Cobalt complex compounds isomers

Extension of the NMR studies described in Section 58.2.3.3(i)(b) to the compounds (136 R = N02 and R = Bu ) led the authors to conclude that one Na- and two No-coordinated cobalt complexes were produced. These were assigned the partial structures (137), (138) and (139), which involve coordination of the ligand in its azo form and in the two alternative hydrazone forms. In general (Section 58.2.3.2), azophenols exist exclusively in the azo form and serious doubts must exist regarding structure (139). Similarly, in view of the reported equilibration between the various isomers in solution, assignment of discrete tautomeric structures must be open to question. As-... 

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) ... 

Ethoxy(l-alkynyl)carbene complex la forms a stable cobalt complex 182, whose structure has been elucidated by X-ray analysis.1453 11 Cobalt complexes of similar type have been derived from 0-allyl-147 and iV-allyl(l-alkynyl)carbene complexes 135 (Scheme 53).145 In contrast to thermolysis of compounds 135 (Scheme 53), thermolysis of compound 182 affords isomers 183 (structure based on X-ray analysis) and 184 (structure based on spectroscopic evidence only) in 48% total yield (Scheme 76). 

In these syntheses, Peppard had to rely on the very weak trans effect in the cobalt complex [Co(NH3) i (SO3) 2 ]-. The cis-dichloro-cis-diammineethylenediaminecobalt(III) ion is asymmetric, and Peppard partially resolved it into its enantiomeric forms by adsorption on quartz ground to 100 mesh. This method of resolution is frequently, but not always, effective. Before it can be fully utilized, we will need to learn a great deal more about the principles of adsorption. The preparation of Peppard s isomer s is particularly interesting because cobalt(III) complexes rearrange easily, whereas the platinum compounds do not. 

Phenylethynylfluorocyclotriphosphazenes (193) and (195) react readily with Co2(CO)g to form the corresponding cobalt complexes. In contrast to the inseparable isomer mixture of (195), the isomers gcw-(196), cj5-(197) and transit) could be separated by chromatography. Electrochemical experiments showed that reduction of compound (194) occurs via a reversible one-electron transition, whereas for gem-(196) there were two independent one-electron reductions. ESR spectra of the radical anions point to localization of the unpaired electron on the organometallic moiety. ... 

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... 

No cases of cis/trans isomerism have yet been reported among organo-cobalt(III) complexes, even among compounds not covered by this review. Complexes such as [R2Co(bipy)2] all have the cis configuration steric hindrance would be greatly increased by the coplanarity of the two bipyridyl ligands in the trans isomer 123). 

The simultaneous observation of the two EPR spectra has been reported in particular for several tris(dithiocarbamato)iron(III) complexes [Fe(R2NC(S)S)3] where R = cyclohexyl [143], hydroxyethyl [144], and n-butyl [145, 146]. In addition, a considerable number of iron(III) complexes of the type [Fe" -N402] has been found which show EPR spectra of both the HS and LS isomers. These comprise [Fe(X-SalEen)2] Y2 where X-SalEen is the Schiff-base ligand obtained by condensation of X-substituted salicylaldehyde and IV-ethylethylenediamine [147] and similar compounds [100, 148, 149, 150, 151]. For the cobalt(II) complex [Co(terpy)2] (004)2, it is not completely clear whether the two observed EPR spectra are due to HS and LS states related by a spin-state transformation [152]. 

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