Ligand dissociation displacement reactions

Displacement Equilibria. Species in solution are generally in formation—dissociation equiUbrium, and displacement reactions of any given metal or ligand by another are possible. Thus,... 

Three features of chelation chemistry are fundamental to most of the appHcations of the chelating agents. The first and probably the most extensively used feature is the control of free metal ion concentration by means of the binding—dissociation equiUbria. The second, often called the preparative feature, is that in which the special properties of the chelate itself provide the basis of the appHcation. The third feature comprises displacement reactions metal by other metal ions, chelant by chelant, and chelant by other ligands or ions. An appHcation may be termed defensive if an undesirable property in a process or product is mitigated, or aggressive if a new and beneficial property is induced. 

Randall and Alberly (136) have studied the binding of various ligands to aquocobalamin using stopped flow techniques. This work suffers from the fact that it is not clear if the added ligand is displacing coordinated water or coordinated benzimidazole. One might be led to believe that the reaction studied in this work is in fact displacement of benzimidazole because the kinetics are at least inconsistent with a mechanism in which unimolecular dissociation of coordinated water is the rate limiting step. 

The argument against Scheme 1 is a negative one. Its basis derives from extensive studies carried out on the ligand replacement reactions of oxorhenium complexes of the family MeReO(dithiolate)L (20,34-37). Those studies (Sections V.B and V.C) show that all such processes studied to date proceed by direct displacement reactions without a recognizable intermediate from unassisted Re-L dissociation. (Indeed, in an early work, a dissociative step was written, but that formulation has since been revised see Section V.D.)... 

Because the interaction between biotin and streptavidin is strong (fCa = 0.6 X 10 L mol ) with a relatively fast association rate (k+i = 2.4 x 10 L mol s ) and slow dissociation rate (k i = 0.4 x 10 s ), the reaction times are fast, i.e. 10-20 s. Furthermore, the addition of reporter ligand is performed only after the analyte protein reaction has taken place in coil I, avoiding a displacement reaction that would substantially increase the overall reaction time. 

While the rate of dissociation of Fe(phen)i" is not sensibly affected by hydrogen ions, hydroxyl (200) as well as cyanide and azide ions 201) greatly accelerate the rate of displacement of the diimine ligand. For the reaction with OH, the activation energy for the catalized path is lowered by ca. 6 kcal/mole 202). The preferred mechanism 201) consists in the replacement of water molecules in one of the pockets between the hgands, by the nucleophilic ion. Interaction between the i r-orbitals of the latter and the antibonding metal i-orbitals is believed to destabilize the metal-diimine bonds. 

Reactions involving the gain or loss of a ligand These reactions deal with the addition or subtraction of a ligand to or from the metal center and include ligand dissociation and substitution, oxidative addition, reductive elimination, and nucleophilic displacement. 

In these and related reactions, the ether and alcohol ligands dissociate readily. The coordinated THF dissociates from the zirconium methyl complex to allow olefin polymerization, albeit more slowly than in the absence of THF. The ether dissociates from palladium and nickel to allow olefin to bind to the cationic palladium and nickel species, and alcohol and water are easily displaced from related Pt(II) complexesby hydrocarbons prior to C-H activation processes. Likewise, the THF and phosphine oxide ligands reversibly dissociate from the zirconium knido complex (Equation 2.18) prior to [2+2] additions with alkynes. - ... 

In terms of ligand displacement reactions, exposure to ancillary bidentate ligands is more likely to lead to dissociation of the carbene than exposure to monodentate ligands, thus these species should be avoided as additives in catalytic reactions employing NHC complexes. ° ... 

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