Cobalt complexes geometrical isomers

Geometrical isomerism Geometrical isomerism is possible only in hexacoordinate complexes and in the case of 2 1 metal, e.g. chromium and cobalt, complexes arises from coordination of the ligand in a meridional (81) or a facial (82) mode in an octahedral complex. In the former case only an enantiomorphic pair of isomers is possible, but in the latter the possibility exists of four enantiomorphic pairs and a centrosymmetric isomer (Figure 1). 

One of the classical tetradentate Schiff bases is the open-chain N2O2 donor ligand salen (137), which has flexibihty sufficient to permit it to form different geometric isomers on complexation. The ability of the ubiquitous salen and close analogues to bind O2 reversibly as its cobalt complex, as in [(02)Co(salen)(py)], has been central to most investigations of its coordination chemistry. Complexes of the [Co(salen)] family also mediate the electrocatalytic reduction of O2 to H2O2, whereas those incorporating chiral centers on... 

HPLC separation of geometrical isomers of / -diketonates was also described. The retention of the fac- and wter-isomers of chromium(in) and cobalt(III) complexes with eight asymmetric / -diketonates was studied iu the 0DS-(Me0H-H20) system. The reteutiou order for two geometrical isomers depends on the structure of the complexing S-diketonates. Thus, the retention order/ac < mer was found for complexes of the fi-diketonates which possess a fluorinated functional group in each molecule. The reverse... 

One of the earliest cases of stereoisomerism among inner complexes, tris(glycinato)cobalt(III), occurs in two forms, violet (a) and red (P). In one of these geometric isomers, all of the amino groups of the glycine molecules are adjacent (fac), while in the other, two of these occupy opposite positions mer). Absorption spectra indicate that the more soluble violet a form is the mer isomer. " ... 

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

It was mentioned above that tris(chelate) complexes of the type [Co(en)jp+ lack an improper axis of rotation. As a result, such complexes can exist in either of two enantiomeric forms (or a racemic mixture of the two). Figure 12.20 illustrates the complex ions [Co(en)3]3+ and [Cr(ox)3]3-, each of which is chiral with D3 symmetry. It is not necessary to have three chelate rings present. The cation dichloro-bis(ethylenediamine)cobalt(lIl) exists as two geometric isomers, cis and trans. The trans isomer has approximate Dlh symmetry (Fig. 12.21b). Because it has three internal mirror planes, it is achiral.40 The cis isomer has C2 symmetry and is chiral (Fig. I2.2la). Since the two chloride ions replace two nitrogen atoms from an eth-... 

The ion-exchange chromatography was first applied to the separation of geometrical isomers of a cobalt(III) complex by King and Walters who separated tram- and cw-[Co(N02)2(NH3)4]. From the cation-exchange resin (Amberlite IR-120), 1 mol/ dm NaCl eluteJ the fra/w-isomer and then the c -isomer was eluted with 3 mol/dm NaCl. This difference was attributed to that the m-isomer was more firmly held because of its larger dipole moment. 

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. 

Chromatography with D(-l-)-lactose hydrate columns was used in the partial resolution of tris(acetylacetonato)cobalt II) complex partial resolution of [Co(acac)3], and mer- and c-[Co(bzac)3] (bzac = benzoylacetonate) is also effected by column chromatography on (H-)-lactos . The [Co(bzac)3] complex prepared from Na3[Co(C03)3] 3 H2O was separated into its mer and fac geometrical isomers by column chromatography on Florisil (Florodin Co., suj rior to alumina), prior to the resolution. For the resolution, (-l-)-lactose dried at 110 °C and then sieved to 100 mesh, and a chromatographic column (3.9 x 230 cm), which narrowed to 3.0 cm at... 

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... 

Pt(NH3)2Cl2], for example, the chloro ligands can be either adjacent to or opposite each other ( Figure 23.21). (We saw an earlier example of this type of isomerism in the cobalt complex of Figure 23.8, and we will return to that complex in a moment.) This form of stereoisomerism, in which the arrangement of the atoms is different but the same bonds are present, is called geometric isomerism. The isomer on the left in... 

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