Complexes cobalt-cyano

Fig. 8-37. Determination of the cobalt-cyano complex in a gold plating bath. — Separator column IonPac NS1 eluent 0.002 mol/L TPAOH/acetonitrile (90 10 v/v) flow rate 1 mL/ min detection suppressed conductivity injection 50 uL sample (1 400 diluted).

In isolation the electron distribution in the trivalent chromium (III) ion consists of three unpaired electrons in the d shell, as indicated in line (a) of Table 5.1. In line (b) the six electron pairs donated to the central chromium atom by oxygen atoms of water molecules give rise to sp3d2 hybridisation. This is characteristic of an octahedral structure. A similar situation arises with the trivalent cobalt(III) complex in line (e), where each of the three t2g levels is doubly occupied by an electron pair from each cyano ligand. 

The above examples illustrate the principal methods used in the syntheis of cyano complexes (especially in recent work), but many others have found occasional use, and details of them may be found in the reviews cited. The recent appearance of a comprehensive article24 on the preparation of the numerous mononuclear substituted cyano complexes of cobalt(lll), in which both the rationale and the details of the methods employed are discussed, is especially noteworthy. 

Formation of Ni(CO)4 or Co(CO)4- by the cyanide method depends upon the stepwise substitution of the anion of the cyano complex by the iso-electronic carbon monoxide molecule. By treating Co2(CO)8 with potassium cyanide we obtained cyanocarbonyls of cobalt of low oxidation number (83). In reactions of the nitrosyl carbonyls of iron and cobalt, Behrens (86) substituted all the CO groups with CN to give K3[Co(NO)(CN)3] or... 

Recently it has been found that the cyano complex of Co(II) may be converted (by the action of such powerful reducing agents as potassium amalgam or NaBH4) to the ion HCo(CN)73. The cobalt in this complex... 

Two sorts of metal—cyano complexes have been studied. The first contains an atom of zinc, cadmium, mercury or copper as the central atom, and the nitrogen quadrupole coupling is only a little higher than it is in nitriles (4.0—4.2 MHz against 3.8—4.0 MHz). In the second sort, the central atom is either platinum or cobalt and the observed couplings are markedly lower (3.47—3.68 MHz). 

The cyano complexes of cobalt(—I) are restricted to nitrosyl and carbonylnitrosyl mixed ligand complexes, all of which obey the 18-electron rule. 

Bis(camphorquinone-a-dioximato)cobalt(II) (10) has been developed as a catalyst for enan-tioselective cyclopropanation reactions. It allows selective carbene transfer from diazoacetic esters to terminal C-C double bonds which are in conjugation with vinyl, aryl, alkoxycarbonyl or cyano groups, but not to alkyl-substituted alkenes, cycloalkenes, 1,3-dienes and al-lenes. The unusual chemoselectivity and some other experimental observations make the two mechanistic pathways proposed vide supra) questionable for these special carbene-transfer reactions. In contrast, the cobalt(II) complex 11 allows not only the cyclopropanation of styrene but also of oct-l-ene, a nonactivated alkene (ethyl diazoacetate, 35 °C, 3mol% of catalyst yield 50-60%). ... 

The ability of chiral bis(camphorquinone-a-dioximato)cobalt(Il) complexes (Section 1.2.1.2.4.2.6.3.1.) to catalyze carbene transfer from diazocarbonyl compounds (diazoacetic esters, 2-diazo-l-phenylethan-l-one) to terminal alkenes conjugated with vinyl, aryl, carbonyl, and cyano groups, has already been mentioned. The ee-values are 75-88 /o at best, often lower. The highest values are again obtained with bulky diazoacetic esters. The significance of these catalysts, however, is their ability to promote cyclopropanation of electron-deficient alkenes, such as acrylates and acrylonitriles, in contrast to the rhodium and copper catalysts discussed above. 

Brink NG, Kuehl FA and Folkers K (1950). Vitamin Bi2 The identification of vitamin B12 as a cyano-cobalt coordination complex. Science, 112, 354. 

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