Cobalt , in oxidation

Cyclopentadienyl complexes of cobalt exist mainly in three oxidation states Co Co, and Co. Co is represented by complexes of the type CpCoL2, where L is CO, alkene, alkyne, or phosphine. Apart from derivatives of cobaltocene, half-sandwich complexes (see Half-sandwich Complexes) (CpCoX)2 or CpCoLX with cobalt in oxidation state II are known. CpCo occurs in various CpCoL Xm compositions (Section 9.2). 

The organometallic chemistry of cobalt in oxidation state III is more extensive than that of iron and nickel. This observation is certainly coimected to the high stability of octahedral Co coordination complexes. These compounds are of types I-V as depicted in Figure 16. 

E19.4 The oxides Fe04 and C02O9 contain iron and cobalt in oxidation states +8 and +9 respectively, which are not stable for either element. The massive amount of energy required to ionize either Fe to -t8 or Co to +9 cannot be offset by the lattice energy of the oxides, and the two compounds do not exist. Fe(+8) and Co(+9) would be a sufficiently strong oxidizing agent to oxidize some of the 0 anions back to O2. 

Stable and isolable compounds containing cobalt in oxidation state IV are rare, with a well-known example being octahedral [CoF6] . However, a larger number of examples of reactive formally cobalt(IV) species have been proposed to be generated by one-electron electrochemical oxidation of their parent cobalt(III) complexes. One important, but contentious, case detailed in... 

Cobalt(in) oxidizes 2-mercaptoethylamine (HMea) in [Co(en)2(Mea)] to the corresponding co-ordinated disulphide complex by pathways involving Co + and [CoOH] +. An outer-sphere mechanism is suggested by the activation entropy (—3.1 cal K mol ) for reaction with Co and the reaction with [CoOH] + is substitution controlled. Redox proceeds by formation of a co-ordinated radical complex [Co(en)2(Mea)] +,... 

Cobalt has an odd number of electrons, and does not form a simple carbonyl in oxidation state 0. However, carbonyls of formulae Co2(CO)g, Co4(CO)i2 and CoJCO),6 are known reduction of these by an alkali metal dissolved in liquid ammonia (p. 126) gives the ion [Co(CO)4] ". Both Co2(CO)g and [Co(CO)4]" are important as catalysts for organic syntheses. In the so-called oxo reaction, where an alkene reacts with carbon monoxide and hydrogen, under pressure, to give an aldehyde, dicobalt octacarbonyl is used as catalyst ... 

The conversion of CO to CO2 can be conducted in two different ways. In the first, gases leaving the gas scmbber are heated to 260°C and passed over a cobalt—molybdenum catalyst. These catalysts typically contain 3—4% cobalt(II) oxide [1307-96-6] CoO 13—15% molybdenum oxide [1313-27-5] MoO and 76—80% alumina, JSifDy and are offered as 3-mm extmsions, SV about 1000 h . On these catalysts any COS and CS2 are converted to H2S. Operating temperatures are 260—450°C. The gases leaving this shift converter are then scmbbed with a solvent as in the desulfurization step. After the first removal of the acid gases, a second shift step reduces the CO content in the gas to 0.25—0.4%, on a dry gas basis. The catalyst for this step is usually Cu—Zn, which may be protected by a layer of ZnO. 

Ocean Basins. Known consohdated mineral deposits in the deep ocean basins are limited to high cobalt metalliferous oxide cmsts precipitated from seawater and hydrothermal deposits of sulfide minerals which are being formed in the vicinity of ocean plate boundaries. Technology for drilling at depth in the seabeds is not advanced, and most deposits identified have been sampled only within a few centimeters of the surface. 

Aeration must be avoided since it can oxidize and resolubiUze the cemented (precipitated) impurities. Filter presses are used after each step and the cakes are leached to recover various values. For example, cadmium is dissolved, recemented with zinc, and recovered on site either electrolyticaHy or by distillation. A copper residue of 25—60% copper is sold for recovery elsewhere. The other impurities cannot be recovered economically with the exception of cobalt in some plants. 

