Dissociative ligand Kinetics

Activation volumes for aquation of Schiff base complexes [Fe(C5H4NCH=NHR)3]2+ (R = Me, Et, nPr, nBu) are between +11 and +14 cm3 mol-1 (107), and thus within the range established earlier (108) for (substituted) tris-l,10-phenanthroline-iron(II) complexes, viz. +11 to +22 cm3 mol-1. These positive values are consistent with dissociative activation. Kinetic studies of the reaction of a CH2S(CH2)3SCH2 -linked bis(terpy) ligand (L6) with [Fe(terpy)2]2+ showed a very slow two-step process. The suggested mechanism consisted of slow loss of one terpy, rapid formation of [Fe(terpy)(L6)], and finally slow displacement of the second terpy as the partially-bonded L6 becomes hexadentate (109). 

A comparison with cross-linker 4a proves the underlying dynamics are controlled by metal-ligand dissociation. Ligand exchange kinetics for 4a are substantially faster than for 4b but the association thermodynamics are very similar, and the effect of those kinetics is dramatic. At 5% cross-linker, the dynamic viscosity of lOOmgmL 4a-PVP is only 6.7 Pa s, a factor of 80 less than that of the isostmctural network 4b PVP. Although the association constants are not identical, the effect of the thermodynamics would be to increase the viscosity of 4a PVP relative to 4b PVP, the opposite direction of that observed. The kinetics dominate even the extent of cross-linking 5% 4a PVP is less viscous by a factor of 5 than is 2% 4b PVP. 

In the kinetic trans effect, the departure of the trans ligand is probably aided by a stabilization of the transition state via the same mechanisms operative for the trans influence.114 Both associative and dissociative ligand substitution processes seem to be facilitated in this way.117... 

To confirm fhat a dissociative ligand exchange mechanism is in operation, the kinetics of fhe rapid reaction of fhese complexes wifh efhyl vinyl ether was studied by NMR and UV/VIS spectroscopies. The results were in complete agreement with a dissociative hgand exchange mechanism for aU complexes in this series, with the possible exception of fhe diiodo derivatives [101]. 

Fig. 9.3 Kinetics of ligand dissociation for LdisPBP2. Data were obtained by equilibrating isolated binding-protein-ligand complex (BP.L) in fresh buffer and separating protein-bound ligand from dissociated ligand at various time points (see text). Ligands tested were +) disparlure 7R, 8S) c/a -7,8-epoxy-2-methyloctadecane) and the enantiomer, (-) disparlure. The dissociation is exponential and biphasic, with a rapid phase (koff i) and a slow phase (koff 2)- The association rates given were calculated from the measured dissociation constants, Ka, and the rapid off rate (koff 1). The half-life for each rate dissociation is given as ti/2 (calculated from the initial value measured and the individual exponential decay).

General Features of the Kinetics of Dissociative Ligand Substitution... 

Ligand substitution on the d , 18-electron, tetrahedral complex Ni(CO) is a classic example of a dissociative ligand substitution. - Kinetic studies show that the rate is first order in nickel and independent of [L]. These data are consistent with rate-determining dissociation of CO (Equation 5.21). 

Studies have been conducted on the factors that accelerate the rate of tiie sequential [2+2] and retro-[2+2] reactions that constitute the olefin metathesis process. The ruthenium system of Grubbs has been amenable to kinetic analysis. The starting complex is a 16-electron species, but the 14-electron species formed by ligand dissociation reacts with olefin in the [2+2] process. Therefore, faster reactions will occur with systems in which dissociation of an ancillary ligand is favored and in which ttie remaining ancillary ligand on the 14-electron intermediate promotes the [2+2] addition, relative to reassociation of the dissociated ligand. 

The kinetics of dissociation and of formation of the pentacyanofer-rate(II) complexes of cysteine, penicillamine, glutathione, and 2-mercapto-ethylamine have been established pyrazine was used as incoming ligand in the dissociation studies. The pH dependence of both dissociation and formation can be ascribed to protonation equilibria of the ligands. Kinetics both of dissociation and of formation have also been studied for the 2-methyl pyrazine complex [Fe(CN)s(2Mepz)] , 2. Dissociation was... 

In general, the kinetics of most allosteric modulators have been shown to be faster than the kinetics of binding of the tracer ligand. This is an initial assumption for this experimental approach. Under these circumstances, the rate of dissociation of the tracer ligand (pA t) n the presence of the allosteric ligand is given by [11, 12]... 

It will not have escaped the reader s attention that the kinetically inert complexes are those of (chromium(iii)) or low-spin d (cobalt(iii), rhodium(iii) or iridium(iii)). Attempts to rationalize this have been made in terms of ligand-field effects, as we now discuss. Note, however, that remarkably little is known about the nature of the transition state for most substitution reactions. Fortunately, the outcome of the approach we summarize is unchanged whether the mechanism is associative or dissociative. 

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