Cobalt asymmetry

The experimental dichroism is seen to have its greatest magnitude some 5 eV above threshold, where 0.10. This corresponds to an asymmetry factor in the forward-backward scattering of y 20%. Such a pronounced PECD asymmetry from a randomly oriented sample looks to comprehensively better the amazingly high 10% chiral asymmetry recorded with highly ordered nanocrystals of tyrosine enantiomer [25] or the spectacular 12.5% asymmetry reported from an oriented single crystal of a cobalt complex [28]. 

The ability of cobalt(II), nickel(II), and copper(II) to exhibit a greater tendency than Zn(II) towards bidentate coordination is further illustrated by structural comparisons within a series of bridging carbonate complexes (188). For example, of the complexes [TpPr 2]M 2(/x-C03) (M = Mn, Fe, Co, Ni, Cu, Zn), only the zinc derivative does not exhibit bidentate coordination at both metal centers (151,153). Furthermore, the carbonate ligand in the complexes [TpPr 2]M 2(/x-C03) (M = Mn, Fe, Co, Ni, Cu) also exhibits varying degrees of asymmetry that closely parallel the series of nitrate complexes described earlier (Fig. 47 and Table IX). 

After the resolution of 1-2-chloro-ammino-diethylenediamino-cobaltie chloride many analogous resolutions of optically active compounds of octahedral symmetry were carried out, and active isomers of substances containing central cobalt, chromium, platinum, rhodium, iron atoms are known. The asymmetry is not confined to ammines alone, but is found in salts of complex type for example, potassium tri-oxalato-chromium, [Cr(Ca04)3]K3, exists in two optically active forms. These forms were separated by Werner2 by means of the base strychnine. More than forty series of compounds possessing octahedral symmetry have been proved to exist in optically active forms, so that the spatial configuration for co-ordination number six is firmly established. 

Optical Activity in the Series.—Another type of isomerism is possible in the series, for the as-diehloro-salts present a case of molecular asymmetry similar to that observed in 1-, 2-dinitro-diethylenediamino-cobalt salts. Two configurations are possible, the one being the mirror image of the other, thus ... 

Isomerism due to Asymmetric Cobalt Atoms.—Werner established his formulas for the cobalt-ammines by proving the fact suggested by his theory that certain of the cobalt atoms in the ammines were centres of asymmetry, and therefore optical activity should be possible. Having established this for some of the simple cobalt-ammines, he then showed that in many of the polynuclear compounds optical activity exists. Thus he prepared optically active isomers of tetraethylenedianaino-... 

One of these is the apparently preferred conformation of pyrophosphate [P207]4-, when chelated to cobalt(III). In that all atoms in this ligand might be considered approximately tetrahedral, a parallel with the conformational properties of a 1,3-propanediamine chelate could be expected with two forms, chair and skew-boat, being produced, but with the former preferred.23,166-168,177 However, the crystal structures of [CoHP207(en)2] (35)231 (phosphorus asymmetry due to proton attachment has been ignored) and [CoHP207(NH3)4] (41)232,233 both show just the skew-boat... 

Previous work in Baldwin s group based around cobalt(I)-mediated cyclization had led to syntheses of (-)-a-kainic acid 2, (+)-allokainic acid 3,34 and acromelic acid A 5.35 A cobaloxime-mediated cyclization of 27 gave the separable pyrrolidines 28 and 29, suitable for conversion to (-)-a-kainic acid 2 and (+)-allokainic acid 3, respectively.34 In this instance, the required stereoisomer 28 for the preparation of (-)-a-kainic acid 2 predominated in a ratio of 28 29,1.7 1 (Scheme 6). Both 28 and 29 were carried through to the respective kainoids 2 and 3. In this case, asymmetry was introduced at a very early stage in the synthesis via a Sharpless asymmetric epoxidation. 

Asymmetry of the Cobalt Atom.—Optically active compounds of cobalt have been produced, indicating that their structure is asymmetric.1... 

Since the copper complexes, [Cu(NN)2]+ and [Cu(NN)(PR3)2]+ (NN = 1,10-phenanthroline, 2,2 -bipyridine, and their derivatives) were applied to stoichiometric and catalytic photoreduction of cobalt(III) complexes [8a,b,e,9a,d], one can expect to perform the asymmetric photoreduction system with the similar copper(l) complexes if the optically active center is introduced into the copper(I) complex. To construct such an asymmetric photoreaction system, we need chiral copper(I) complex. Copper complex, however, takes a four-coordinate structure. This means that the molecular asymmetry around the metal center cannot exist in the copper complex, unlike in six-coordinate octahedral ruthenium(II) complexes. Thus we need to synthesize some chiral ligand in the copper complexes. 

These cobalt(III) complexes have the same charge ( + 3) and essentially the same electronic state to each other, but they differ with respective to size, shape, and stereochemical properties for instance, [Co(en)3]3+ has only one molecular asymmetry about the metal center, but [Co(en)2(R,R-chxn)]3 +, [Co(en)(R,R-chxn)2]3 +, and [Co(R,R-chxn)3]3+ have chiral centers on the ligand besides the chirality about the metal center. In the excited state of [Eu(dpa)3]3" and [Eu(cda)3]6, A form - A form interconversion occurs with the first order rate constants of 15.8 and 29.6 s respectively. These values are much smaller... 

Interesting results on the separation of aromatic hydrocarbons using Werner complexes [82-84] as stationary phases have been described [85, 86], Interesting results on the separation of p- and w-xylene isomers have also been obtained on a column containing M(4-methylpyridine)4 (NCS)2, where M = Ni, Co or Fe. For example, the relative retentions for the above xylene isomers are 2.42 (M = Ni, 80°C), 2.10 (M = Co, 90°C) and 2.50 (M = Fe, 80°C). However, the para-isomer, which is the last to leave the column, forms an asymmetric chromatographic zone, the asymmetry of which increases for stationary phases containing the above metals in the order iron > cobalt > nickel. 

A detailed discussion of the symmetry properties of 0/Co(0001) is difficult since oxygen adsorbs in a disordered state on this surface. In the case of an ordered structure (O/Fe(001)) Huang and Hermanson [66] calculated an induced magnetic moment of oxygen to be about 0.24 Moreover, the photoelectron intensity differences (i.e. the MCDAD asymmetry) in the Co 3d band near the Fermi level decrease after oxygen exposure due to the chemical interaction of oxygen with the topmost cobalt layer. The reduced asymmetry displays a reduced magnetic moment of cobalt. 

In the cobalt induced structure (at a kinetic energy of about 11 eV) the asymmetry decreases without changing sign with increasing exposure. This means that minority electrons dominate directly at the surface for both the bare and the oxygen covered one. Therefore, this behavior is markedly different from iron. 

Ellison MK, Scheldt WR (1998) Tilt/asymmetry in nitrosyi metalloporphyrin complexes the cobalt case. Inorg Chem 37 382-383... 

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