The lego way: curve crossing diagrams as general models in physical organic chemistry

The whole may be understood by its reconstruction from building block components: this is a LEG0 principle. A curve crossing diagram is a model for the reconstruction of energy profiles from Valence Bond (VB) building blocks, and may be considered as the quantum chemical analog of Robinson's curved arrow mnemonic. There are two archetypal diagrams which describe chemical transformations: (a) a twc-curve diagram in which the spine is the crossing of the two spin-paired covalent forms of the bonds which interchange during the transformation, and (b) a three-curve diagram in which the principal curves (in (a)) are horizontally crossed by an intermediate curve. The two archetypal diagrams constitute a unifying framework for conceptualization and prediction of reactivity patterns, reaction mechanisms and structural trends related to "hypervalency" and "hypercoordination". Examples are discussed.