Rate constants ligand substitution

Mercury(II) reacts with organochromium complexes by electrophilic substitution. Rate constants have been reported for Hg2+ attack at a series of alkylchromium complexes with the macrocyclic ligand 1,4,8,11-tetrazacyclotetradecane, CrR(H20)([14]aneN4). The Hammett relationship established for a series of meta and para substituted benzyl analogues is consistent with attack of the Hg2+ at a-carbon (89). 

Ten years ago Rorabacher (13) observed the substitution rate constants for aquonickel(II) ion with different amines (Table II). There is a decrease in the rate constants by a factor of 14 in going from ammonia to dimethylamine. If nickel-(II) substitution reactions are dissociative, then why is the effect this large Is this a steric effect with some associative contribution or is it an outer-sphere effect There has been surprisingly little investigation of the nature of the entering ligand so far as its bulk or its nucleophilicity is concerned even for what have been generally considered as simple substitution reactions. 

For several metal ions like Ni2+, the exchange rate constant s entropy of activation is positive, and ligand properties do not appreciably affect the rate of substitution then, the solvent exchange rate is a characteristic of the metal ion, and the mechanism is dissociative (D). In contrast, associative (A) mechanisms have negative entropies of activation the substitution rate constants vary with ligand, and a characteristic substitution rate constant is not a meaningful concept. 

Table 9.9, Dependence of Substitution Rate Constants, k, on Leaving Ligands, X, and trans Ligands, Y, in [Rh2(pL-02CCH3)4XY] ...

If the rate constant for the oxidation-reduction reaction is larger than the rate of ligand substitution on either metal, then an outer-sphere mechanism is required. For example, V(OH2)6 " has a water exchange rate of 90 s" and substitution is by an mechanism, so that the substitution rate constants should be <100 M" s". For the following reactions, the rate constant is much larger than this. 

As shown in Table 2.27 the rate constants for displacement reactions of acetonitrile in Os3(CO)i i (NCMe) are independent of the nature as well as of the concentration of the substituting ligand. The rate constants determined for the substitution of acetonitrile by triphenylphosphin at different ratios [MeCN]/[PPh3] illustrated in the plots in Fig. 2.59 agree with the rate equation. 

Ultrasonic relaxation absorption has been observed " in aqueous solutions of Zn(II) and Cd(II) edta complexes. The absorption is ascribed to a rapid structural change of the complex itself involving penta- and hexacoordinated forms of the ligand, and in both cases the rate constant (5 — 6) is close to the water substitution rate constant of the metal ion. NMR spectroscopy has been used " to demonstrate that a similar dynamic equilibrium exists in the edta complexes of Co ", Ni ", and Cu ", although in the case of the Mn " complex the ligand is present only in the pentacoor-dinate form. In the reaction of Mg ", Ca ", Sr ", Ba ", Ni ", Cu ", or Zn ... 

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

Several authors have suggested that the pathway may prove to be the most common mechanism in substitution reactions of octahedral complexes generally. However, the D path can be clearly demonstrated in some cases including at least two examples from Co(III) chemistry. The path (I - III - IV, Fig. 7) through the fivecoordinate intermediate would lead, in the case of rate studies in the presence of excess anionic ligand, to observed first-order rate constants governed by equation (13)... 

The extraction system which was measured by the HSS method for the first time was the extraction kinetics of Ni(II) and Zn(II) with -alkyl substituted dithizone (HL) [14]. The observed extraction rate constants linearly depended on both concentrations of the metal ion [M j and the dissociated form of the ligand [L j. This seemed to suggest that the rate determining reaction was the aqueous phase complexation which formed a 1 1 complex. However, the observed extraction rate constant k was not decreased with the distribution constant Kj of the ligands as expected from the aqueous phase mechanism. 

Fig. 1. Mean lifetimes of a single water molecule in the first coordination sphere of a given metal ion, th2o> and the corresponding water exchange rate constants, h2o- The tall bars indicate directly determined values, and the short bars indicate values deduced from ligand substitution studies. References to the plotted values appear in the text.

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