Complex substitution reactions

The more precise formulation of the transition complex of n complex substitution reactions makes it possible to write a reaction scheme [Eq. (17)] showing the possible interconnection of a number of hitherto unrelated hydrogenation and exchange mechanisms. 

The trans effect illustrates the importance of studying the mechanisms of complex substitution reactions. Before continuing with a discussion of mechanisms, the distinction between the thermodynamic terms stable and unstable and the kinetic terms labile and inert should be clarified. Consider the following cyano complexes [Ni(CN)4]2-, [Mn(CN)6]3-, and [Cr(CN)6]3-. All of these complexes are extremely stable from a thermodynamic point of view is yet kinetically they are quite different. If the rate of exchange of radiocarbon labeled cyanide is measured, we find that despite the thermodynamic stability, one of these complexes exchanges cyanide ligands very rapidly (is labile), a second is moderately labile, and only [Cr(CN)6]3 can be considered to be inert ... 

Although in the previous section the basic concepts related to substitution reactions were explained with reference to octahedral complexes, substitution reactions are also common in square planar complexes. Studies on these complexes have resulted in a great deal of knowledge of the mechanisms of these reactions, so a brief description of the topic is presented next. 

The A-nitroimidazolcs are stable for a time even in the presence of water, but treatment with concentrated sulfuric acid cleaves the N-nitro group, and strong base opens the ring. Much of the interest in such compounds is related to their multistep complex substitution reactions in which sequential nucleophilic addition of arylamines, ring opening, ring closure, nitroamide elimination, and rearomatization gives l-aryl-4-nitroimidazoles, e.g., 840 839. This method can also be used to prepare isotopically labeled imidazoles when labeled amino acids are used as the amine. 

The solution kinetics involves three fundamental types of experiment, namely (i) rate law, (ii) stereochemistry and (iii) variation of rate constant with structure or environment. Each one permits the analysis of slightly different aspects of a reaction, and then develops a scheme of reaction classification which throws light on the behaviour of a reaction in each experimental situation. It is useful to briefly discuss the role of each of the three types of experimental investigation in the study of complex substitution reactions. 

An added group is attached at the same time to the complex and to a group in the second sphere of the original complex (substitution reaction in the second sphere). 

Ligand Exchange via Electron Exchange. When a metal atom forms cations in two oxidation states, one giving labile complexes and the other inert complexes, substitution reactions of the latter can be accelerated by the presence of trace quantities of the former. For example, the reactions of type (21-36) that are catalyzed by a trace of Cr2+ must occur as shown in... 

Entering group effect platinum complexes substitution reactions, 494 EXAFS spectroscopy cadmium complexes, 929 copper(II) complexes, 720 zinc complexes, 929... 

Lanthanide shift reagents silver complexes, 806 Leaving group effect platinum complexes substitution reactions, 494 Leucine aminopeptidase zinc, 1005 Ligases zinc, 1002... 

Platinum(Tv) Complexes. Substitution reactions at this centre are generally ligand replacements, but with the added feature of marked catalysis by platinum(n) compounds. Indeed, this area is as much one of inner-sphere redox reactions as of substitution. Recent kinetic studies include those of substitution at /ra/ij-[PtCl2L4] +, where L = NHj, amine,... 

Interference from Ring-closure.—In 1966, Kustin, Pasternack, and Weinstock published a paper entitled Steric effects in fast metal complex substitution reactions , in which they reported a temperature-jump study involving the nickel(ii) and cobalt(ii) complexes with a- and jS-alanine. With a-alanine, the substitution at cobalt was significantly faster than at nickel, but with the jS-isomer, whereas the Ni + substitution rate was approximately the same as before, substitution at Co + was significantly slower. The rate constants shown in Table 4 were obtained for the 1 1... 

A number of reviews contain relevant material such as those on dimolybdenum and ditungsten complexes, substitution reactions of chromium complexes, the reactivity of complexes containing multiple bonds between metal atoms, model reactions of coupling ieactions2 5, and in a review principally on ruthenium chemistry.216... 

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