Anation mechanism

It was also found that the degree of hydration of the complex affected the rate of racemization. Generally, the hydrated complexes reacted faster than anhydrous samples. It was also found that reducing the particle size increased the rate of racemization, but when the iodide compound was heated with water in a sealed tube, the racemization was slower than for the hydrated solid from which the water could escape. The fact that the hydrated samples racemized faster could indicate that an aquation-anation mechanism is involved, but the results obtained in the sealed tube experiments do not agree with that idea. 

This promise has been only partially fulfilled because of the difficulty of interpreting anation mechanisms where second order kinetics, first order in entering anion and first order in complex, are often found because of ion association which contributes a term in anion concentration to the rate law. A further difficulty, emphasised by Archer in his recent review on the stereochemistry of octahedral substitution reactions, is found in cobalt(III) chemistry because of the difficulty in isolating trans solvent-containing species. This results in continued doubt in the study of such systems as ... 

Halides are ubiquitous co-ligands for cobalt(III), and are met throughout this review. Anation of (solvent)cobalt(III) complexes by halide has been examined from time to time. An example is substitution of coordinated acetonitrile in [Co(L)(MeCN)2]3+ (L = tetraaza-macrocycle) by Cl-and Br-.1096 A mechanism involving interchange from within tight ion pairs was proposed. Halo-bridged polymeric complexes are well known with both classical and organometallic complexes. 

In proposed mechanism I, the loss of water from the complex is the rate-determining step, but removal of water from the coordination sphere of the metal ion should be independent of the nature of the anion that is not part of the coordination sphere of the metal ion. On the other hand, if mechanism II is correct, the entry of X into the coordination sphere of the metal would be dependent on the nature of the anion, because different anions would be expected to enter the coordination sphere at different rates. Because there is an observed anion effect, it was concluded that the anation reaction must be an Sn2 process. However, it is not clear how a process can be "second-order" when both the complex cation and the anion are parts of the same formula. As discussed in Chapter 8, it is not always appropriate to try to model reactions in solids by the same kinetic schemes that apply to reactions in solutions. 

Several interesting observations have been made on this reaction. First, the rate of isomerization was found to be the same as the rate of dehydration. All attempts to dehydrate the starting complex by conventional techniques were found to lead to isomerization. On the basis of this and other evidence, the mechanism proposed involves the aquation in the complex followed by anation. In this process, water first displaces Cl- in the coordination sphere and then is displaced by the Cl-, possibly by an SN1 mechanism. A trigonal bipyramid transition state could account for the Cl- reentering the coordination sphere to give an cis product. The rate law for this reaction is of the form... 

Potassium nitroacetate 53a reacts with Me3SiCl in aprotic solvents to give SENA (51a) in moderate yield. At the same time, the introduction of yet another electron-withdrawing group (N02 or CC>2Me) stabilizes the anion of salt (53) to an extent that it does not react with Me SiCl by the Sm mechanism without electrophilic assistance. Hence, K or Na salts 53b, C are inert with respect to halosilanes, and silver or mercury salts are required for the preparation of the corresponding nitronates. The latter salts are much safer to use as diox-anate complexes. These complexes react with halosilanes in inert aprotic solvents... 

As the above discussion indicates, assigning mechanisms to simple anation reactions of transition metal complexes is not simple. The situation becomes even more difficult for a complex enzyme system containing a metal cofactor at an active site. Methods developed to study the kinetics of enzymatic reactions according to the Michaelis-Menten model will be discussed in Section 2.2.4. 

The aquated Co(III) ion is a powerful oxidant. The value of E = 1.88 V (p = 0) is independent of Co(III) concentration over a wide range suggesting little dimer formation. It is stable for some hours in solution especially in the presence of Co(II) ions. This permits examination of its reactions. The CoOH " species is believed to be much more reactive than COjq Ref. 208. Both outer sphere and substitution-controlled inner sphere mechanisms are displayed. As water in the Co(H20) ion is replaced by NHj the lability of the coordinated water is reduced. The cobalt(III) complexes which have been so well characterized by Werner are thus the most widely chosen substrates for investigating substitution behavior. This includes proton exchange in coordinated ammines, and all types of substitution reactions (Chap. 4) as well as stereochemical change (Table 7.8). The CoNjX" entity has featured widely in substitution investigations. There are extensive data for anation reactions of... 

Before concluding the discussion of the anation reaction, some consideration should be given to alternative formulations of the reaction mechanism. In parti-... 

The aquation of the various CoCCN -3 complexes must occur by a reaction path which is merely the reverse of that followed in the anation. If the proposed mechanism for the anation reaction is valid, the reverse of Reactions 1 and 2 may be used to describe the equation. In any given experiment the rate of approach to equilibrium may be characterized by a first-order rate constant k which is related to the other kinetic parameters by Equation 3. When krfa/kz k (X ), as it is in... 

The solvolysis of hexaaquachromium(HI) in DMSO proceeds via the series of complexes [Cr(DMS0)n(H20)6 ]3+ (n = 1 to 6).945,946 The anation and solvolysis of several chromium(III) complexes in DMSO have been studied 947,948 in general the reactions proceed by /d mechanisms. A direct electrochemical synthesis of [Cr(DMSO)6][BF4]3 has been reported.949 A comprehensive review of metal ion complexation by DMSO contains an extensive section on chromium(III).950... 

Because of the inertness of Co(III) and Cr(III) complexes, their substitution reactions were the first among those of octahedral complexes to be extensively studied. Most evidence supports the fd mechanism for substitution in Co(fll) complexes. First, there is little dependence of reaction rates on the nature of the incoming ligand, if bond making were of significant importance, the opposite would be expected. Data are presented in Table 13.4 for the anation reaction of penta-ammineaquacobaltdll) ... 

Bowen, J., Hinchliffe, J.R., Horder, T.J., Reeve, A.M.F. (1989). The fate map of the chick forelimb-bud and its bearing on hypothesized developmental control mechanisms. Anat. Embryol. 179, 269-283. 

One mechanistic study7S worth describing here concerns the photoreactivity of [Pt(diethylenetriamine)Br]+. Photolysis in the presence of N02 accelerates the substitution of Br to yield [Pt(diethylenetriamine)N02 ]+. The reaction was shown to proceed via [Pt(diethylenetriamine)OH2 ]2+ which is rapidly anated by either Br or N02 The essential evidence rests in the fact that photolysis in basic solution yields only [Pt(diethylenetriamine)OH]+ even in the presence of N02 This result prompts the postulate that a dissociative interchange mechanism obtains as proposed for Co(CN) -. 6S ... 

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