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 d(0) 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.