Hydrogen Shift Reactions of Rhenium Hydrido Carbyne Complexes

Rhenium hydrido carbyne complexes Re­(CCHC­(CMe3)­CCR)­HCl­(PMe2Ph)3 (R = H, n-pentyl) undergo 1,2-hydrogen shift reactions from the metal to the carbyne carbon atom to give complexes Re­(HCCHC­(CMe3)­CCR)­HCl­(PMe2Ph)3, which have two isomeric forms, namely, a metallabicyclo[3.1.0]­hexatriene complex, in which the chloride is cis to the metal-bonded CH, and an alkyne–carbene complex, in which the chloride is trans to the metal-bonded CH. In contrast, a similar transformation does not occur for the analogous complex Re­(CCHC­(CMe3)­CCSiMe3)­HCl­(PMe2Ph)3, which has a SiMe3 group on the CC moiety. A computational study suggests that the difference in the reactivity of the hydrido carbyne complexes is related to steric effects in the corresponding hydride-shift products. Formation of Re­(HCCHC­(CMe3)­CCSiMe3)­HCl­(PMe2Ph)3 is not favored, mainly due to the steric interactions of the SiMe3 group with CMe3 and one of the phosphine ligands in the resulting metallabicyclo[3.1.0]­hexatriene complex, and of the SiMe3 group with the chloride ligand in the resulting alkyne–carbene complex.