Ligand exchange reactions kinetic data

Stability constants are not always the best predictive tool for measuring the ease and the extent of chemical reactions involving complexes nor their stability with time, because their kinetic behavior can often be even more crucial. For example, when ligand exchange reactions of ML (e.g., [FeEDTA]) with other metal ions (e.g., Zn2+ or Ca2+) are ki-netically slow, they do not significantly influence ligand speciation. Another typical example of the thermodynamics vs kinetics competition is the fact that the degradability of some metal complexes (e.g., metal-NTA) is related to their kinetic lability, rather than to their thermodynamic stability constants. Kinetic rather than thermodynamic data are then used to classify metal complexes as labile, quasi-labile, slowly labile, and inert (or stable). See Section 3.2.6. 

Kinetic Data for X Ligand Exchange Reactions Involving T1(EDTA)X in... 

Schwlng at al. [7] developed a potentlometric-voltammetrlc system with digital data acquisition and treatment for the kinetic determination of mixtures of metal Ions based on ligand-exchange reactions. The resulte obtained from potentlometrlc measurements are more precise than those found with ampero-metrlc measurements (the typical relative errors are 5% and 5-10, respectively). 

The third and last example in this chapter illustrates a case where kinetic data were used to derive relative enthalpies for a series of similar reactions. Consider the ligand (phosphine) exchange reaction 15.15 (see figure 14.5 forthe structure of the complex),... 

A careful compilation of as many kinetic parameters as possible can lead to overwhelming support for a mechanism. Only occasionally are such comprehensive data available. The occurrence is nicely illustrated by the exchange reaction (M = Nb, Ta and Sb X = Cl and Br L = variety of neutral ligands) ... 

CN-stretching frequencies, 12 387 kinetic data for, 12 413 ligand substitution reactions, 12 406 Pentacyanocobalt(II) exchange reactions of, 10 201, 202 kinetic data for, 10 202... 

Dale Margerum Ralph Wilkins has mentioned the interesting effect of terpyridine on the subsequent substitution reaction of the nickel complex. I would like to discuss this point—namely the effect of coordination of other ligands on the rate of substitution of the remaining coordinated water. However, before proceeding we should first focus attention on the main point of this paper-which is that a tremendous amount of kinetic data for the rate of formation of all kinds of metal complexes can be correlated with the rate of water substitution of the simple aquo metal ion. This also means that dissociation rate constants of metal complexes can be predicted from the stability constants of the complexes and the rate constant of water exchange. The data from the paper are so convincing that we can proceed to other points of discussion. 

The reactivity of the encounter complexes between protonated and acetylated (R)-and (5)-2-butyl acetate, (CH3COOsBu) M+ (sBu = (/ )-, (5)-, or ( )-2-butyl M = H (n= 1,2) CH3CO (n = 1)), and (S, S,, S )-tri-sec-buty I borate has been measured by FT-ICR-MS. The relevant ion patterns are shown in Scheme 12.475 The kinetic data of Table 20 reveal some differences in both the overall reactivity of chiral (CH3COOsBu) M+ ions toward (iS, iS, S )-tri-sec-butylborate (kUtl, k M, and k5) and the relative extent of the competing addition/elimination (k and k4), proton transfer (k2), and ligand exchange (fc3) channels. They clearly indicate that (S,S,S) tri-sec-butylborate reacts more efficiently with the homochiral (5)-2-butyl acetate ions, than with the heterochiral (R)-2-butyl acetate ones. As expected, the reaction efficiency of the racemate ( )-2-butyl acetate ions falls in between. 

For kinetics studies, it was most convenient to measure the mole fraction (i.e., area fraction) of water or Hq in the gas phase. Fig. 3.3 shows the consumption of water and the production of Hq in the hydrolysis of Alq3 at 125°C. The data fit a rate law which shows a power-law dependence on the mole fraction (or partial pressure) of water in the gas phase 93 = P 20 where n 2-6. To explain this, a mechanistic model (Fig. 3.4) was developed considering the sorption and desorption of the volatile components. The first step of the reaction is the absorption of water into the solid and the formation of an intermediate where water hydrogen bonds with the electronegative oxygen atoms on the ligand. The reaction then follows where the water exchanges position with Hq and the products can be desorbed (or desolvated). 

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