New Keys for Old Keywords. Case Studies within the Updated Paradigms of the Hybridization and Aromaticity

Summary.

The recovery of the basic qualitative concepts of the chemical language into the quantum treatment is a rather challenging problem because the desirable simplicity is easily lost in the black box of technical mechanisms of computation, e.g. among the ingredients of post-orbital methods of correlation effects. We present here several clues for bringing the hybridization and aromaticity concepts in a nontrivial and non-redundant conjunction with both quantum chemical quantities and the chemists’ tools of intuition. A rather inedited structural correlation that works as an experimental proof of the hybridization is discussed. The main idea is to exploit the intermediate symmetries, where the hybridization is no longer tautological because of the symmetry. For instance in a MA₂B₂ tetrahedron, if the hybridization not existed as real engine of the stereochemistry, the AMA and BMB angles should be arbitrarily independent. In turn, if the hybridization acts, there is a certain trigonometric relationship between the two angles. The existence of this correlation is noticed and discussed in prototypic series. A new understanding and analysis of strain in cycles and clusters (with particular emphasis on organometallic structures) is constructed as continuation of discussed models. The problem of aromaticity in chelates, rings and clusters is approached by means of specific models. The careful attention devoted to the structural details rewards the theory with an extended understanding of various bonding regimes and at the same time enriches the experimentalists’ perspectives beyond the geometric description and taxonomic systematization of X-ray molecular structure and crystal packing data.