Abstract:
Metallacyclobutane (MCB) intermediates of Grubbs and Schrock olefin metathesis catalysts are wellknown
for their unusually short single bond-like metal to Cβ distance and unusually long CαCβ distances.
From the analysis of structural, bond order, electron density and 13C NMR data of a large variety of MCB
systems, we show that the Cβ of the metallacycle possesses pentacoordinate geometry due to the agostic
type interaction of the metal with the CαCβ bonds. The pentacoordination of Cβ to the metal center is
characterized by a catastrophe ring critical point (RCP) in the quantum theory of atoms-in-molecule
(QTAIM) analysis. Fine tuning of the ligand environment changes the catastrophe point to a fifth bond critical
point (BCP) which is clearly brought out in the case of two ruthenium olefin metathesis systems. Several Ru
and W agostic MCB complexes exhibiting pentacoordinate Cβ as well as their non-agostic isomers have
been reported at the BP86/def2-TZVPP level of DFT. The agostic systems showed a significant bond order
between metal and Cβ (0.17–0.36), single bond-like electron density values at the catastrophe RCP/BCP and
a significantly large difference in 13C NMR chemical shift values between Cα and Cβ atoms
