Optically active coordination compounds

Alfred Werner s research on optically active coordination compounds. G. B. Kauffman, Coord. Chem. Rev., 1974,12, 105-149 (376). 

Optically active coordination compounds. S. Kirschner, Prep. Inorg. React., 1964,1,29-57 (140). 

The resolution of optically active coordination compounds, a feat which shook chemistry to its innermost foundations ,72 gained for the coordination theory the widespread recognition for which Werner had been striving for so long. Nor was the theory s founder neglected, for two years later, largely in recognition of the most brilliant confirmation of [his] stereochemical views ,73 Werner was awarded the Nobel Prize in chemistry for 1913.74... 

Note. Inasmuch as marked rotary dispersion is encountered with many optically active coordination compounds, it is advisable to designate the wave length of light at which the rotation was measured. This is especially important because the sign of rotation actually reverses within the visible spectrum for many coordination compounds. 

I would suggest that the formation of metal chelate complexes, with a four or six-coordinate metal partly bound to an optically active protein and partly bound to a substrate molecule can explain this stereospecificity. The optically active coordination compounds of metals, such as cobalt, have extraordinarily high molecular rotation, and so the difference in chelation powers of the d and I forms of a substrate may be very great. As Dr. Chaberek has pointed out (Lecture 33), this chelation may involve both metals of constant valency, e.g.. Mg, Zn, and those of variable valency. Metallic ions of both types are proven essential trace metals in biological systems. 

Many optically-active organic molecules are present in plants and animals, and they can often be isolated and obtained in a pure form. In recent years, considerable success has been achieved in the selective synthesis of individual isomers. However, laboratory preparations of compounds that can exhibit optical activity normally yield 50-50 (racemic) mixtures of the two optical isomers and hence produce an optically-inactive material (Section 3.4). Therefore, the basic step in the laboratory preparation of an optically-active coordination compound is separation from its optical isomer. For example, racemic [Co(en)3] is readily prepared by the air oxidation of a cobalt(II) salt... 

Octahedral coordination compounds having the general formula ML2X2, where L is a bidentate ligand and X is a halide, will also exhibit optical isomerism. Werner and his students were among the first chemists to synthesize an optically-active coordination compound. Specifically, Werner was able to resolve the optical isomers of ds-[CoCI(NH3)(en)2] , which are shown in Figure 15.17, by precipitation with silver d-a-bromocamphor- r-sulfonate. 

Figure 18. Victor L. King (1886—1958), codiscoverer in 1911 with Alfred Werner of optically active coordination compounds, Universitat Zurich, 1910. (Courtesy of Victor R. King.) (Reproduced from reference 19, p 132. Copyright 1975 American Chemical Society.)...

Figure 19. The polarimeter used by Werner to measure the rotation of optically active coordination compounds (Franz Schmidt Haensch model no. 8142). (Reproduced with permission from reference 10, facing p 178. Copyright 1968 Dover Publications.)...

Kauffinan, G. B. "The Discovery of Optically Active Coordination Compounds—A Milestone in Stereochemistry, Isis 1975.65,38. 

In a classic paper on optically active coordination compounds, Mills and Quibell reported the synthesis of a Pt complex containing one molecule of meso-stilbenediamine (H2N-CHPh-CHPh-NH2) and one molecule of isobutylenedi-amine (H2N-CH2-CMe2-NH2). Also, the complex was successfully resolved into its enantiomers. Determine whether the coordination around the metal ion is tetrahedral or square planar. Also illustrate these modes pictorially. 

The mechanisms involved in the racemization of optically active coordination compounds (mainly tris(chelates) such as [Cr(ox)3] ) have been discussed. Kinetic data for the thermal degradation of [M(LL)3]- H20 (M = Al, Cr, Fe LL = lapachol) to the metal oxides have been reported. ... 

Although reactions of coordination compounds in the solid state have been known since the time of Werner, detailed studies of the kinetics and mechanism of such reactions remain limited. A recent review discusses mechanisms for the racemization of optically active coordination compounds in the solid state and should help to revive interest in this area. 

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