| Abstract
| - Reaction pathways are identified for Na-, Al-, and Y-induced acetylene (HCCH)−vinylidene (CCH2)rearrangements in the gas phase. Density functional and coupled cluster calculations are performed withbasis set extrapolations. The rearrangement barriers decrease from 44.0 kcal/mol in the metal-free reaction to41.3 (Na), 19.1 (Al), and 16.1 (Y) kcal/mol in the metal-induced reactions. This decrease results from thestrengthening of the M−C bonds (Na−C < Al−C < Y−C) in the M(HCCH) and M(CCH2) complexes.Natural bond orbital analysis reveals the metallacyclopropene and metallaallene character of several of theM(HCCH) and M(CCH2) complexes. In addition, analysis of the transition states provides a detailed pictureof the redistribution of bonding and nonbonding electrons along the reaction pathway. The metal-free andNa-induced rearrangements proceed via 1,2-hydride shifts, whereas the Al- and Y-induced reactions proceedvia 1,2-hydrogen shifts. The latter reactions involve homolytic bond cleavage and formation, with the α andβ electron densities undergoing redistribution in opposing directions.
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