Cobalt/ions/salts

Cobalt(II) is also easily oxidised in the presence of the nitrite ion NO2 as ligand. Thus, if excess sodium nitrite is added to a cobalt(II) salt in presence of ethanoic acid (a strong acid would decompose the nitrite, p. 244), the following reaction occurs ... 

Here, effectively, the Co " (aq) is being oxidised by the nitrite ion and the latter (in excess) is simultaneously acting as a ligand to form the hexanitrocobaltate(III) anion. In presence of cyanide ion CN. cobalt(II) salts actually reduce water to hydrogen since... 

Cobalt U) sulphide is precipitated as a black solid by addition of sulphide ion to a solution of a cobalt(II) salt, in alkaline solution. 

Cobalt exists in the +2 or +3 valence states for the majority of its compounds and complexes. A multitude of complexes of the cobalt(III) ion [22541-63-5] exist, but few stable simple salts are known (2). Werner s discovery and detailed studies of the cobalt(III) ammine complexes contributed gready to modem coordination chemistry and understanding of ligand exchange (3). Octahedral stereochemistries are the most common for the cobalt(II) ion [22541-53-3] as well as for cobalt(III). Cobalt(II) forms numerous simple compounds and complexes, most of which are octahedral or tetrahedral in nature cobalt(II) forms more tetrahedral complexes than other transition-metal ions. Because of the small stabiUty difference between octahedral and tetrahedral complexes of cobalt(II), both can be found in equiUbrium for a number of complexes. Typically, octahedral cobalt(II) salts and complexes are pink to brownish red most of the tetrahedral Co(II) species are blue (see Coordination compounds). 

Discussion. An excellent method for the colorimetric determination of minute amounts of cobalt is based upon the soluble red complex salt formed when cobalt ions react with an aqueous solution of nitroso-R-salt (sodium 1-nitroso-2-hydroxynaphthalene-3,6-disulphonate). Three moles of the reagent combine with 1 mole of cobalt. 

The liquid-phase autoxidation of cyclohexane is carried out in the presence of dissolved cobalt salts. A lot of heterogeneous catalysts were developed for this process but most catalysts lacked stability. The incorporation of cobalt ions in the framework of aluminophosphate and aluminosilicate structures opens perspectives for heterogenization of this process. CoAPO (cobalt aluminophosphate) molecular sieves were found to be active heterogeneous catalysts of this oxidation.133 Site isolation was critical to get active catalysts.134... 

The question about the competition between the homolytic and heterolytic catalytic decompositions of ROOH is strongly associated with the products of this decomposition. This can be exemplified by cyclohexyl hydroperoxide, whose decomposition affords cyclo-hexanol and cyclohexanone [5,6]. When decomposition is catalyzed by cobalt salts, cyclohex-anol prevails among the products ([alcohol] [ketone] > 1) because only homolysis of ROOH occurs under the action of the cobalt ions to form RO and R02 the first of them are mainly transformed into alcohol (in the reactions with RH and Co2+), and the second radicals are transformed into alcohol and ketone (ratio 1 1) due to the disproportionation (see Chapter 2). Heterolytic decomposition predominates in catalysis by chromium stearate (see above), and ketone prevails among the decomposition products (ratio [ketone] [alcohol] = 6 in the catalytic oxidation of cyclohexane at 393 K [81]). These ions, which can exist in more than two different oxidation states (chromium, vanadium, molybdenum), are prone to the heterolytic decomposition of ROOH, and this seems to be mutually related. 

The autoxidation of hydrocarbons catalyzed by cobalt salts of carboxylic acid and bromide ions was kinetically studied. The rate of hydrocarbon oxidation with secondary hydrogen is exactly first order with respect to both hydrocarbon and cobalt concentration. For toluene the rate is second order with respect to cobalt and first order with respect to hydrocarbon concentration, but it is independent of hydrocarbon concentration for a long time during the oxidation. The oxidation rate increases as the carbon number of fatty acid solvent as well as of cobalt anion salt are decreased. It was suggested that the cobalt salt not only initiates the oxidation by decomposing hydroperoxide but also is responsible for the propagation step in the presence of bromide ion. 

Both of the above approaches employed a metal ion as a template about which the corrin cyclization was performed, but the nickel or cobalt ions could not subsequently be removed. In order to obtain metal-free corrins, a new route was therefore devised (67AG865) which employed the novel principle of sulfide contraction (Scheme 22). Thus the sodium salt of the precorrin (284) (Scheme 23) was transformed into the thiolactam (285), and loose complexation with zinc(II) ions caused cyclization to give (286), which was treated with benzoyl peroxide and acid to give the ring-expanded compound (287). Contraction with TFA/DMF gave the corrins (288) and (289), and the major of these (289) was desulfurized with triphenylphosphine and acid to give (288). Finally, demetallation with TFA gave the required metal-free corrin (290), a source for a whole variety of metal derivatives. 

