Cobalt rapid complexation

With a view to determining the equilibrium constant for the isomerisation, the rates of reduction of an equilibrium mixture of cis- and rra/i5-Co(NH3)4(OH2)N3 with Fe have been measured by Haim S . At Fe concentrations above 1.5 X 10 M the reaction with Fe is too rapid for equilibrium to be established between cis and trans isomers, and two rates are observed. For Fe concentrations below 1 X lO M, however, equilibrium between cis and trans forms is maintained and only one rate is observed. Detailed analysis of the rate data yields the individual rate coefficients for the reduction of the trans and cis isomers by Fe (24 l.mole sec and 0.355 l.mole .sec ) as well as the rate coefficient and equilibrium constant for the cw to trans isomerisation (1.42 x 10 sec and 0.22, respectively). All these results apply at perchlorate concentrations of 0.50 M and at 25 °C. Rate coefficients for the reduction of various azidoammine-cobalt(lll) complexes are collected in Table 12. Haim discusses the implications of these results on the basis that all these systems make use of azide bridges. The effect of substitution in Co(III) by a non-bridging ligand is remarkable in terms of reactivity towards Fe . The order of reactivity, trans-Co(NH3)4(OH2)N3 + > rra/is-Co(NH3)4(N3)2" > Co(NH3)sN3 +, is at va-... 

Anionic complexes of iron and cobalt rapidly react with permethylated a,a>-dihalopolysilanes to give disubstituted polysilanes [1,2]. Because of the low nucleophilicity of pure carbonyl-cobaltate, we substituted one of the CO ligands with PPh3, whereupon the reactivity increased dramatically. 

In 1984 Sargeson and coworkers [31] have detected that the dinuclear, p-nitrophenyl phosphate-bridged cobalt(III) complex 1 rapidly hydrolyzes in water at pH 10 and 25 °C (k = 2 x 10 2 s-1). Based on a detailed kinetic investigation the authors suggest intramolecular attack of coor-... 

A variety of geometries have been established with Co(II). The interconversion of tetrahedral and octahedral species has been studied in nonaqueous solution (Sec. 7.2.4). The low spin, high spin equilibrium observed in a small number of cobalt(Il) complexes is rapidly attained (relaxation times < ns) (Sec. 7.3). The six-coordinated solvated cobalt(ll) species has been established in a number of solvents and kinetic parameters for solvent(S) exchange with Co(S)6 indicate an mechanism (Tables 4.1-4.4). The volumes of activation for Co " complexing with a variety of neutral ligands in aqueous solution are in the range h-4 to + 1 cm mol, reemphasizing an mechanism. 

Although Co(III) is often considered the classical representative of inert behavior, there are a number of cobalt(III) complexes that react rapidly enough to require that the rates be determined by flow methods. Table 8.11 shows a representative selection of such labile complexes. 

Photoreduction of cobalt(III) complexes in nonaqueous solvent systems has been little studied because of the limited solubility of cobalt(III) complexes and their tendency to photooxidize the solvent. Irradiation with 365-mjj. light of cis- or trans-Co(en)2C 2 + and Co(en)2Cl(DMSO)2+ in dimethylsulfoxide (DMSO) leads rapidly to production of a green tetrahedral cobalt(II) product apparently with concurrent solvent oxidation.53,71 Irradiation with 365-mjx light of the molecular Co(acac)3 in benzene rapidly gives a red precipitate which may be the cobalt(II) acetylacetonate.53... 

Aqueous solutions containing cobaltous cyanide complexes (the prevalent complex in a freshly prepared solution is believed to be Co(CN)6 ) absorb hydrogen rapidly ( 5 min.) at temperatures as low as 0°, the total uptake corresponding to that required to reduce all the cobalt to the +1 state. It has been suggested (Mills et al., 50) that the mechanism of the reaction involves the heterolytic splitting of hydrogen, i.e.. 