Whereas finely divided cobalt is pyrophoric, the metal in massive form is not readily attacked by air or water or temperatures below approximately 300°C. Above 300°C, cobalt is oxidized by air. Cobalt combines readily with the halogens to form haUdes and with most of the other nonmetals when heated or in the molten state. Although it does not combine direcdy with nitrogen, cobalt decomposes ammonia at elevated temperatures to form a nitride, and reacts with carbon monoxide above 225°C to form the carbide C02C. Cobalt forms intermetallic compounds with many metals, such as Al, Cr, Mo,... 

Cobalt cannot be classified as an oxidation-resistant metal. Scaling and oxidation rates of unalloyed cobalt in air are 25 times those of nickel. The oxidation resistance of Co has been compared with that of Zr, Ti, Fe, and Be. Cobalt in the hexagonal form (cold-worked specimens) oxidizes more rapidly than in the cubic form (annealed specimens) (3). 

The scale formed on unalloyed cobalt during exposure to air or oxygen at high temperature is double-layered. In the range of 300 to 900°C, the scale consists of a thin layer of the mixed cobalt oxide [1308-06-17, Co O, on the outside and a cobalt(Il) oxide [1307-96-6] CoO, layer next to the metal. Cobalt(Ill) oxide [1308-04-9] maybe formed at temperatures below 300°C. Above 900°C, Co O decomposes and both layers, although of... 

Cobalt metal is significantly less reactive than iron and exhibits limited reactivity with molecular oxygen in air at room temperature. Upon heating, the black, mixed valence cobalt oxide [1308-06-17, Co O, forms at temperatures above 900°C the oHve green simple cobalt(II) oxide [1307-96-6] CoO, is obtained. Cobalt metal reacts with carbon dioxide at temperatures greater than 700°C to give cobalt(II) oxide and carbon monoxide. 

Cobalt compounds can be classified as relatively nontoxic (33). There have been few health problems associated with workplace exposure to cobalt. The primary workplace problems from cobalt exposure are fibrosis, also known as hard metal disease (34,35), asthma, and dermatitis (36). Finely powdered cobalt can cause siUcosis. There is Htfle evidence to suggest that cobalt is a carcinogen in animals and no epidemiological evidence of carcinogenesis in humans. The LD q (rat) for cobalt powder is 1500 mg/kg. The oral LD q (rat) for cobalt(II) acetate, chloride, nitrate, oxide, and sulfate are 194, 133, 198, 1700, 5000, and 279 mg/kg, respectively the intraperitoneal LD q (rat) for cobalt(III) oxide is 5000 mg/kg (37). 

Cobalt in Catalysis. Over 40% of the cobalt in nonmetaUic appHcations is used in catalysis. About 80% of those catalysts are employed in three areas (/) hydrotreating/desulfurization in combination with molybdenum for the oil and gas industry (see Sulfurremoval and recovery) (2) homogeneous catalysts used in the production of terphthaUc acid or dimethylterphthalate (see Phthalic acid and otherbenzene polycarboxylic acids) and (i) the high pressure oxo process for the production of aldehydes (qv) and alcohols (see Alcohols, higher aliphatic Alcohols, polyhydric). There are also several smaller scale uses of cobalt as oxidation and polymerization catalysts (44—46). 

Cobalt-cataly2ed oxidations form the largest group of homogeneous Hquid-phase oxidations in the chemical industry. 

Cobalt in Driers for Paints, Inks, and Varnishes. The cobalt soaps, eg, the oleate, naphthenate, resinate, Hnoleate, ethyUiexanoate, synthetic tertiary neodecanoate, and tall oils, are used to accelerate the natural drying process of unsaturated oils such as linseed oil and soybean oil. These oils are esters of unsaturated fatty acids and contain acids such as oleic, linoleic, and eleostearic. On exposure to air for several days a film of the acids convert from Hquid to soHd form by oxidative polymeri2ation. The incorporation of oil-soluble cobalt salts effects this drying process in hours instead of days. Soaps of manganese, lead, cerium, and vanadium are also used as driers, but none are as effective as cobalt (see Drying). 

A wide variety of greens ranging from blue to yellow in shade ate based on cobalt in combination with chromium, aluminum, titanium, nickel, magnesium, antimony, or zinc. These are brighter than the chromium oxides. 

