Transition-metal-coordinated alkynes protonation

Orbital interaction diagram and EHT calculations show that the 1,2 intramolecular shift of hydrogen is symmetry disfavored [10]. In presence of a transition metal fragment to which the alkyne coordinates, the activation energy is considerably lower. This has been attributed to the tendency of H to shift as a proton rather than as a hydride. 

Several routes that lead to formation of alkenylidene type complexes are known [130]. A route involving the side-on coordination of an alkyne to a low valent transition metal complex (10) may lead to a hydrido alkynyl complex (12) through the activation of the C-H bond in the coordinated alkyne (11) as shown in Scheme 7.23. Proton 1,3-migration in the alkynyl-hydrido complex to the alkynyl carbon in 12, or the direct 1,2-transfer of the proton in 11 gives the alkenylidene complex 13 [131]. It is also possible that the alkenylidene complex is produced by protonation of the alkynyl complex. 

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