Deformation density and energy decomposition to describe interactions between (η5-C5H5)M and highly reactive molecules C4H4 and (C3H3)-.

Using DFT calculations, an energy decomposition analysis (EDA) combined with natural orbitals for chemical valence (NOCV), EDA-NOCV approach was used to describe the nature of the interaction between η5-cyclopentadienyl metal complexes (η5-C5H5)M, with M=Co, Rh, and cyclobutadiene (Cb) and cyclopropenyl anion (C3H3)- molecules, which are highly reactive molecules in their free state. EDA-NOCV draws a covalent picture for these interactions. With this interpretation of interactions, the character of aromaticity could be the result of the delocalization of six electrons in π orbitals of the (η5-C5H5)M fragment and Cb/C3H3(-1) ligand. This description of the bonding interaction might also justify the experimental observation that, in complexes of CpM-Cb (M=Co, Rh), the viability of the Friedel-Crafts acylation and other electrophilic substitutions on the four-membered ring is greater than that of the five-membered ring.