Exchange Processes Involving Dissociated Species

The first and most extensively studied alkyl exchange process is that for trimethylaluminum dimer (6, 25, 27, 37, 41, 51, 55). The mechanism favored for this exchange process is the dissociation as indicated in Eq. 

Further studies on this system, and studies on the exchange between MegAl and MegGa in which random recombination and exchange occurs as shown in Eq. (2), provide convincing evidence that dissociation occurs as the rate-determining step (6, 25, 27, 51). 

Main Group Metal-Transition Metal Bonded Compounds 

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The existence of a simple exchange process implies that the adsorbed intermediate, whether it is of the dissociative type, C Hm i, or the associative type, CnHm+i, must be a comparatively stable entity on the surface of the catalyst. The species must have little chance of undergoing further reaction involving the introduction of a second deuterium atom into the molecule during its lifetime on the surface, because this would give rise to the formation of products with more than one deuterium atom and the exchange would cease to be a simple exchange. 

The ligand-exchange process has been applied as a mobile-phase-additive technique for enantioseparations. It involves the formation of a dissociable diastereoisomeric complex between a homochiral additive and a heterochiral solute about a central metal ion (Fig. 28). The mobile phase contains both the homochiral ligand and the metal ion as additive components. These species probably exist as the fully complexed species with at least two molecules of the homochiral... 

It appears that ligand exchange in pentacoordinate silicon species involves many pathways. Beside a possible intramolecular isomerization arising from Berry pseudorotation of a trigonal bipyramidal silane, several other processes may take place. The multiplicity of mechanisms comes from the possibility for pentacoordinate silanes to react by associative or dissociative pathways. Isomerization and exchange processes can involve a reduction of the coordination number to four or an expansion to six. 

A similar dissociation process is displayed in another homoleptic complex [Cu2(dppf-P,F )2(f "dppl)][ F4]2 1 [30]. A single broad resonance at 300 K is resolved into an AB2 multiplet at 233 K. An exchange mechanism involving limited dissociation of the dimeric complex to two monomeric species via bridge splitting has been suggested. The monomers could then undergo facile mutual interconversion. 

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