Dissociation kinetic chelate effect

Hence, dissociation of tiron occurs in a concerted fashion when the porphyrin is incorporated into the first coordination sphere. Obviously, if proton transfer from H2P has been established, the series of substitutions occurring at MA are rather fast steps dominated by kinetic chelate effects. 

The first dissociation (1) is expected to be slower than a similar dissociation of ammonia, because the ethylenediamine ligand must bend and rotate to move the free amine away from the metal. The reverse reaction associated with the first dissocation is fast. Indeed, the uncoordinated nitrogen is held near the metal by the rest of the ligand, making reattachment more likely. This kinetic chelate effect dramatically reduces aquation reaction rates. 

It is of interest now to consider the kinetics of complex formation (kf) and dissociation (kd) of chelate, macrocyclic, and cryptate complexes and how these are related to the type of ligand stmcture. Table 10 lists kf and kj valnes for chelating ( 1-3), macrocyclic ( 4 and 5), and cryptand ( 6-8) ligands and the appropriate reference compounds. For the chelate effect, the reactions of Ni " with py, bipy, and terpy show very little difference in kf values. On the other hand, the valnes of kj decrease by factors of 2 x 10 and 2 X 10, for bipy and terpy, respectively. A similar effect is seen in a comparison of the reactions of Cn + ion with the thioether macrocycle [14]aneS4 (ane denoting a saturated carbon macrocycle and S4 (he four sulfur donor atoms) and its acyclic... 

In 1959, the coordinated mercaptide ion in the gold(III) complex (4) was found to undergo rapid alkylation with methyl iodide and ethyl bromide (e.g. equation 3).9 The reaction has since been used to great effect particularly in nickel(II) (3-mercaptoamine complexes.10,11 It has been demonstrated by kinetic studies that alkylation occurs without dissociation of the sulfur atom from nickel. The binuclear nickel complex (5) underwent stepwise alkylation with methyl iodide, benzyl bromide and substituted benzyl chlorides in second order reactions (equation 4). Bridging sulfur atoms were unreactive, as would be expected. Relative rate data were consistent with SN2 attack of sulfur at the saturated carbon atoms of the alkyl halide. The mononuclear complex (6) yielded octahedral complexes on alkylation (equation 5), but the reaction was complicated by the independent reversible formation of the trinuclear complex (7). Further reactions of this type have been used to form new chelate rings (see Section 7.4.3.1). 

Similar ion-pair competition has been reported for the reaction of CN" with [Fe(phen)j] + in water where addition of I" inhibits the reaction by formation of a relatively inert iodide ion pair." In this instance there is independent evidence for ion-pair formation with iodide. There is, however, criticism of ion-pair mechanisms although the observed kinetic behaviour is not in accord with the proposed alternative mechanism involving a preequilibrium to form a bis chelate. A recent lengthy paper dealing with reaction of several [Fe(LL)3] + complexes with CN" highlights the importance of other effects in some instances. [FelLL),] " (LL = phen, bipy), in the presence of long chain alkyl sulfonates, are readily extracted into chloroform, and dissociation and racemiza-tion rates arc enhanced via specific interactions. ... 

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