Cobalt complexes stability

The formally d5 cobalt complexes stabilized by coordination as in cationic [CpCoL(dt)]+ or neutral [CpCoX(dt)]... 

Azopyrazolone, o-hydroxy aryl-chromium complex geometrical isomerism, 70 cobalt complexes stability, 52... 

Similar effects are observed in the iron complexes of Eqs. (9.13) and (9.14). The charge on the negatively charged ligands dominates the redox potential, and we observe stabilization of the iron(iii) state. The complexes are high-spin in both the oxidation states. The importance of the low-spin configuration (as in our discussion of the cobalt complexes) is seen with the complex ions [Fe(CN)6] and [Fe(CN)6] (Fq. 9.15), both of which are low-spin. 

A number of metal porphyrins have been examined as electrocatalysts for H20 reduction to H2. Cobalt complexes of water soluble masri-tetrakis(7V-methylpyridinium-4-yl)porphyrin chloride, meso-tetrakis(4-pyridyl)porphyrin, and mam-tetrakis(A,A,A-trimethylamlinium-4-yl)porphyrin chloride have been shown to catalyze H2 production via controlled potential electrolysis at relatively low overpotential (—0.95 V vs. SCE at Hg pool in 0.1 M in fluoroacetic acid), with nearly 100% current efficiency.12 Since the electrode kinetics appeared to be dominated by porphyrin adsorption at the electrode surface, H2-evolution catalysts have been examined at Co-porphyrin films on electrode surfaces.13,14 These catalytic systems appeared to be limited by slow electron transfer or poor stability.13 However, CoTPP incorporated into a Nafion membrane coated on a Pt electrode shows high activity for H2 production, and the catalysis takes place at the theoretical potential of H+/H2.14... 

A modification of the HSAB approach was first explained by C. K. Jorgensen in connection with the stability of a cobalt complex. Under normal circumstances, Co3+ is a hard Lewis acid. However, when... 

Ethylenediaminetetraacetic acid, analogs, complexes of, 3 277 chelation by, 3 276-277 cobalt complex of, 3 281 complexes, 3 277-278 formation constant of, 3 273-274 -nickel, 3 17-18 stability of, 3 266-267 reaction with metal ions, 3 62 Ethylene dibromide, irradiation of, 5 196 4,5-Ethylenedithio-1,3-dithiole-2-thione based supramolecular complexes, 46 200-204 Ethylene glycol, 32 4... 

Porphyrins, 21 14, 36, 135 -based manganese complexes, 46 400-402 as cobalt complex ligants, 44 284-290 compared to phthalocyanines, 7 75 complexes, 19 144, 145, 147 complex stability, 42 135-137 degeneracy lifting, 36 206 metalloporphyrins, DNA cleavage and, 45 271-283... 

If a chiral cobalt complex, instead of Co(salen)2, is used, enantioselectivity during the ring-opening process is induced (at —20°C the optimal ee was 76%) <2002CC28>. The application of stabilized Horner-Wadsworth-Emmons phosphonates represents a viable alternative to ylides in the cyclopropanation reaction <2002JOC3142>. 

It should be recognized that the stability of cobalt complexes under carbon monoxide can be enhanced by the addition of ligands, as is the case for phosphine-modified cobalt hydroformylation catalysts (57, 58). The stability will also probably depend on properties of the solvent employed. Nevertheless, the plot shown in Fig. 4 appears to be quite useful for assessing long-term cobalt stability under H2/CO in the absence of strongly coordinating solvents or ligands. 

The stability of soluble ruthenium carbonyl species toward decomposition to metal is a function of both carbon monoxide partial pressure and reaction temperature, similar to the situation described earlier for cobalt complexes and shown in Fig. 4. However, a quantitative study of these variables on ruthenium stability has not yet been reported. 

There are ammoniates of PtCl2, of halides of other platinum metals and of cobalt and nickel, too, some of which have been mentioned before in, Section 50. The cobalt complexes clearly show the importance of the completed d shells for the stability of the complex. Non complex compounds of trivalent cobalt are very unstable. Solutions of divalent cobalt in ammonia, however, are readily oxidized by air, because the NH3 complex of trivalent cobalt Co(NH3)6 3+ClT has eighteen electrons used in bond formation, whereas the ion Co(NH3) + would have nineteen electrons. 

