The QTAIM Approach to Chemical Bonding Between Transition Metals and Carbocyclic Rings: A Combined Experimental and Theoretical Study of (η5-C5H5)Mn(CO)3, (η6-C6H6)Cr(CO)3, and (E)-{(η5-C5H4)CF=CF(η5-C5H4)}(η5-C5H5)2Fe2

Experimental charge densities for (C5H5)Mn(CO)(3) (2), (eta(6)-C6H6)Cr(CO)(3) (3), and (E)-{(eta(5)-C5H4)-CF=CF(eta(5)-C5H4)}(eta(5)-C5H5)(2)Fe-2 (4) have been obtained by multipole refinement of high-resolution X-ray diffraction data at 100 K. The resultant densities were analyzed using the quantum theory of atoms in molecules (QTAIM). The electronic structures of these and related pi-hydrocarbyl complexes have also been studied by ab initio, density functional theory calculations, and a generally good agreement between theory and experiment with respect to the topological parameters was observed. The topological parameters indicate significant metal-ring covalency. A consistent area of disagreement concerns the topology of the metal-ring interactions. It is shown that because of the shared-shell bonding between the metal and the ring carbons, an annulus of very flat density p and very small del rho is formed, which leads to topologically unstable structures close to catastrophe points. This in turn leads to unpredictable numbers of metal-C bond paths for ring sizes greater than four and fewer M-C bond paths than expected on the basis of the formal hapticity. This topological instability is a general feature of metal-pi-hydrocarbyl interactions and means that a localized approach based on individual M-C-ring bond paths does not provide a definitive picture of the chemical bonding in these systems. However, other QTAIM indicators, such as the virial paths, the delocalization indices, and the source function, clearly demonstrate that for the n-hapto (eta(n)-CnHn)M unit, there is generally a very similar level of chemical bonding for all M-C-ring interactions, as expected on the basis of chemical experience.