MECHANISM OF THE ACETYLENE-VINYLIDENE REARRANGEMENT IN THE COORDINATION SPHERE OF A TRANSITION-METAL

Quantum mechanical calculations at the CCSD(T) level of theory using B P86-optimized geometries indicate that the high-valent d(0) tungsten a cetylene complex [F2W(HCCH)] (1) is 10.4 kcal/mol lower in energy than the isomeric vinylidene complex [F4W(CCH2)] (2). Two energetically hi gh-lying reaction pathways are calculated for the tautomerization reac tion 1 --> 2. The direct 1,2-hydrogen migration has a barrier of 84.8 kcal/mol and proceeds via the transition state TS1, which has a nonpla nar C2H2 moiety. TS1 resembles the transition states for the rearrange ment of the free C2H2 species in the triplet state and as an anion. Th e alternative rearrangement involves the alkynyl(hydrido) complex 3 as an intermediate, which is 50.5 kcal/mol higher in energy than 1. The rate-determining step of the two-step process 1 --> 3 --> 2 is the 1,3 -hydrogen migration 3 --> 2, which has an energy barrier of 85.5 kcal/ mol with respect to 1. The high barriers for the two alternative pathw ays of the tautomerization reaction 1 --> 2 make it unlikely that they play a role in the acetylene polymerization reaction, which is cataly zed by high-valent transition-metal compounds. The analysis of the bon ding situation using the NBO and CDA methods show that 1 and 2 should not be considered as acetylene and vinylidene complexes but rather as metallacyclopropene and metallaallene, respectively.