Associative substitutions square-planar, 16-electron

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

Another impetus to mechanistic studies arose from the recognition that compounds of these d ions were those on the energy borderline between stable 18-electron and 16-electron molecules (1) and that the reactions involving transitions between these states are those encountered in catalytic cycles based on these compounds. Nucleophilic ligand substitution, involving association of an entering nucleophile with a square-planar compound, is just one example of the easy 16- 18- 16... 

Organometallic dissociative substitutions, like their associative counterparts for square-planar complexes, have been investigated in detail primarily for octahedral transition metal carbonyl complexes." They involve the initial rate-limiting separation of one ligand (X) from the substrate to give an electronically unsaturated (16-electron) distorted tbp intermediate, followed by rapid binding of the nucleophile (Y) to give the substituted product (Equation (3), where kx, > k ). 

Eighteen-electron complexes react more slowly than similar complexes with either more or less electrons. The eighteen-electron rule explains why some reactions are associative and others dissociative. Complexes in which the metal has sixteen or less valence electrons tend to react by associative mechanisms, since the metal has vacant low-energy orbitals which can be used to form a bond with the entering ligand. This orbital can accept an electron pair from an entering ligand and provide a path for associative substitution. Substitution reactions in square planar complexes illustrate this point, reaction (40). 

In Equation (19.8), a similar mechanism occurs for the 18-electron [(> 6 6H6) Mo(CO)3] molecule, with the coordinated benzene ring slipping to an i/ -linkage to allow room for the initial PR3, where the intermediate [(>/4-C6H6)Mo(CO)3(PR3)] has been isolated and characterized in the solid state. Sometimes an associative mechanism will also involve a second term in its kinetics rate law involving a competition between addition of Y and addition of solvent, as was observed in Chapter 17 for square planar substitution reactions. In this case, the solvent term will follow pseudo-first-order kinetics. 

Kinetically labile and inert complexes Dissociation, association and interchange Activation parameters Substitution in square planar complexes Substitution in octahedral complexes Racemization of octahedral complexes Electron-transfer processes... 

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