Using hybrid density functional theory calculations with the B3LYP functional, the reaction mechanisms for cleavage of R2N−CN (R  H, Me) and MeO−CN bonds in the presence of an unsaturated iron(II) silyl complex, CpFe(CO)SiMe3, were studied. The following sequence of reactions was shown to be favorable: (i) coordination of a nitrile through the lone pair of electrons on the nitrile N atom (NCN) to form an end-on complex, (ii) isomerization of the endon complex to a side-on complex, (iii) migration of the silyl group to NCN facilitated by the hypervalent character of the Si atom and its electrostatic attraction with NCN to form a stable Fe−C−NCN three-membered-ring intermediate with an Fe−NCN dative bond, (iv) dissociation of the NCN atom from Fe and coordination of an amino N atom (NNR2) or methoxy O atom to Fe leading to an Fe−C−NNR2 or Fe−C−O three-membered-ring intermediate, and (v) cleavage of the R2N−C or MeO−C bond to form a silyl isocyanide ligand. Step iv possesses the largest activation energy in the sequence of reactions. The activation energies for the reactions of H2NCN, Me2NCN, and MeOCN were calculated to be 29.9, 28.0, and 19.1 kcal/mol, respectively, on the basis of potential energies with zero-point energy correction. This accounts for the experimental observation that the intermediates formed by silyl group migration can be isolated. The effects of the amino and methoxy groups are discussed by comparing their reaction profiles with that for the reaction of acetonitrile. Localized orbital analysis showed that in the three-membered-ring intermediates formed in step iv, the R2N−C and MeO−C bonds are activated by ring strain, whereas the Me−CN bond is activated by interaction of the Me−C bond with the vacant coordination site that is produced in the dissociation of NCN.