The transformation of a rhodium(I) eta(2)-alkyne model complex RhCl(PH3)(2)(HC=CH) (A) into the vinylidene form RhCl(PH3)(2)(C=CH2) (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(PH3)(2)(H)(C=CH) (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, C alpha, and C beta 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((PPr3)-Pr-i)(2)(H)(C=CH), 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((PPr3)-Pr-i)(2)(HC=CR), 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.