Electron density plot for the open-arm intermediate for C-H activation in a Rh[PCO] pincer complex.
[From: B. Rybtchinski, S. Oevers, A. Vigalok, M. Montag, H. Rozenberg, J. M. L. Martin, and D. Milstein,
" Comparison of Steric and Electronic Requirements for C-C and C-H Bond Activation. Chelating vs Nonchelating Case",
J. Am. Chem. Soc. 123, 9064-9077 (2001).]
[Visualization by S. Oevers using SPARTAN 5.1.] While the geometry, and particularly the metal-carbon distances, would suggest an eta-4 metal-ring interaction, the plotted electron density clearly shows that the interaction is best described as eta-2. If the reaction is carried out in methanol or a similar solvent, coordination of solvent molecules dramatrically lowers the C-H activation barrier and leads to a competition between closed-arm and open-arm C-H activation products. Upon evaporation, a closed-arm C-C activation product is obtained exclusively.
