Contour Plot of the B3LYP/LANL2DZ electron density
in the Cipso-Hipso-Rh plane
[From: A. Vigalok, O. Uzan, L. J. W. Shimon, Y. Ben-David, J. M. L. Martin, and D. Milstein,
" Formation of eta-2 C-H agostic rhodium arene complexes and their relevance to electrophilic bond activation",
J. Am. Chem. Soc. 120, 12539-12544 (1998).]
The picture of the electron density between the ipso-C-H bond and the Rh atom is strongly suggestive of an agostic interaction. Additional evidence for an eta-2 agostic structure, rather than an arenium (Wheland) character, include the Löwdin bond orders involving {Rh, Cipso and Hipso}, the weak alternation of the ring bond lengths and the computed bond orders in the ring, the NPA charges which reveal that most of the positive charge is concentrated on the metal and its ligands rather than on the ring, and the small deviation from planarity of the ring.
