Octahedral complexes optically active ligands

Optical activity two forms of an octahedral complex. The bidentate ligands are represented by a... 

A molecule is optically active when it cannot be superimposed on its mirror image. Although this condition is met by an octahedral complex such as MLaLbLgLdLgLf it is rare indeed to be able to resolve such a complex. In practice, optical activity is largely confined to octahedral complexes of chelating ligands. Optical activity has also been observed for chelated tetrahedral and square planar complexes but only rarely. It is necessary for the chelated complex to be stable kinetically to permit resolution, it must retain its configuration for at least a matter of minutes. This confines attention to complexes of a few ions, of which cobalt(III), chromium(III)... 

Octahedral Complexes Containing Optically Active Ligands... 

Optical activity in metal complexes may also arise either if one of the ligands bound to the metal in the first co-ordination sphere is itself optically active or if the complex as a whole lacks a centre of inversion and a plane of symmetry. Thus all octahedral cts-complexes of the tris-or bis-chelate type have two isomeric forms related by a mirror plane, the d- and /-forms. These species have circular dichroism spectra of identical intensities but opposite in sign. The bands in the circular dichroism spectrum are, of course, modified if ligand exchange occurs but they are also exceedingly sensitive to the environment beyond the first co-ordination sphere. This effect has been used to obtain association constants for ion-pair formation. There also exists the possibility that, if such compounds display anti-tumour activity, only one of the mirror isomers will be effective. 

In describing a stereoisomer, it is perhaps most important initially to define whether or not it is chiral. The origins of chirality (optical activity) in coordination compounds and important experimental results have been recently reviewed.112,113,121,122 The classical example of chirality or enantiomerism in coordination chemistry is that of octahedral complexes of the type [M-(bidentate)3]. These exist in the propeller-like,123 non-superimposable, mirror-image forms (13a) and (13b). Synthesis of this type of complex from M and the bidentate ligand in an achiral environment such as water results in an equimolar mixture of the two stereoisomers. The product... 

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. 

Oxalate (ox, 204 ) complexes of Cr° have been known since the very beginning of coordination chemistry. Thus, the resolution of chiral [Cr(ox)3] with strychninium counterion by Werner in 1912 prodnced the first optically active anionic coordination compound. There also exists a series of bis(oxalato) complexes of the type [Cr(ox)2X2] + , where X can be any of a variety of nentral donors (e g. H2O, NH3, etc.) or anionic ligands (e g. SCN, N3, etc.). These compounds have been used to study the mechanism of cis/trans isomerization and racemization of optically active octahedral coordination compounds. 

Werner must have felt satisfied that the octahedral arrangement of ligands in 6-coordinate complexes was firmly established, but some critics objected because the resolved complexes contained carbon, and optically active carbon compounds were well known. Werner silenced this objection by preparing and resolving a completely inorganic complex (71), [Co (OH)2Co(NH3)4 3] , in which the chelate ligands around the central Co(III) are the complex ions c2s-[Co(NH3)4(OH)2]. With this accomplishment, the major points of Werner s coordination theory for 6-coordi-nate complexes were firmly established long before modem structural methods were available. 

Although the resolution into their enantiomeric forms of octahedral metal complexes bearing bidentate ligands was accomplished at an early stage,19 the application of such materials for the preparation even of dinuclear complexes was greatly delayed.20 Only recently has the preparation of a particular stereoisomer of a diruthenium complex been accomplished by the resolution of the bis-o-phenanthroline complex (9). The subsequent use of the optically active material in a stereoselective reaction with 2,5-bis(2-pyridyl)pyrazine (10) yielded the dinuclear com-... 

In the ligand polarization mechanism for optical activity, the potential of the electric hexadecapole component, Hxy(x>-y>), produces a determinate correlation of the induced electric dipole moment in each ligand group which does not lie in an octahedral symmetry plane of the [Co Ng] chromophore (Fig. 8). The resultant first-order electric dipole transition moment has a non-vanishing component collinear with the zero-order magnetic moment of the dxy dxj yj transition in chiral complexes, and the scalar product of these two moments affords the z-component of the rotational strength, RJg, of the Aj -> Ti octahedral excitation. 

So far, we have introduced the concept of optical activity (or chirality) in a complex arising as a result of the way ligands are arranged around the central metal. For six-coordination octahedral, there are actually several ways in which dissymmetry (chirality) can arise, namely from ... 

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