Valence bond diagrams and chemical reactivity

Eighteen years after the first publication, valence bond (VB) diagrams have developed into a unified system of thought on fundamentals of chemical reactivity: barriers and reaction mechanisms. In this review the reader is led into the model, in a manner which enables to build know-how, through a gamut of applications from the elementary problem of bond breaking to more complex mechanisms where barriers and intermediates are involved in stepwise processes. How does a bond undergo heterolysis, and what is electrostatic catalysis by metal ions? What is the rate-enhancing factor in the in situ DNA repair mechanism? When do 'forbidden' reactions become facile, and why do some 'allowed' and highly exothermic reactions possess very large barriers? What is the mechanism of C-H activation by CRO₂Cl₂? How do lanthanide cations and other metal cations activate C-F bonds? How can we derive stereoselection rules for reaction mechanism and transition state stereochemistry, or stereoselection and mechanistic selection rules for the reactivity of ion radicals and for radical reactions? What are the electronic mechanisms by which complex molecules find low-energy pathways for otherwise high-barrier transformations? What is the difference between nucleophilic substitutions on silicon and carbon, or between concerted, nucleophilic and electron transfer pathways in polar cycloaddition? What are the origins of the novel S(RN)2 and S(RN)2(c) reaction mechanisms? What are the origins of reactivity reversals and reactivity zigzags? And what are entangled mechanisms? These are part of the problems which are addressed by use of VB diagrams.