Measurements of the equilibrium constants of the reactions imply that the stabilities of the monosubstituted complexes are predominantly determined by steric effects of the ligand, reflecting a very crowded space around the metal in this system. The large ligands PPh3 or P(c-C6H,) do not react with the cobalt complex. 

Electronic effects become apparent in the M—NCS/M—SCN linkage switches observed in a series of Pd11 (iso)thiocyanate complexes. Ligands positioned trans to the pseudohalide and that are suited to accept electron density from the metal into empty orbitals (backbonding) stabilize the Pd—NCS linkage isomer. However, this rationale is contradicted by the trend in Co—(NCS) bonding in a series of cobalt complexes (see ref. 204b for a review). 

Chromium and cobalt are the metals most commonly used in dyestuffs for polyamide fibres and leather because of their kinetic inertness and the stability of their complexes towards acid. Since the advent of fibre-reactive dyestuffs, chromium and cobalt complexes have also found application as dyestuffs for cellulosic fibres, particularly as black shades of high light-fastness. Copper complexes are of more importance as dyes for cellulosic fibres and are unsuitable for polyamide fibres because of their rather low stability towards acid treatments. 

The stability of 1 1 cobalt complex dyestuffs of this type varies considerably according to the nature of the metallizable system in the azo compound. For example, neutral aqueous solutions of 1 1 cobalt complexes of o-hydroxyarylazopyrazolones are stable almost indefinitely at 60 °C. Under similar conditions, 1 1 cobalt complexes of o.o -dihydroxydiarylazo compounds slowly decompose with loss of ammonia to give the symmetrical 2 1 cobalt(III) complex of the azo compound. The corresponding complexes of u-carboxy-o -hydroxydiarylazo compounds decompose rather more rapidly at 60 °C, and those of o-carboxyarylazopyrazolones are unstable in aqueous solution at room temperature in the absence of excess ammonia. None is sufficiently stable to be of value as a dyestuff in its own right but the isolation of complexes of this type opened the way to the production51 of pure, unsymmetrical 2 1 cobalt(III) complex dyestuffs for the first time (e.g. 42). 1 1 Cobalt complex dyestuffs have also been prepared by the interaction of tridentate metallizable azo compounds and cobalt(II) salts in the presence of inorganic nitrites. These too are reported52 to react with an equimolar quantity of a different tridentate metallizable azo compound to yield a pure unsymmetrical 2 1 cobalt(III) complex. 

Incorporation of nickel or cobalt complexes into a silver-behenate-based imaging system is reported to stabilize it to the effects of heat and humidity.198 Among the complexes disclosed are Co(acac)2 and bis(salicylaldoximato)nickel(II). 

Cobalt complexes find various applications as additives for polymers. Thus cobalt phthalocyanine acts as a smoke retardant for styrene polymers,31 and the same effect in poly(vinyl chloride) is achieved with Co(acac)2, Co(acac)3, Co203 and CoC03.5 Co(acac)2 in presence of triphenyl phosphite or tri(4-methyl-6- f-butylphenyl) phosphite has been found to act as an antioxidant for polyenes.29 Both cobalt acetate and cobalt naphthenate stabilize polyesters against degradation,73 and the cobalt complex of the benzoic acid derivative (12) (see Section 66.4) acts as an antioxidant for butadiene polymers.46 Stabilization of poly(vinyl chloride)-polybutadiene rubber blends against UV light is provided by cobalt dicyclohexyldithiophosphinate (19).74 Here again, the precise structure does not appear to be known. 

Nickel complexes are of considerable importance as stabilizers and antioxidants for polymers of various kinds. The nickel(II) complex of the benzoic acid derivative (12) (see Section 66.4) acts as a stabilizer against oxidation of polybutadiene,46 but is less effective in this respect than the manganese and cobalt complexes. Complex (20) is effective in decreasing the rate of photooxidation of two-phase poly(vinyl chloride)-polybutadiene rubbers 74... 

Sulfonated Cobalt Complexes. Unlike triaqua 1 1 chromium complexes, the corresponding cobalt complexes can not be prepared in acidic solution. The 1 1 cobalt complexes are only stable in the presence of a surplus of nitrogen donor groups. Sandoz has developed sufficiently stabilized 1 1 cobalt complexes by using nitrite ions as N ligands [29], The coordinatively bound nitrite ions can easily be displaced by another tridentate azo dye [30],... 

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