The transformation of a rhodium(I) η 2 -alkyne model complex RhCl(PH 3 ) 2 (HCtCH) (A) into the vinylidene form RhCl(PH 3 ) 2 (CdCH 2 ) (E) has been examined by ab initio theoretical calculations using MP2 level geometry optimizations and localized molecular orbital (LMO) analysis. The vinylidene form E has been found to be 7.8 kcal/mol more stable than A. The previously found intraligand 1,2-hydrogen shift mechanism in the Ru(II)-coordinated alkyne-vinylidene isomerization is not relevant for the present Rh system. The reaction proceeds via the oxidative addition product RhCl(PH 3 ) 2 (H)(CtCH) (C), followed by a bimolecular hydrogen shift from the metal to the terminal carbon of a second molecule rather than by intramolecular 1,3-hydrogen transfer. The LMO analysis of the transition state of the unimolecular 1,3-hydrogen shift indicates that the hydrogen moves as a proton while it interacts with the three centers simultaneously, i.e., Rh, CR, and C in the transition state. The hydrogen was analyzed to migrate also as a proton in the bimolecular mechanism. The barrier of the bimolecular pathway has been further calculated for a more realistic system with substituted phosphines, RhCl(P i Pr 3 ) 2 (H)(CtCH), using the integrated MO + MM (MP2:MM3) method. It was concluded that in the real system with substituents on both the phosphines and the alkyne, RhCl(P i Pr 3 ) 2 (HCtCR), the bimolecular hydrogen shift is still favored by ca. 15 kcal/mol in free energy of activation; unimolecular 1,3-H migration should become important in special cases like solid state isomerizations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.