Octahedral complexes absolute configuration

There are more examples of a second type in which the chirality of the metal center is the result of the coordination of polydentate ligands. The easiest case is that of octahedral complexes with at least two achiral bidentate ligands coordinated to the metal ion. The prototype complex with chirality exclusively at the metal site is the octahedral tris-diimine ruthenium complex [Ru(diimine)3 with diimine = bipyridine or phenanthroline. As shown in Fig. 2 such a complex can exist in two enantiomeric forms named A and A [6,7]. The bidentate ligands are achiral and the stereoisomery results from the hehcal chirality of the coordination and the propeller shape of the complex. The absolute configuration is related to the handness of the hehx formed by the hgands when rotated... 

The discovery by Bijvoet (22) of the method of anomalous dispersion of X-rays gave a means of establishing the absolute configurations of particular enantiomers, and, in 1955, the first report of the absolute stereochemistry of an octahedral complex appeared. This concerned the important ion (+)-[Co(en)3]3+ which had the configuration (10) shown in (II). 

Figure 1-19 Trischelate octahedral complexes (actual symmetry D ) showing how the absolute configurations A and A are defined according to the translation (twist) of the helices.

The absolute configurations of some octahedral complexes can be described using either the skew-line reference system (Section IR-9.3.4.11) or the C/A system. The first is used more commonly, but the C/A system is more general and may be used for most complexes. The skew-line reference system is only applicable to tris(bidentate), bis(bidentate) and closely related systems. 

Such an interpretation is also supported by the 13C-NMR data. An ortho, para electronic shift of this type has recently been proposed to explain the optical spectral differences between free and metal-bound phenoxides in the transferrins (67). In enterobactin, however, this tautomerism is of fundamental conformational importance as it lowers the energy barrier to rotate the amide bond to —47° from the planar trans configuration (Fig. 6), as is required in order to form a mononuclear octahedral complex. Upon reorientation, the amide proton becomes sterically more shielded from interaction with the solvent and this is reflected in its temperature coefficient which (in absolute value) drops from 4.7 X 10-3 ppm/deg in enterobactin to 1.3 X 10-3 ppm/deg in the Ga3+ chelate. Conversely, the driving force to form the mononuclear complex will favor the electronic delocalization implied in the above tautomerism. 

Stereospecific formation of the chiral heterodinuclear 3d-4f complexes (A-A)-[(acac)2Cr(Ox)Dy(Tp)2] (where (A-A) means the absolute configuration around the octahedral Cr and square antiprismatic Ln moiety, respectively) is reported.634... 

In fact, chiral ligands are often used in synthesizing molecules of this type, both for observing sieric effects and for aid in solving the X-ray crystal structures. See the discussion of absolute configuration of the previous complex with C.N. = 5 and octahedral complexes with C.N. = 6 (page 495). 

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