-311++G(3df, 2p)//MP2/6-311++G** level to be 32 kcal/mol, which is ca. 30 kcal/mol smaller than that for the inversion pathway. The predicted exclusive retention stereochemistry and the calculated activation energy are in good agreement with the experimental results for a silylmethyl ketone with an optical active silyl group. Remarkable differences in the retention transition structures between the 1,3-silyl migrations in formylmethylsilane and the 1,3-silyl migrations in allylsilane are revealed by detailed analysis of geometries, natural bond orbitals, and the Laplacian (del(2)rho) of the wave function. The retention 1,3-silyl migration in formylmethylsilane is best described as an intramolecular nucleophilic substitution at silicon, while the corresponding 1,3-silyl migration in allylsilane is as an electrocyclic sigmatropic rearrangement controlled by subjacent orbital interactions. For related 1,3-migrations in HC(=O)CH2MH3 (M = C, Si, Ge, Sn), the E-a values for the retention pathway are much lower than the inversion pathway and they decrease in the following order: M = C much greater than Si > Ge > Sn (as expected from the relative stability of the pentacoordinate structure among the metals). A facile 1,3-migration from carbon to nitrogen in iminomethylsilane is predicted to occur with retention stereochemistry via an intramolecular nucleophilic substitution, which is similar to that in formylmethylsilane.