The mechanism of the isocyanide migratory insertion into the metal-carbon bond of monocylopentadienyltantalum dimethyl derivatives with [OSO] 2- tridentate phenolate ligands has been investigated with DFT calculations. The presence of both a cyclopentadienyl and a tridentate ligand complicates a usually simple reaction, the migratory insertion reaction being coupled with a fac → mer rearrangement of the tridentate ligand. Two routes have been explored for the overall migratory insertion process, depending on the order of the fac → mer and insertion steps. Calculations show that the dissociative (first fac → mer rearrangement, then migratory insertion) and the associative (first migratory insertion, then fac → mer rearrangement) pathways are in principle competitive. However, electronic effects of the phenyl substituents can favor one of the pathways. The study also points out the influence of the donor atom, due to the inversion at the donor atom required in order to achieve the fac → mer interconversion. © 2012 American Chemical Society.