Cobalt complexes thermodynamic effects

Although the vast majority of cobalt(III) complexes are six-coordinate, several groups of five and even four-coordinate complexes are known in which the cobalt may be considered as having the formal oxidation state(III). These complexes represent the extreme case where the thermodynamic effects are so great that no ligand, even H2O, can form a stable bond to the cobalt, i.e., there is a decrease in coordination number. 

Effective protocols have also been developed for the extraction of nickel and cobalt from chloride and ammoniacal process streams derived from leaching sulfidic ores or mattes.103 175 214 224 270-279 The greater thermodynamic stability of the Co11 chloroanionic complexes such as [CoCl4]2-over analogous Ni11 species has been exploited to effect the separation of nickel and cobalt via an... 

Exactly the same problem arises with the recent studies of NiO solubility by Tremaine and Leblanc (25) and again the thermodynamic data on the aqueous anionic species at 300 C are likely to be more reliable than on the Ni + ion. There is good spectroscopic evidence for complex formation in chlorides of nickel (II), (26) cobalt (II) (27), and copper (II) (28) at 300°C and above. Most of the work was done at rather high Cl concentrations but qualitatively the effects of dielectric constant and concentration are as expected. A noteworthy feature (which estimation procedures will have to allow for) is the change from 6 to 4 coordination at the lower pressures (150-300 bar) and the higher Cl concentrations. This change appears to take place with only 2 or 3 Cl ions coordinated to the metal (at least in the case of Ni(II)). 

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