In their first publication on this subject,59 Werner and Miolati showed that the molecular conductances (fx) of coordination compounds decreased as successive molecules of ammonia were replaced by acid residues (negative groups or anions). For cobalt(III) salts, they found that fi for luteo salts (hexaammines) > fi for purpureo salts (acidopentaammines) > /t for praseo salts (di-acidotetraammines). The conductance fell almost to zero for the triacidotriammine Co(N02)3-(NH3)3 and then rose again for tetracidodiammines, in which the complex behaved as an anion. By such measurements, Werner and Miolati determined the number of ions in complexes of cobalt(III), platinum(II) and platinum(IV). They not only found support for the coordination theory, but they also elucidated the process of dissociation of salts in aqueous solution and followed the progress of aquations. 

The resolution of racemic amino acid mixtures via coordination to a metal ion has been a popular field of study. [Cu(L-aa)2] complexes can be used to resolve DL-Asp, dl-G1u and DL-His.58,59 (—)-[Co(EDTA)] has been used to resolve DL-His having first resolved the racemic [Co(EDTA)] ion using the L-histidinium cation.60 Schiff base complexes of both Co111 and Ni11 have also been used to resolve amino acids.61,62 A more esoteric finding is that the bacterium Enterobacter cloacae prefers to metabolize the A-( —) isomer of/ac-[Co(GlyO)3] rather than the A-(+) form,63 an observation reminiscent of that made by Bailar using tris(ethylenediamine)cobalt(III) salts. 

It was not until 1965 that 1 1 cobalt(III) complexes of tridentate azo compounds were prepared49 by the interaction of the azo compound and a cobalt(II) salt in aqueous medium in the presence of excess ammonia under an inert atmosphere. In every case, e.g. (41), the coordination sphere of the cobalt ion was completed by three molecules of coordinated ammonia and oxidation to the cobalt(III) state occurred at the expense of the azo compound, some of which was reduced. The scope of the reaction is wide and 1 1 cobalt(III) complexes of this type have been prepared from a wide range of tridentate metallizable azo compounds. 

Salt. When compared on a molar basis, the mineral ion salts (ammonium, calcium, rubidium, copper, silver, lead, manganese, cobalt, potassium, and sodium, and cyclohexylamine salt) were as effective as the free gibberellic acid in promoting stem elongation (10,14). As shown in Figure 1, the potassium and zinc salts of A3 were as active as the acid in promoting the growth of d-1 dwarf maize. 

The different behavior of strontium and cobalt ions is not surprising. The former is an ordinary second-row element with strong tendencies to salt formation, whereas the latter is a transition metal with partly filled 3d-orbitals, which significantly will be involved in the complex formation ... 

Hexamminecobalt(III) salt cannot be used as a precipitant in the oxalato complex precipitation system because it precipitates as hexamminecobalt(III) oxalate. Besides the hexaureachro-mium(III) salt, hexamminechromium(III), tris(ethylenediamine)cobalt (III) or tris(trimethylenediamine)cobalt(III) salts can be used as precipitants. Hexamminechromium(III) and tris(ethylenediamine) cobalt (III) salts form precipitates with actinide(IV) or (VI) oxalato complex ions, whereas tris(trimethylenediamine)co-balt(III) salt forms precipitates with Th(IV) or U(VI) oxalato complex ions leaving Pu(IV) ion in the supernatant solution.Therefore, this reagent plays the role of both a separating agent and a precipitant and is applicable for the separation of Pu(IV) ion from Th(IV) or U(VI) ion. 

Clathrochelates comprise a new type of coordination compound containing a metal ion both coordinately saturated and encapsulated by a single ligand.1 5 The ligands of one class of clathrochelates are derived from dioximes and various boron compounds.1,4 Typical examples of iron and cobalt clathrochelates of this structural class are shown in Fig. 19 and Table I. These complexes are readily prepared from iron(II) or cobalt(II) salts,... 

The synthesis of pentaammine(carbonato)cobalt(III) salts is readily accomplished by an air-oxidation method starting with cobalt(II) nitrate and ammonium carbonate in aqueous ammonia.1 However, a suitable general method for making other types of monodentate amino(carbonato) metal salts is not available in the literature. The air-oxidation technique can be applied only to complexes where the ligands are all ammonia molecules or where the central metal ion is cobalt. 

The use of metal ions as templates for macrocycle synthesis has an obvious relevance to the understanding of how biological molecules are formed in vivo. The early synthesis of phthalocyanins from phthalonitrile in the presence of metal salts (89) has been followed by the use of Cu(II) salts as templates in the synthesis of copper complexes of etioporphyrin-I (32), tetraethoxycarbonylporphyrin (26), etioporphyrin-II (78), and coproporphyrin-II (81). Metal ions have also been used as templates in the synthesis of corrins, e.g., nickel and cobalt ions in the synthesis of tetradehydrocorrin complexes (64) and nickel ions to hold the two halves of a corrin ring system while cycliza-tion was effected (51), and other biological molecules (67, 76, 77). 

The chemistry of cobalt involves mainly its +2 and +3 oxidation states, although compounds containing cobalt in the 0, +1, or +4 oxidation states are known. Aqueous solutions of cobalt(II) salts contain the Co(H20)62+ ion, which has a characteristic rose color. Cobalt forms a wide variety of coordination compounds, many of which will be discussed in later sections of this chapter. Some typical cobalt compounds are listed in Table 20.8. 

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