Cobalt aquation reactions

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. 

Wemer andMiolati s first publication on this subject showed that the molecular conductances (/r) of coordination compounds decrease as molecules of ammonia are successively replaced by acid residues (negative groups or anions). For cobalt(III) salts, /r for luteo salts (hexaammines) > /r for pur-pureo salts (acidopentaammines) > /r for praseo salts (diaci-dotetraammines). The conductance decreases almost to zero for the triacidotriammine Co(N02)3(NH3)3 and then increases for tetraacidodiammines, in which the complex behaves as an anion. Wemer and Miolati used these measurements to determine the number of ions in cobalt(III) and platinum(II) and -(fV) complexes, which supported the coordination theory and also elucidated the process of dissociation of salts in aqueous solution and the progress of aquation reactions. 

Although aquation (or acid hydrolysis) reactions of cobalt(III) complexes have been studied more extensively than any other octahedral substitutions, it still is impossible to assign detailed mechanistic paths for them. An aquation reaction takes place at a pH less than 4 and is one in which a ligand is replaced by a molecule of water, such as that shown by Equation 1. 

Al , Ga , and In catalyse the aquation reaction of malonatopenta-ammine-cobalt(ni). The kinetic data may be rationalized in terms of the reaction sequence (17) and (18) " ... 

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. 

Cobalt(lII) (High-pressure studies of the anation of [M(NH3)5(OH2)] + ions (M = Co, Rh, or Cr) gave the AF+ values shown in Table 1. These results, together with those for the corresponding aquation reactions enable values for A to be estimated, and the values obtained... 

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. 

Aquation reactions involve the replacement of a ligand (other than water itself) with a water molecule. A considerable amount of work has been done on reactions of this type involving a variety of inert cobalt(III) complexes. For example. Equation (5.32) represents the aquation reactions of various pentaammine(ligand) cobalt(III) cations ... 

A plot of log K versus log k for a variety of pentaammine (ligand)cobalt(III) complex cations. The rate constants k refer to the following aquation reactions ... 

Other aquation reactions give further evidence concerning the most favored mechanism of octahedral substitution reactions. Consider the data for the aquation of various bidentate amine complexes of cobalt(III) as shown in Equation (5.33) and Table 5.3 ... 

Compare and rationalize the rate constants for the aquation reactions of the following ra j-cobalt(III) complexes ... 

Table 7.3. Activation Volumes (AV ) and Reaction Volumes (AV) for Several Aquation Reactions of Cationic Cobalt(III) Complexes ...

The earliest investigation of the exchange reaction between the aquated ions of Co(III) and Co(II) was carried out by Hoshowsky et al., using the isotopic method ( Co). When sulphate salts ( 10 M) were employed, complete exchange was observed between the two oxidation states of cobalt, in a time of less than two min. Two separation methods were employed (a) adsorption on an alumina column, and (b) precipitation of the Co(III) as the cobaltinitrite. 

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... 

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... 

Half-reactions Involving Aquated Cobalt(II) and Cobalt(III). 532... 

Half-reactions Involving Aquated Cobalt(ll) and Cobalt(lll)... 

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