Transition-metal catalysis 18-electron rule

Indeed, the application of transition-metal catalysis in organic synthesis is built around many such formalisms. In addition to the formal oxidation state, these include coordinative unsaturation, coordination number, and coordination geometry, hydride formalism and the 18-electron and 16-18-electron rules, also referred to as electron bookkeeping. 

Hydrogenation of olefins is a good example for demonstrating the roles of the surface atoms in catalysis. The orbital symmetry rule in chemical reactions suggests that the highest occupied molecular orbital (HOMO) of one reaction partner and the lowest unoccupied molecular orbital (LUMO) of the other should meet the symmetry requirements. In this respect, a concerted addition of an H2 molecule to the double bond of an olefin, that is, a molecular addition reaction, is a forbidden process. Adsorption of olefin on transition metal surfaces undoubtedly changes the population of electrons in the HOMO (7tu) and the LUMO (re ) as shown schematically in Fig. 1. In spite of such perturbation of the electron densities of the HOMO and the... 

A convenient tool for understanding organometallic catalysis mechanisms is the 16 and 18 electron rule, whereby valence electrons are counted in order to ascertain whether or not complexes are coordinatively unsaturated. An 18 electron complex possesses an inert gas configuration and must first undergo dissociation to achieve the coordinative unsaturation necessary for reactivity. The number of valence electrons for various transition metals is readily seen from their position in the periodic table (e.g., Mn has 7, Fe has 8). The number counted for a particular metal is independent of its oxidation state. 

The design and synthesis of new chiral ligands that coordinate to transition metals is a vital aspect of asymmetric catalysis research. A compilation of structural and electronic attributes that provide a blueprint for the generation of potent asymmetric catalysts is not available. The factors that are required for a catalyst to display a combination of high efficiency and high enantioseleetivity remain nebulous and are often reaction specific. Rational or de novo design of effective chiral ligands continues as the exception rather than the rule. 

Olefin insertions into transition metal-carbon and transition metal-hydrogen bonds are fundamental reactions in homogeneous catalysis. With unsymmetrically substituted olefins, a remarkable regioselectivity is frequently observed, whereby the orientation of the olefin depends on the metal, the ligands, and the olefin itself. Empirical rules of regioselectivity are given, and interpreted on the base of the electronic structure of the reaction partners. 

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