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Roles of π‐Alkyne, Hydride–Alkynyl, and Vinylidene Metal Species in the Conversion of Alkynes into Vinylidene: New Theoretical Insights
Author(s) -
PerézCarreño Enrique,
Paoli Paola,
Ienco Andrea,
Mealli Carlo
Publication year - 1999
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/(sici)1099-0682(199908)1999:8<1315::aid-ejic1315>3.0.co;2-7
Subject(s) - chemistry , acetylide , acetylene , hydride , alkyne , ab initio , computational chemistry , ligand (biochemistry) , crystallography , photochemistry , metal , organic chemistry , catalysis , biochemistry , receptor
The transformation of acetylene into vinylidene, as promoted by the metal fragment [(pp 3 )Co] + [pp 3 = P(CH 2 CH 2 PPh 2 ) 3 ], is unimolecular and features the hydride–acetylide species as an intermediate. The paper describes a detailed ab initio study of the reaction, in particular with regard to the step involving 1,3‐H shift. The best computational results are obtained by mimicking the pp 3 ligand with actual ethylenic chains rather than with single PH 3 molecules. The keypoints along the two‐step reaction path (π‐acetylene, hydride–acetylide, and vinylidene complexes, as well as intermediate transition states) have been optimized for Co I and Rh I derivatives at the MP2 level. For the fragment [(pp 3 )Co] + , the barrier associated with transformation of the hydride–acetylide intermediate to vinylidene (20.6 kcal/mol) is easier to surmount compared to that for reversion to the reactants (28.6 kcal/mol). The situation is reversed for the analogous Rh I system, with the initial π‐acetylene adduct being slightly more stable. Although higher in energy, the hydride–acetylide species is the experimentally detected product of the reaction of acetylene with the fragment [(pp 3 )Rh] + . The salient chemical aspects of the 1,3‐H shift are discussed in terms of perturbation theory arguments. Parallel EHMO calculations, which have provided a relatively good consistency with the ab initio results, allow the proposal of an orbital rationale for the mode of migration of the hydride ligand along the substantially linear Co–C α –C β grouping.