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

TLDR: The QTAIM indicators clearly demonstrate that for the n-hapto (eta(n)-C(n)H(n))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.

Abstract: Experimental charge densities for (C5H5)Mn(CO)3 (2), (η6-C6H6)Cr(CO)3 (3), and (E)-{(η5-C5H4)CF═CF(η5-C5H4)}(η5-C5H5)2Fe2 (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 π-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 ρ and very small ▽ρ is formed, which leads to topologically unstable structures close to catastrophe points. This in turn leads to unpredictable numbers of m...