Are there missing bond paths in Trimethylenemethane-Iron-tri-carbonyl [(CO)3Fe-(C4H6)] complex?

TL;DR

This paper challenges the notion that bond paths are necessary indicators of chemical bonds in the (CO)3Fe-TMM complex, demonstrating that bond paths can appear in non-equilibrium geometries and clarifying the relationship between QTAIM analysis and traditional chemical concepts.

Contribution

It clarifies the flawed assumption that bond paths are equivalent to chemical bonds and shows that bond paths can exist in accessible non-equilibrium geometries, refining the interpretation of QTAIM results.

Findings

Bond paths can appear in non-equilibrium geometries accessible at room temperature.

QTAIM analysis supports the presence of Fe-C bonds in the complex.

No strict one-to-one correspondence between bond paths and chemical bonds.

Abstract

In a recent paper [J. Organomet. Chem. (2013) doi: 10.1016 /j.jorganchem. 2013.03.047] analyzing the bonding mode of Trimethylenemethane (TMM) with some metal carbonyls, Mousavi and Frenking have declared the absence of bond paths, the so-called missed bond paths, between metal atoms and terminal carbon atoms in several complexes. In this communication, it is first demonstrated that the presupposition of the equivalence of a bond path and a chemical bond within the context of the quantum theory of atoms in molecules (QTAIM) is superficial and basically flawed which is not only against with the recent strict declaration on the contrary [R.F.W. Bader, J. Phys. Chem. A 113 (2009) 10396], but also in odd with the foundations of the QTAIM. Then, it is demonstrated that the so-called missed bond paths indeed appear in molecular graphs of some non-equilibrium geometries that are energetically quite accessible at room temperatures. To emphasize on the importance of this observation the term passionate neighbors is coined referring to the atomic basins that do not share an inter-atomic surface at the equilibrium geometry but are neighbors, share an inter-atomic surface, at non-equilibrium geometries accessible by nuclear vibrations. Using the delocalization index as well as other evidence from previous literature it is demonstrated that the QTAIM analysis is indeed compatible with the presence of chemical bonds between iron metal and terminal carbons in (CO)3Fe-TMM complex. Finally, some general discussions are done to unravel the delicate relationship between the QTAIM proposed concepts, e.g. bond paths and molecular graphs, and some orthodox concepts of chemistry, e.g. chemical bonds and chemical structures, emphasizing that there is no one-to-one relationship between these two categorizes.