The 18-Electron Rule for Transition Metal Bonding

Usha Please do not forget that the first ones to apply the technique to solve the structures of hemoglobin and myoglobin were Max Perutz and John Kendrew. They also got the Nobel Prize, and they are also mentioned in The Double Helix. Racheli And let s not forget Watson and Crick, who used the X-ray information generated by Rosalind Franklin to determine the structure of DNA. What a great technique  

Sason Thank you, Usha and Racheli, for these comments. 

Transition metals form fascinating molecules because their valence shell can accommodate more electrons than that of main group elements can, and therefore they bind to ligands that possess many available electrons like benzene or to many different ligands having available lone pairs. 

SCHEME 9.1 (a) The periods of transition metals and their family numbers (the first two atoms in each period are main group elements), (b) Some neutral transition metals and transition metal cations with their valence electron counts given in parentheses, (c) The transition metal (TM) is bonded to a ligand (L), which is a modular fragment that consists of a molecule or an atom. 

Since the number of electrons required to achieve Nirvana is so large for transition metal species, the stability differences of other electron counts are not too forbidding. Consequently, the 18e rule is softer than the octet rule, and we may expect to find relatively persistent radical complexes with 17 electrons, and complexes with 16e or even less. These cases are in fact extremely interesting because the electron-deficient complexes can serve as catalysts that activate other molecules. Despite all these qualifications, the Law of Nirvana for transition metal compounds is a very useful guide for constructing transition metal complexes and for considering their reactivity (propensity to react) and properties. Let us see how the rule is applied along with the click bond method. 

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