Mechanisms of substitution

Kinetic experiments are frequently carried out with large excess of the incoming reagent, Y. This simplifies the analysis of the progress of the reaction for each kinetic run, but requires a number of runs at different concentrations of Y to determine the order of the reaction with respect to Y. 

FIGURE 12-2 Energy Profiles for Dissociative and Associative Reactions, (a) Dissociative mechanism. The intermediate has a lower coordination number than the starting material, (b) Associative mechanism. The intermediate has a higher coordination number than the reactant. 

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Mechanisms of substitution reactions of metal complexes. F. Basolo and R. G. Pearson, Adv. Inorg. Chem. Radiochem., 1961, 3, 1-89 (132). 

Kinetics and mechanisms of substitution reactions of octahedral macrocyclic amine complexes. C. K. Poon, Coord. Chem. Rev., 1973,10,1-35 (130). 

The mechanism of substitution on an electron-rich benzene ring is electrophilic substitution, electrophilic attack on an atom and the replacement of one atom by another or by a group of atoms. The fact that substitution occurs rather than addition to the double bonds can be traced to the stability of the delocalized 7T-electrons in the ring. Delocalization gives the electrons such low energy—that is, they are bound so tightly—that they are unavailable for forming new cr-bonds (see Sections 2.7 and 3.12). 

For each of these reactions kinetic data were obtained. The reactions were first order in complex concentration, and zero order in isocyanide, as expected. The complex Ni(CNBu )4, and presumably other Ni(CNR)4 complexes as well, undergo ligand dissociation in solution. In benzene solution, a molecular weight determination for this compound gives a low value (110). This is in accord with the presumed mechanism of substitution. 

The mechanism of substitution of anionic ligands on [Cr(NH3)5H20]3+ has received both Id and Ia assignments from various authors (23). This dichotomy of viewpoint comes about substantially... 

Under (i) the square and pyramidal complexes are often easier to substitute than the octahedral complexes for the obvious reason that they have open residual coordination sites, looking upon all the complexes as derived from an octahedron. The mechanism of substitution can then be the typical organic Sn2 attack. More usually the reactions of complex ions proceed by predissociation, SnI, so that the important consideration is that c and d should be at least relatively good leaving groups. 

A review of recent advances in chromium chemistry (82) supplements earlier comprehensive reviews of kinetics and mechanisms of substitution in chromium(III) complexes (83). This recent review tabulates kinetic parameters for base hydrolysis of some Cr(III) complexes, mentions mechanisms of formation of polynuclear Cr(III) species, and discusses current views on the question of the mechanism(s) of such reactions. It seems that both CB (conjugate base) and SVj2 mechanisms operate, depending on the situation. The important role played by ionpairing in base hydrolysis of macrocyclic complexes of chromium(III) has been stressed. This is evidenced by the observed order, greater... 

Kinetics and mechanisms of substitution at Pt(IV) are occasionally mentioned in relation to those complexes which may have anti-tumor properties. An article on molecular modeling of interactions between platinum complexes and nucleotides or DNA includes a brief mention of Pt(IV) (178). 

Kinetics and mechanisms of substitution at Pt(II) and Pd(II) have been reviewed and compared with respect to reactions of nitrogen bases such as imidazole, pyrazole, inosine, adenosine, and guanosine-5 -monophosphate with ammine, amine, pyridine carboxylate, and... 

The mechanism of substitution at these centers can conveniently be probed through CO exchange at the cis-[M(CO)2X2] anions (M =Rh, Ir X = Cl, Br or I). The rate law for these exchanges, and the activation parameters shown in Table IX, suggest the operation of a limiting A mechanism (265). 

Ingold, I. Dostrovsky, and E. D. Hughes, "Mechanism of Substitution at a Saturated Carbon Atom. Pt. XXXII. 

See C. K. Ingold, I. Dostrovsky, and E. D. Hughes, "Mechanisms of Substitution," JCS 149 (1946) 173194 and esp. Ingold, "Les reactions de la chimie organique (quatre conferences), Actualites Scientifiques et Industrielles, no. 1037 (Paris Hermann, 1948), 3238. These lectures were given at the Faculty of Sciences in Paris in May 1946 on the invitation of Edmond Bauer, Jean Perrin s successor at the Sorbonne and the Laboratoire de Chimie Physique. 