In general, then, metal ions in solution form complexes (frequently six coordinate) with the solvent molecules, their counterions, and other donor molecules that happen to be in the solution. For example, in ammo-niacal aqueous solution, Ag+ forms Ag(NH3)2+ (as noted above), Cu2+ forms a series of aquaammines but most notably the royal blue trans-Cu(NH3)4(OH2)22+, and cobalt(II) forms Co(NH3)i(H20)6-i2+ complexes which react quite rapidly with oxygen in air to give the strawberry-red cobalt(III) complex Co(NH3)5OH23+ or (if much chloride ion is present) the Co(NH3)5C12+ ion mentioned above. 

The rate of the spin state change for the octahedral cobalt(II) complexes is expected to be faster than that observed for the iron(II) and iron(III) complexes. In the cobalt(II) case the spin state change involves only one electron, that is AS = 1. The 2E and 4T states are directly mixed by spin-orbit coupling (10, 163). The spin state transition should be adiabatic, with k = 1, without any spin-forbidden barrier. Furthermore, the coordination sphere reorganization involves a change in bond length of 21 pm along only two bonds, instead of all six bonds as in iron complexes. Both of these factors lead to the prediction of rapid spin state interconversion. 

The dynamics of spin equilibria in solution are rapid. The slowest rates are those for coordination-spin equilibria, in which bonds are made and broken even these occur in a few microseconds. The fastest are the AS = 1 transitions of octahedral cobalt(II) complexes, in which the population of the e a antibonding orbital changes by only one electron these appear to occur in less than a nanosecond. For intramolecular interconversions without a coordination number change, the rates decrease as the coordination sphere reorganization increases. Thus the AS = 2 transitions of octahedral iron(II) and iron(III) are slower than the AS = 1 transitions of cobalt(II), and the planar-tetrahedral equilibria of nickel(II) are slower again, with lifetimes of about a microsecond. 

Similar considerations apply to the role of spin equilibria in electron transfer reactions. For many years spin state restrictions were invoked to account for the slow electron exchange between diamagnetic, low-spin cobalt(III) and paramagnetic, high-spin cobalt(II) complexes. This explanation is now clearly incorrect. The rates of spin state interconversions are too rapid to be competitive with bimolecular encounters, except at the limit of diffusion-controlled reactions with molar concentrations of reagents. In other words, a spin equilibrium with a... 

In contrast, the oxidation potential of the 1 1 cobalt(II) complex dyestuff in which the coordination sphere of the metal is completed by three molecules of ammonia is such that oxidation to the kinetically inert 1 1 cobalt(III) complex occurs rapidly (equation 6a). 

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. 

The enzyme aconitase catalyzes the isomerization of citric acid to isocitric acid via the intermediate cis-aconitic acid (Scheme 46),530 and various attempts have been made to model this reaction.21 The cobalt Ill) complexes derived from methyl maleate (171) and methyl fumarate (172) have been prepared531 to study intramolecular attack by coordinated hydroxide on the alkene. Generation of the hydroxo species of the maleic acid complex leads to rapid cyclization to give the... 

For the cobalt(III) complexes this method offers the additional advantage of rapid, controlled oxidation of the cobalt(II) intermediate complex to the cobalt(III) complex through the direct use of the elemental halogen corresponding to the anion of the desired salt, which eliminates the long air oxidation and the possible undesirable side reactions. 

The reduction behaviour of the alkylidene adduct of a cobalt-dithiolene complex (423) has been examined548 and the study has shown that, when the alkylidene-bridged structure (423) is reduced by one electron, it isomerizes rapidly and quantitatively to the ylide form (424). This represents the first example of reversible isomerization of the metal-carbon bond in a cobaltadithiolene complex. A surprising cis- to tra .s-dihydride isomerization which is unprecedented for 18-electron six-coordinate complexes has been observed549 in an octahedral iridium-c7.y-di hydride complex. 

Piacenti et al. suggested that the different results at low and high carbon monoxide pressure were due to different catalytic intermediates (A and B) under the two sets of conditions. Thus at low pressures A caused a rapid olefin isomerization and the formation of similar product distributions of aldehydes from 1- and 2-pentene. At high pressures little olefin isomerization occurred and 1-olefin yielded significantly more straight-chain aldehyde than 2-olefin. This would seem consistent with Heck and Breslow s mechanism (62) if A were an acylcobalt tricarbonyl in equilibrium with isomeric olefin-cobalt hydrocarbonyl complexes and B were an acylcobalt tetracarbonyl. 

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