Shorter Nonbonded Than Bonded Contacts or Nonclassical Metal-to-Saturated Carbon Atom Interactions?

A Rhodium Complex with One Rh⋅⋅⋅CC and One Rh⋅⋅⋅HC Agostic Bond

“…Indeed recent AIM analysis of the titanium complex A, [7] which contains a short Ti···CC contact in the solid state, indicates that there is no agostic interaction between the Ti and the C À C single bond. This result was contrary to that from previous DFT calculations, [5] for which a natural orbital analysis was used as evidence for an agostic interaction, and highlights the power of methods, such as AIM, which use criteria solely based on a topological analysis of the electron distribution to characterize bonding interactions. AIM analysis of 1 (see Figure 2 and Supporting Information) shows the expected (3, À1) bond critical points (BCPs) between C25ÀC21 (BCP 1), RhÀC15 (BCP 14), RhÀC11 (BCP 15), and furthermore, the manifestation of an agostic Rh···CC interaction is observed through the presence of (3, À1) BCPs between RhÀC25 (BCP 8) and RhÀC21 (BCP 9).…”

“…[2] Complexes showing such M···CC interactions are extremely rare, and are limited to intramolecular (agostic) interactions that have been reported for complexes A-D; [3][4][5][6] although in one case (A) the bonding mode has been queried. [7] The lack of well-characterized examples of such species is in contrast with M···HC agostic complexes that are well established; [2,[8][9][10] and further examples of genuine, fully-characterized examples of M···C À C s interactions are of clear importance regarding the fundamental study of CÀC activation processes.…”

C–C σ complexes of rhodium

“…A transition metal complex with a C-C⅐⅐⅐M agostic bond characterized both crystallographically and in solution was reported by Ernst and colleagues (30,31) (Scheme 2, complex II). On the basis of very short Ti⅐⅐⅐C distances to saturated carbon atoms, lower than expected 13 C- 13 C coupling constants, and a theoretical natural bond order (NBO) analysis, agostic C-C⅐⅐⅐Ti interactions were proposed, although the presence of these have been questioned since (32).…”

CC versus CH Activation and versus Agostic CC Interaction Controlled by Electron Density at the Metal Center

“…While agostic interactions of CÀH bonds, [14] particularly those stabilizing cationic metal centers, [15] are well documented, examples of CÀC agostic interactions are extremely rare. [8,16] CÀH agostic metal complexes are normally viewed as intermediates in CÀH oxidative addition. [14] Since, as we have shown, C À H and C À C bond activation processes by Rh I have similar electronic requirements, [5] complex 7 can be viewed as a 'frozen' intermediate toward CÀC bond activation.…”