In another case, cobalt(II) oxide can be prepared by heating the carbonate in the absence of air. ni2+ O2- o2-... 

A detailed study of the dehydrogenation of 10.1 l-dihydro-5//-benz[6,/]azcpinc (47) over metal oxides at 550 C revealed that cobalt(II) oxide, iron(III) oxide and manganese(III) oxide are effective catalysts (yields 30-40%), but formation of 5//-dibenz[7),/]azepinc (48) is accompanied by ring contraction of the dihydro compound to 9-methylacridine and acridine in 3-20 % yield.111 In contrast, tin(IV) oxide, zinc(II) oxide. chromium(III) oxide, cerium(IV) oxide and magnesium oxide arc less-effective catalysts (7-14% yield) but provide pure 5H-dibenz[b,/]azepine. On the basis of these results, optimum conditions (83 88% selectivity 94-98 % yield) for the formation of the dibenzazepine are proposed which employ a K2CO,/ Mn203/Sn02/Mg0 catalyst (1 7 3 10) at 550 C. 

A third access to isocorroles was found7 when a tetrapyrrole 11 having an acrylaldehyde side chain was cyclized in presence of copper(II) or cobalt(II) salts. In this case isocorrole-9-carb-aldehydes 12 are formed with copper and cobalt in the oxidation state + III. The copper compound can easily be demetaled by hydrochloric acid to yield the metal-free isocorrole. In contrast, the cyclization of the tetrapyrrole in the presence of palladium(II) gives the isopor-phycene (see Section 1.7.1.). 

Corrosion products include iron oxide (Fe203), ferrosoferric oxide (Fe304), nickel oxide (NiO), cobalt oxide (CoO), and complex Fe, Ni, and Co oxides. Cobalt in particular may present a problem (as cobalt59, a naturally occurring isotope), and when present as a contaminant in nickel alloys (such as Inconel 800), may enhance the development of an outer-core radiation field (see Section 7.4.1). 

Electrobalances suitable for thermogravimetry are readily adapted for measurements of magnetic susceptibility [333—336] by the Faraday method, with or without variable temperature [337] and data processing facilities [338]. This approach has been particularly valuable in determinations of the changes in oxidation states which occur during the decompositions of iron, cobalt and chromium oxides and hydroxides [339] and during the formation of ferrites [340]. The method requires higher concentrations of ions than those needed in Mossbauer spectroscopy, but the apparatus, techniques and interpretation of observations are often simpler. 

A procedure involving catalytic oxidation of sulphones has also been developed218. In this case the sulphone is mixed with sodium carbonate and cobalt(II) oxide and the mixture is burned in a stream of oxygen. This method works very well for nitrogen-containing sulphones and requires no expensive equipment. 

Cobalt ores are often found in association with copper(II) sulfide. Cobalt is a silver-gray metal and is used mainly for alloying with iron. Alnico steel, an alloy of iron, nickel, cobalt, and aluminum, is used to make permanent magnets such as those in loudspeakers. Cobalt steels are hard enough to be used as surgical steels, drill bits, and lathe tools. The color of cobalt glass is due to a blue pigment that forms when cobalt(II) oxide is heated with silica and alumina. 

Several complexes with cobalt in the unusually high oxidation state of (-(- 4) were reported (198,202,280) in 1974. All of the complexes reported were prepared by reaction of [Codlidtcls] with BF3 or Et20BF3 in the presence of air. The complexes were formulated (202, 280) as [Co(R2dtc)3]BF4 (R = Me, Et, Pr, or cyclohexyl), but Hendrickson and Martin (198) suggested that dimeric [Co2(R2dtc)5]BF4 (Rj = Me2, Et2 pyrrolidyl, MeBu", or Bzj), Co(III) complexes, form that are analogous with the ruthenium(III) complex discussed earlier. 

Because ammine ligands are neutral molecules, the oxidation state of each metal is the same as the charge on the complex. Iron loses two of its eight valence electrons to reach the +2 oxidation state, leaving six electrons for the d orbitals. Likewise, cobalt in its +3 oxidation state has six d electrons. 

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. 

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