Cobalt pentacyanide

At low values of R (the mole ratio of total cyanide to total cobalt) there is a precipitate of pink cobaltous dicyanide which dissolves as more KCN is added. At E = 5 (under most experimental conditions) a clear solution is obtained that may be straw-colored, or greenish-yellow. The work of Hume and Kolthoff (3) and of Adamson (4) shows that this is a solution of cobaltous pentacyanide ion and suggests that Co does not form a hexacyanide. Adamson (4) finds that pentacyanide ion in solution is paramagnetic and largely or entirely in the monomeric form [Co (CN)6] . 

The rate expressions are derived as follows The total concentration of cobalt, pentacyanide ion, dicyanide in solution, total dicyanide, and free cyanide are [Co], p, D, d, and c, respectively. When the catalyst concentration is small, by definition... 

Certain groups of organocobalt(III) complexes have been dealt with in previous reviews. The organo-corrinoids have been mentioned in all reviews on vitamin B, 2 since 1961, when the coenzyme form was identified as an organometallic compound [see, for example, (79, 178) and references therein]. The literature on the corrinoids is too extensive to be treated comprehensively here and for details and references readers are referred to the book on The Inorganic Chemistry of Vitamin B,2 (136)certain other aspects of the organometallic chemistry of cobalt corrinoids are treated elsewhere (137). The pentacyanides were reviewed in 1967 (105), the DMG complexes (cobaloximes) in 1968 (145), and some aspects of salen, BAE, and related complexes in 1970 (17). 

A series of complexes in which the cyanide ligands are modified or replaced arises from the decomposition of methyl and pyridiomethylcobalt(111) pentacyanide derivatives in acid solution. The reactions include protonation of a cyanide ligand, insertion of a cyanide ligand between the organic group and the cobalt atom to produce an imine (see Section VI,D), decomposition of this imine to an acyl product, and replacement of a cyanide ligand by water 100, 101). The products are listed in Table III, 29. 

When the rates at which Hj reacts with aqueous mixtures of CoCL and KCN at 1° are determined as a function of the mole ratio R (CN/Co), it is found that there are two maxima, the first at 12 = 3.9 and the second at R > 100. The homogeneous reaction which occurs at i2 > 5 is catalyzed by the small amount of cobaltous hexacyanide ion that is in equilibrium with colbaltous pentacyanide ion. 

Potentiometric titration and visual evidence show that there are conditions under which practically all cobaltous dicyanide is redissolved when R reaches 4.5. Since all of the cobalt cannot be present as the pentacyanide ion at this value of R, it is concluded that an appreciable amount exists as a complex in which the CN/Co ratio lies between 2 and 5. Of the possible complexes tricyanide ion, tetracyanide ion, and [C02 (CN)9] , the last accords best with the experiments which have been described. 

Of the organometallic nitrosyls which have been reported, the great majority are simple substitution products of cobalt nitrosyl carbonyl, Co(NO)(CO)3 and of iron nitrosyl carbonyl, Fe(NO)2(CO)2. Recently, however, there has been research activity on cyclopentadienyl nitrosyls, nitrosyl pentacyanides, and alkyl and allyl nitrosyls. 

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