Abstract:
The formation of metallacyclobutadienes (MCBs
from chloro-ligated alkylidyne complexes of group 4, 5, and 6 transition metals (MCln(C3H3)) has been studied at the BP86/
def2-TZVPP level. All the MCBs showed M−Cβ distances (∼2.1 Å) very close to M−Cα distances (1.8−2.0 Å), suggesting a
bonding interaction between the metal and the β-carbon (1,3-MC bond). Energy decomposition analysis using C2v symmetric
structures revealed that a b2 orbital composed of mainly metal dπ and Cβ pπ overlap and an agostic a1 orbital contributed to the orbital interaction of the 1,3-MC bond. The bond order of the 1,3-
MC bond is a minimum of 0.26 for M = Cr and a maximum of 0.43 for M = Ta. Further, all the MCBs showed a characteristic δ
orbital interaction through an a2 orbital, which contributed to the double-bond character of M−Cα bonds (bond order 1.27−
1.44). Although the formation of b2 and a2 orbitals increased the M−C interactions, they significantly reduced the π interactions
within the C3H3 fragment (C−C bond order 1.09−1.18). 1,3-MC bonding suggested a planar tetracoordinate configuration for
Cβ, as the Cα−Cβ bonds possessed largely formal Csp2−Csp2 single-bond character. Electron density analysis showed a
“catastrophic” character of the 1,3-MC bond. In groups 4 and 5, MCBs were more stable than the isomeric η3-structures
(metallatetrahedranes). A mechanistic study on the reaction between acetylene and alkylidyne complex MClnCH showed that a
nearly barrierless and exothermic pathway exists for MCB formation (exothermic value 75−102 kcal/mol for groups 4 and 5; 6−
27 kcal/mol for group 6). The rich metathesis chemistry associated with Mo and W is attributed mainly to the moderate
activation energy required for the alkyne disproportionation step of metathesis. A mechanistic possibility other than Katz's is also proposed for alkyne metathesis that showed that the 1,3-MC bonded MCB complex can act as a metathesis catalyst by reacting
with alkyne to form a bicyclic intermediate and subsequently disproportionating to yield the alkyne and the MCB. For this
mechanism to be effective, rearrangement of the bicyclic intermediate to a more stable cyclopentadienyl complex has to be prevented.
