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Stereoisomeric Relationships

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Z. Afrer an aqueous solution containing Co(OH)3 and three equivalents of alanine, H2NCH(CH3)C02H2, is heated four neutral, six-coordinate compounds having the formula {Co[(H2NCH(CH3)CO2]3} can be isolated. Two of these compounds are red and have nearly identical absorption spectra, which suggests they have the same stereochemical arrangement of donor atoms about the cobalt ion. Two of these compounds are red and have nearly identical absorption spectra, which suggests they have the same stereochemical arrangement of donor atoms about the cobalt ion. The two red isomers each exhibit a single meythl resonance in their proton NMR spectra. (The methyl resonance is a doublet as a result of coupling to the methine proton). The other two compounds are violet and have nearly identical absorption spectra again indicating that the stereochemical arrangement of donor atoms about the cobalt ion is the same in both isomers. However, both of the violet compounds exhibit three methyl resonances(als odoublets) in their proton NMR spectra.

a) Structures of two of the four isomers of tris(alaninato)cobalt(III) are shown below. Expand on this generic to indicate the absolute configuration (A or delta) of the metal center, the stereo-chemistry of the donor set (fac or mer) and the absolute configuration (R or S) of the chirality centers in the bidentate ligand.
b) What is the stereoisomeric relationship between these structures, i.e., constitutional isomers, diastereoisomers, enantiomers, etc.
c) Name the two stereoisomers whose structures are not shown in the same rnanner as done in a)?
d) What would be be the name of the enantiomer of L-mer-tris(S-alaninato)colbalt(Ill)?
e) Indicate which isomers have a single methyl doublet in their proton NMR specta and which display three methyl doublets.

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a) The isomers are redrawn below in the same order as given. Facial isomers are when two similar ligands are 90 degrees from each other while in meridian they are 180 from each other. Check this file for explanation of the metal center ...

Solution Summary

The solution discusses stereoisomeric relationships.

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