The electronic and structural characteristics of the reaction interconverting a 1-alkyne complex into a vinylidene via a 1,2 hydrogen shift are examined. In a mononuclear system, initial slippage of the alkyne to an q' geometry is indicated. The subsequent step is shown to be analogous to the isomerization of a methylvinyl cation. We conclude that an alternative route involving a hydrido-acetylide species will be of much higher energy. A concerted shift in binuclear and trinuclear systems is ruled out, based on the loss of a strong bonding interaction between the alkyne and the metallic piece, in the transition state. In trinuclear systems, a mechanism for the isomerization is suggested involving prior oxidative addition of the C-H bond across a metal-metal bond. The metallic piece in this case assists the transformation. The discussion is extended to other reactions featuring hydrogen shifts; these include the intramolecular formation of a binuclear vinylidene from a 1,2-hydrido-acetylide complex, the isomerization of a binuclear p-alkylidyne into a pvinyl geometry and the transfer of a bridging hydrogen onto a capping hydrocarbon fragment in trinuclear cluster complexes.Transition metal elements have the remarkable ability to coordinate organic fragments or molecules. Beside the fascinating world of the structural chemistry thus generated, a crucial consequence of thls ability to bind is essential modification of the organic species' chemical behavior. It is, of course, up to us, the chemists, to understand how these alterations operate. We would like ultimately (and ambitiously) to be able to monitor and tune these changes toward an obvious synthetic purpose.An attractive illustration of these considerations may be found in reaction ( 1 ) in which an acetylene-vinylidene rearrangement has occurred, mediated by, or centered on, a transition metal fragment ML,. The transformation could be alternatively viewed as the isomerization of a transition metal alkyne into a vinylidene complex via a 1,2 H shift. This at first sight puzzling reaction has been mentioned several times in the recent literature, particularly with a mononuclear ML, [l]') and is described as a way to obtain vinylidene complexes [4]. Intriguingly, binuclear frameworks appear to have no propensity for this type of reaction. In fact to the best of our knowledge only one case has been reported, with the mandatory presence of a halogen atom as the migrating group IS]. However, a somewhat related H shift was experimentally demonstrated by Cusey and coworkers; the ') For cases where q2-alkyne was suggested as intermediates, see [2]. For cases where the vinylidene formation from an q2-alkyne was suggested as a mechanistic step, see [3].

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