Aquation cobalt complexes

Kinetic parameters for aquation at corresponding Cr(III) and Co(III) complexes have been compared for a series of complexes cis-[ML4XY]"+, where L4 = (NH3)4 or (en)2, X = Cl- or H20, and Y=an uncharged leaving group (DMSO, DMF, or DMAC). The uniformly negative activation volumes (AV between —2 and —11 cm3 mol-1) for the chromium complexes contrast with uniformly positive activation volumes (A V between +3 and +12 cm3 mol-1) for the cobalt complexes - AV values provide a more clear-cut contrast than AS values here (22). 

The major contributions falling within the scope of this section are those of Frankel, Langford, King and others,who have concentrated on the thermal and photochemical reactions of chromium(III) complexes. Additional interesting results relevant to cobalt(III) aquation reactions have been produced by Barraclough and McTigue " while a recent review by Amis on the solvation of a range of main block elements is relevant. 

As the oxidation mechanism the Shibata brothers proposed the activation of water molecules which were replaced by the aquation reaction of anionic ligands in a complex ion. They summarized the results of their early 16 papers, and in 1936 they published a book Katalytische Wirkungen der Metallkomplexverbindungen 34). This book was not frequently cited in the chemical literatures of English- speaking countries probably because it was written in German, and the publication was close to the outbreak of World War II. In 1974, however, 40 years after the publication of this book, Eastman Kodak Co. requested a US patent on the use of a cobalt complex such as [Co(NH3)6]Cl3 as an amplifier in the development of color films (55). Although no reference is made to the works of Shibata, it is supposed that the chemists at Eastman Kodak Co. studied the works of Shibata and co-workers. 

The use of activation volumes in the diagnosis of mechanism has continued to provide much valuable information. Activation volumes for substitution at octahedral complexes have formed the subject of a well-referenced review,in which the importance both of intrinsic and of solvation contributions is recognized. The topics of most relevance to this chapter include isomerization and racemization reactions of cobalt(III) complexes, aquation of cobalt(III) and of iron(II) complexes, and base hydrolysis of cobalt(III) complexes. Merbach s continuing investigations into the effects of pressure on rates of solvent exchange at 2-h and 3+ transition metal cations, while not being always strictly... 

As in organic chemistry where m values for bromides are rather lower than for chlorides, for example m — 0.92 for t-butyl bromide compared with m = 1.00 for t-butyl chloride, m values for cobalt(ra)-amine-bromide complexes are, at around 0.2, rather lower than for the analogous chlorides. Whereas in this work the effect of solvent structure on reaction rates has been used to gain further insight into reaction mechanisms, the opposite approach has also been used, in a study of aquation of tra/u-[Co(en)2Cl2]+ in alcohol-water mixtures, in which variation of rate with solvent composition has been used as a probe of solvent structure variation. Rates of aquation of both cis- and trans-[Co ea)2C have been determined in aqueous acetonitrile (0 < mole fraction MeCN < 0,104). For both complexes aquation rates decrease only slightly as the proportion of acetonitrile increases, with the cir-complex slightly more sensitive to solvent variation. The kinetic effects observed here are smaller than those observed in t-butyl alcohol-water solvent mixtures. ... 

Reasoning on the basis of the greater polarity of ND3 alone, we must take the isotope effect as evidence against an SnI mechanism, in which considerable positive charge is induced on the central atom It is reasonable for an Sn2 reaction, but only if bond formation contributes more effectively than bond rupture to stabilization of the transition state. The isotope effect thus constitutes additional support for Ingold s (134) interpretation of the aquation of cobalt complexes. 

Rate Constants for Aquation and for Base Hydrolysis of Selected penta-ammine-Cobalt(III) Complexes a... 

Hydrolysis of coordinated ligands is a special case of nucleophilic attack. Two examples involving inorganic ligands have already been given in Section II. A on aquation of cobalt(III) complexes. Many further examples will be found in the following Section VII.B on catalysis of hydrolysis of organic substrates by metal ions and complexes. 

The chemical reactivity of crown-ether complexes with neutral molecules has received little attention. Nakabayashi et al. (1976) have reported crown-ether catalysis in the reaction of thiols with l-chloro-2,4-dinitrobenzene. The catalytic activity was attributed to deprotonation of thiols by dicyclohexyl-18-crown-6 in acetonitrile solution. Blackmer et al. (1978) found that the rate of aquation of the cobalt(III) complex [333] increases on addition of... 

Many aquatic organisms exhibit an ability to concentrate a variety of trace elements and this ability has been identified as a function of the tendency of the elements to be complexed by ligands (159). The alkaline earth metals are poorly com-plexed in relation to the transition metals, copper, nickel, cobalt, zinc and manganese. The actinides should be regarded as members of an intermediate group. It has been suggested by Martin (160) that at least five mechanisms may regulate the uptake of metals by marine biota. These are... 

The plot of log kf vs log X, is linear over a wide range of rate constants (Fig. 2.7). Obviously, the faster the aquation, the more the reaction goes to completion The sloped is 1.0 and this indicates that the activated complex and the products closely resemble one another, that is, that has substantially separated from the cobalt and that therefore the... 

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... 

When acido groups, halides, thiocyanate, azide, acetate, and nitrate are present in the coordination sphere of cobalt(III), they appear to be oxidized in preference to coordinated ammonia. Many of the radicals thus produced are capable of oxidizing ammonia released from the complex or of interfering in other ways with the reduction process, and these systems have proved very difficult to understand even in general terms. Quantum yield and other data for a number of acidopentammine and certain other complexes are given in Table IV the data on the aquation reactions of some of the complexes are considered in Section I1I-D. 

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