The behavior of an acetylene molecule in the coordination sphere of transition metals has been probed by the reactions of RuX2(PPh3)3 (X = C1, Br) with tert-butylacetylene to give vinylidene complexes of the formula RuXz(PPhl)2(C=CHtBu). IR and NMR data have indicated that the initial product of this reaction is a mixture of two complexes each of which has a vinylidene unit and nonequivalent cis-bis(phosphine) ligands. In solution, these kinetic products gradually isomerize to the final trans-bis(phosphine) complex. The structure of this five-coordinated and thermodynamically stable complex (X = Br) was determined by X-ray crystallographic analysis to have a quasi trigonal-bipyramidal conformation with the two phosphines occupying axial positions. The potential surface for the transformation of coordinated acetylene to vinylidene was calculated by the ab initio molecular orbital method. The primary process was concluded to be a slippage of the $-CC coordinated alkyne to the q2-CH coordinated complex via a transition state with an VI-acetylene and a side-on acetylene. The 92-CH complex undergoes 1,Zhydrogen migration within the acetylene unit, whose transition state is the highest point of the whole process, giving finally the thermodynamically metastable vinylidene complex with a cis-bis(phosphine). The isomeric vinylidene ruthenium(I1) complex with trans-bis(phosphine) has been calculated to be the final product and thermodynamically most stable form of this reaction system. The role of the metal in the present rearrangement is discussed on the basis of the localized molecular orbital analysis of the key intermediates.The electronic character and reactivity of organic compounds change, often dramatically, when they interact with transition metals. Since this phenomenon is strongly related to many catalyzed processes, clarification or understanding of this basic problem is an important topic for organometallic as well as theoretical chemists. The relative stability of acetylene and its isomeric vinylidene form provides a good and simple example: free vinylidene, :C=CH2, has been the subject of a number of theoretical and physicochemical studies1 and is proved nowadays to be ca. 44-47 kcal/mol less stable than acetylene, H W H . 1 -3 In contrast, 1 -alkyne to vinylidene tautomerization in the coordination sphere of a transition metal has proved to be a useful entry into vinylidene complexes, apparently with vinylidene being the more stable form in many transition-metal complexes.4.5 Such (2) (a) Ervin, K. M.; Ho, J.; Lineberger, W. C. J. Chem. Phys. 1989,91, 5974. (b) Ervin, K. M.; Gronert, S.; Barlow, S. E.; Gilles, M. K.; Harrison, A. G.; Bierbaum, V. M.; DePuy, C. H.; Lineberger, W. C.; Ellison, G. B. Pirio, N.;Touchard,D.;Toupet,L.; Dixneuf, P. H. J. Chem. SOC., Chem. Commun. 1993, 163. (6) Bianchini, C.; Peruuini, M.; Zanobini, F.; Frcdiani, P.; Albiati, A.