INSERTION, H/D EXCHANGE, AND SIGMA-BOND METATHESIS REACTIONS OF ACETYLENE WITH CL2SCH

Correlated ab initio theoretical calculations at the valence double-ζ plus polarization level are used to study reaction paths for the reaction of acetylene with Cl2ScH. The paths studied are the classic insertion pathway and two σ-bond metathesis pathways, one resulting in H/D exchange and the other resulting in formation of a scandium acetylide. The insertion process is calculated to have a barrier of 6.3 kcal/mol with respect to the complexed acetylene (–9.1 kcal/mol with respect to free acetylene) and to be 36.5 kcal/mol exothermic with respect to free acetylene. The direct H/D exchange reaction is calculated to have a barrier of 13.7 kcal/mol with respect to free acetylene. The acetylide-forming reaction is calculated to have a barrier of 6.2 kcal/mol with respect to free acetylene and to be 15.2 kcal/mol exothermic with respect to free acetylene. Given the reversibility of this reaction, H/D exchange could occur through sequential formation and reduction of the acetylide complex. The insertion pathway is the only one calculated to proceed through a metal acetylene complex. The two σ-bond metathesis pathways are each calculated to proceed through a direct interaction with the Sc–H bond. In addition, the overall reaction energetics are calculated for the insertion and acetylide formation reactions of Cl2ScCH3 with acetylene. In contrast to the hydride reactions, the insertion and acetylide formation reactions for the methyl complex are found to be equivalently exothermic. The insertion reaction is calculated to be 39.1 kcal/mol exothermic and the acetylide formation reaction is calculated to be 40.9 kcal/mol exothermic. © 1990, American Chemical Society. All rights reserved.