The behavior of an acetylene molecule in the coordination sphere of transition metals has been probed by the reactions of RuX(2)(PPh(3))3 (X = Cl, Br) with tert-butylacetylene to give vinylidene complexes of the formula RuX(2)(PPh(3))(2)(C double bond CH(t)Bu). 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 eta(1)-CC coordinated alkyne to the eta(2)-CH coordinated complex via a transition state with an eta(1)-acetylene and a side-on acetylene. The eta(2)-CH complex undergoes 1,2-hydrogen 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). Th isomeric vinylidene ruthenium(II) 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 thee localized molecular orbital analysis of the key intermediates.