From Ingold, Structure and Mechanism in Organic Chemistry, 315. See Ingold, with L. C. Bateman, K. A. Cooper, and E. D. Hughes, "Mechanism of Substitution at a Saturated Carbon Atom. Pt. XIII. Mechanism Operative in the Hydrolysis of Methyl, Ethyl, Isopropyl, and Tert.-Butyl Bromides in Aqueous Solutions," JCS... 

See Shoppee (1972 362) and Ingold (1948 2730). "A great part of the mystery of Walden inversions is dissipated since we have related it to different mechanisms of substitution" (Ingold [1948] 30). Walden made his discovery in 1895 and listed more than twenty cycles of inversion and retention in Optische Umkehrserscheinungen (1919). 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

In recent years there has been a tendency to assume that the mechanisms of substitution reactions of metal complexes are well understood. In fact, there are many fundamental questions about substitution reactions which remain to be answered and many aspects which have not been explored. The question of associative versus dissociative mechanisms is still unresolved and is important both for a fundamental understanding and for the predicted behavior of the reactions. The type of experiments planned can be affected by the expectation that reactions are predominantly dissociative or associative. The substitution behavior of newly characterized oxidation states such as copper-(III) and nickel (III) are just beginning to be available. Acid catalysis of metal complex dissociation provides important pathways for substitution reactions. Proton-transfer reactions to coordinated groups can accelerate substitutions. The main... 

Since we shall not obtain the comparable amount of detailed information on the mechanisms of substitution in octahedral complexes from the studies of more complicated substitutions involving chelation and macrocycle complex formation (Secs. 4.4 and 4.5) it is worthwhile summarizing the salient features of substitution in Werner-type complexes. 

The study of the complexing of macrocycle ligands should be considered for its intrinsic importance rather than for its value in illuminating the mechanism of substitution. Kinetic (but much more thermodynamic °) data are available for the reactions of the different macrocycle ligand types, shown in Fig. 4.5, including azamacrocycles,crown ethers and cryp-tands, and porphyrins. ... 

The V(IV)-H20 exchange is catalyzed by V(V) (Prob. 5(a)). The enhanced reactivity for the base form is observed in dimerization, substitution and redox reactions (below). The mechanism of substitution of remains uncertain. One of the problems is to assess the contribution of the highly reactive VO(OH)+. Rate constants for complexing by VO + are all = lO M s, Ref. 35, consistent with an/ mechanism. By using chelating ligands to tie up... 

The complexes of Fe(II) and Fe(III), the important oxidation states in aqueous solution, have played major roles in our understanding of the mechanisms of substitution and redox pro-... 

We have seen above that the structure of the substrate is the most important feature that dictates the mechanism of substitution reactions. Thus, the Sn2 mechanism is favoured when the reaction takes place at a primary centre, whereas an SnI mechanism is preferred at tertiary centres, or where stable intermediate carbocations can be produced (see Section 6.2.3). 

Mechanisms of Substitution Reactions of Metal Complexes Fred Basolo and Ralph 0. Pearson... 

Mechanisms of Substitution Reactions of Cobalt (III) Cyanide Complexes... 

Leonard Katzin I want to make two comments, one on this last point in relation to the point that Dr. Margerum made about substituents. Chromium (I II) in the hexahyd rated state is quite resistant to penetration of the coordination shell by nitrate ion. Yet if one takes the% violet chromium nitrate hexahydrate in solid state and treats it with liquid tributylphosphate, within a matter of minutes one gets chromium compound in solution by the mechanism of substituting tributylphosphate for water. So this reaction is fast. This initial solution is violet Within the space of an hour or two it is green. And we have had for some years now infrared evidence that this color change is accompanied by penetration of the nitrate ion into the coordination sphere (4). So this again is a matter of the substituent s changing the relationship of the water. 

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