Bond weakening, theory

The suppression of these pathways can result in the isolation of a number of metal-carbon compounds. Since the last volume of these Specialist Periodical Reports a number of transition-metal bicyclo[2,2,lJhept-l-yls (1) have been isolated which are clearly inert to / -elimination, since this would generate a double bond at a bridgehead. Recently it has been suggested that where d-d excited states of the appropriate symmetry exist, bond-weakening distortions are vibronically facilitated and metal-carbon bond homolysis becomes more likely. However, there are a number of exceptions to this theory and the idea has been questioned. ... 

Theories based on cr-effects consider that the more electronegative a ligand the more polarization of the metal occurs, weakening the bond trans to it. This can alternatively be viewed in terms of electronic transmission via a <7-type (p) orbital shared by the two ligands. 

A heterogeneous catalytic reaction begins with the adsorption of the reacting gases on the surface of the catalyst, where intramolecular bonds are broken or weakened. The Appendix explains how this happens on metals in terms of simplified molecular orbital theory. Next, the adsorbed species react on the surface, often in... 

The result is that for minimum activation energy, the bond YZ must be stretched and weakened before the final bond XY is established. Quantitative calculations show that the activation energy evaluated in this manner amounts, at the most, to 7 % of the bond energy YZ. In accordance with this theory, recombinations of atoms of hydrogen, nitrogen, oxygen, chlorine on metal surfaces... 

Strain of various sorts (Huisgen and Ott, 1959) has been suggested as the cause of at least part of the acceleration of rate caused by enzymes. The most popular theory described enzymes as a rack on which the substrate was stretched. Electrostatic or other forces caused strong interactions between the enzyme and two ends of the substrate and pulled or bent it in such a way as to weaken the fissile bond. The strain was postulated to lower the energy required to reach the transition state (Eyring et al., 1954 Lumry, 1959). 

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