Cobalt complexes, optical isomers

Fanali, S., Ossicini, L., Foret, F., and Bocek, R, Resolution of optical isomers by capillary zone electrophoresis study of enantiomeric and distereoisomeric cobalt (III) complexes with ethylenediamine and amino acid ligands, /. Microcol. Sep., 1, 190, 1989. 

The importance of the theory was further demonstrated by the discovery of the existence of optically active inorganic compounds, and the isolation of the exact number of optical isomers theoretically possible for the spatial arrangement of the atoms.1 Friend 2 and others criticised the theory on the grounds that in simple compounds, such as sodium chloride or cobaltous chloride, the chlorine is ionised and yet is attached to sodium or cobalt atom directly, whereas in the ammino-coinpounds the acid capable of ionisation is that which is not directly attached to metal. For instance, in chloro-pentammino-cobaltic chloride, [CoCI(NH3)5]C12, it is the chlorine outside the first zone which is ionised in solution. Also, the dissociable acidic groups are not attached to any point within the complex, but simply hover round the central complex in an indefinite manner. Thus a definite valency for ionisable... 

Co(en)33+ salts are usually prepared, but they can be resolved (i.e., separated from one another) by fractional crystallization of a salt of Co(en)33+ from solution with an anion that is itself an optical isomer, for example, the d-tartrate anion. The fact that optical isomers of Co(en)33+ and other chelates do exist was used by Werner as incontrovertible proof of his theory of octahedral coordination in cobalt(III) and other complexes. 

That chromium forms cPsp8 hyperligating bonds with oxalate in the ion [Cr(Ci04)j]+++ has been inferred by C. H. Johnson (Trans. Faraday Soc. 28, 845) [1932]) from the following facts The chromium trioxalate complex and the cobaltic trioxalate complex can be resolved into optical isomers, whereas... 

It must be concluded, therefore, that the ligands do not become completely detached from the metal ion in isomerization reactions. Comparable results have been observed in the isomerization95 of potassium diaquodioxalatochromium(III) and the racemization96 of optically active potassium tris(oxalato)chromium(III) when no exchange with free ligand in solution occurs. Thus, although it is not practicable to take advantage of the desirable properties of individual isomers of 2 1 chromium and cobalt complexes of tridentate azo compounds because of the facility with which such compounds isomerize in solution, the technically important unsymmetrical 2 1 complexes are capable of practical application because they show little or no tendency to disproportionate in solution. 

Fundamentally, optical isomers of the spiroheterocyclic cobalt complex must exist. Since these compounds could not be separated, we tried to synthesize the dicyano derivative under CO substitution by reaction with KCN in liquid NH3 in the hope that a separation into the optical antipodes might be possible. At 120°C, however, the reaction gave, besides K2[Co(NO)(CO)(CN)2], a complex in which the C(CH2PPh2)4 ligand is tridentate (139) ... 

The optical isomers of cis-Cr(benz)3 are quite stable in dry chloroform in the dark, yet isomerize rapidly in methanolic solution. The trans-isomer is moderately stable in alcohols, with a half-life for racemization of a few days. Due to the instability with respect to decomposition of the neutral tris(benzohydrox-amato)cobalt(III) complex, only the CD spectrum of the anionic species has been recorded (Figure 7). It resembles that of the cis-Cr(hydroxamate)3 complexes. 

Draw structures of all the geometric and optical isomers of each of the following cobalt complexes ... 

The stereochemistry of cobalt(III) complexes with 0,N,0-terdentate ligands of the linear type was studied by Okamoto et al. In a [Co(0,N,0-terdentate)2]-type complex four isomers,/ac-/rans(N), mer-trans(ti), A-c/ (N) and A-cis(N), are shown in Fig. 3.7. When L-alanine-N-monoacetic acid (L-H2alama) participates in coordination, the nitrogen donor atom of the L-alama is optically rtivated, and hence three isomers, RR, RS, and SS, are possible for each of the above rrans(N) and cis(N) forms. 

Yoshikawa and Yamasaki have reviewed the resolution of optical isomers and the separation of geometrical isomers of cobalt(III) complexes on Sephadex ion-exchangers , including the experimental techniques. 

One geometrical isomer of the cobalt (iii) complex of ethylenediamine-iViV -diacetate-iViV -dipropionate (edda) has been prepared by oxidation of C0CI2 with H2O2 in the presence of edda. It has been resolved into optical isomers and spectral evidence suggests that its structure is (107). The structure of K[ds-Co(ida)2],2.5H20 (ida = iminodiacetate) has been determined. Cobalt(iii) complexes of (S)-azetH (108) have been prepared. One isomer of... 

In 1897 Werner recognized that a compound such as [Co(C204)(en)2] should be resolvable into optical isomers, and he discussed this in a publication in 1899 (3). He made many attempts to resolve cobalt complexes. His American student Victor L. King carried out some 2000 fractional crystallizations without resolution. Finally, the resolution of cis -[CoCl(en)2(NH3)] and cis -[CoBr(en)2(NH3)] + was successful in... 

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