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 BP86-optimized geometries indicate that the high-valent d⁰ tungsten acetylene complex [F₂W(HCCH)] (1) is 10.4 kcal/mol lower in energy than the isomeric vinylidene complex [F₄W(CCH₂)] (2). Two energetically high-lying reaction pathways are calculated for the tautomerization reaction 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 nonplanar C₂H₂ moiety. TS1 resembles the transition states for the rearrangement of the free C₂H₂ species in the triplet state and as an anion. The 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 pathways of the tautomerization reaction 1 → 2 make it unlikely that they play a role in the acetylene polymerization reaction, which is catalyzed by high-valent transition-metal compounds. The analysis of the bonding 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.