Studies of Cobalt III Complexes

However, for [Co(H20)6], the longest wavelength band is in the near ir and the third band is not observed. The observed band is ascribed to the Tig (F)- T,g(P) transition. 

Prepare 0.1 M solution of cobalt(Il) nitrate or chloride in water and run its spectrum in a 10 mm cell over the range 400-800 nm. To a portion of the solution add cone. HCl, shake well to attain maximum colour and run the spectrum over the same range. Comment on the spectra, comparing their appearance and intensity. 

Alternatively dissolve an alkali metal chloride in 0.1 M cobalt(lI) chloride hexahydrate for the second spectrum. Run a reflectance spectrum on the last complex prepared above. If an instrument is available, obtain an e.s.r. spectrum. Measure the magnetic susceptibility of a purest Co(II) salt and of the two complexes last prepared and work out the number of unpaired electrons from the effective magnetic moment. Alternatively use the Hg(II) complex to calculate the constant C (equation 2.6) from the result of the Gouy balance. 

Cobalt(III) chemistry is dominated by Co(III) complexes. A large number of these have been prepared and studied during this century and their structure and bonding have been elucidated with the advance in instrumentation. Since most of these complexes are octahedral, low spin, diamagnetic and kinetically inert, their reactions can be conveniently studied. They undergo numerous reactions acid and base hydrolysis, ligand exchange, reduction and, where applicable, isomerisation. 

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Many more recent stoichiometric studies of cobalt(III) complexes have been responsible for most of the developments in this area of research. Cobalt(III) ammine complexes effect hydrolysis of ethyl glycinate in basic conditions via intramolecular attack of a coordinated amide ion hydrolysis by external hydroxide ion attack also occurs (equation 74).341 Replacement of ammonia ligands by a quadridentate or two bidentate ligands allows the formation of aquo-hydroxo complexes and enables intramolecular hydroxide ion attack on a coordinated amino ester, amino amide... 

Yajima, F., Koike, Y., Yamasaki, A. and Fujiwara, S. (1974) Cobalt-59 nuclear magnetic resonance study of cobalt(III) complexes. Empirical rules for the cobalt-59 chemical shifts and line widths of [Com(en)x(NH3) 6 2x—yLy]-type complexes. Bull. Chem. Soc.Jpn, 47, 1442-1446. 

Besides the direct photoreactions reported to date (see Figure 6.6), coordination entities also undergo the photosensitized modes in which they play the role of non-absorbing substrate. The application is usually connected with solving some mechanistic problems, as in the early study of cobalt(III) complexes with organic sensitizers, such as biacetyl, benzophenone, benzaldehyde, and naphthalene, which provided valuable mechanistic information about the excited states responsible for the photophysical, photoredox, and photosubstitution processes observed in cases of these complexes [105],... 

Most CD spectral studies of cobalt(III) complexes have been undertaken to investigate various sources of optical activity such as distribution of chelate rings, conformation of chelate rings, vicinal effect due to asymmetric carbon in an optically active ligand, and vicinal effect due to an asymmetric donor atom. Extensive reviews on these subjects have been written by Fujita and Shimura (1), Hawkins (2), and Mason (3). 

Isomerisation studies of cobalt(iii) complexes cover a variety of compounds. The rate law for isomerisation of [Co(dien)(OH2)a] + indicates some isomerisation via the hydroxo-cation [Co(OH)dien(OHa)2] +. Rates and activation parameters for direct isomerisation of the tris-aquo-complex were determined. Further kinetic results for isomerisation of cis- and rra 5 -[Co(OH)2 en2]+ in strongly basic solution indicate an intramolecular mechanism. Isomerisation of cw-[CoCl2(diars)2] in methanol is also intramolecular, but isomerisation of cw-[Co(02C CH3)2en2]+ in acetic acid occupies an intermediate position between intra- and inter-molecu-larity, for the essential step is solvent-assisted acetate exchange within ion-pairs. This assignment of mechanism results from consideration of kinetics of acetate exchange as well as of isomerisation. ... 

Several reports on X-ray diffraction studies of cobalt(III) complexes of L824 and L825 are available [12-17]. 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

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... 

Intramolecular lactonization has been studied on cobalt(III) complexes (Scheme 44).144 The reaction is catalyzed by general acid and the attack of coordinated water occurs at a greater rate than that of coordinated hydroxide ion. Presumably, the relatively non-nucleophilic water is assisted by hydrogen bonding to the carboxyl group to become a pseudo-hydroxide ion. 

In closely related studies, Lee and coworkers obtained even more dramatic results in attempts to alleviate the generally observed selectivity for the formation of cyclic carbonates from aliphatic epoxides and C02 [53, 54], This was achieved by adding cationic charge to the salen ligands of cobalt(III) complexes, as illustrated in Figure 8.11. 

Barve A, Ghosh S, Kumbhar A, Kumbhar A, Puranik V. Synthesis, DNA-binding and photocleavage studies of cobalt(III) polypyridyl complexes. I Inorg Biochem 2003 96 100. 

Although aquation (or acid hydrolysis) reactions of cobalt(III) complexes have been studied more extensively than any other octahedral substitutions, it still is impossible to assign detailed mechanistic paths for them. An aquation reaction takes place at a pH less than 4 and is one in which a ligand is replaced by a molecule of water, such as that shown by Equation 1. 

Kinetically inert low-spin cobalt (III) clathrochelates are reversibly reduced by accepting one electron to yield kinetically labile cobalt(II) complexes. In the case of the usual amines (for instance, ammonia), the reduction is, as a rule, accompanied by irreversible decay of the amine cobalt complex. This reaction is slower for chelating amines macrocyclic and especially macrobicyclic amines produce complexes with cobalt(II) ion that are stable over a long time. This fact facilitates the study of the reduction of cobalt(III) complexes to cobalt(II) ones. In most cases, the reactions of macrobicyclic ligands do not interfere with this process. 

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