Experimental Separation and Identification of Isomers

Separation of chiral isomers requires chiral counterions. Cations are frequently resolved by using the anions z -tartrate, antimony d-tartrate, and a-bromocamphor-iT -sulfonate anionic complexes are resolved by the bases brucine or strychnine or by using resolved cationic complexes such as [Rh(en)3] . In the case of compounds that racemize at appreciable rates, adding a chiral counterion may shift the equilibrium even if it does not precipitate one form. Apparently, interactions between the ions in solution are sufficient to stabilize one form over the other. 

The best method of identifying isomers, when crystallization allows it, is X-ray crystallography. Current methods allow for the rapid determination of the absolute configuration at costs that compare favorably with other, more indirect methods, and in many cases new compounds are routinely examined this way. 

Gillard, D. J. Shepherd, andD. A. Tarr, J. Chem. Soc., Dalton Trans., 1976, 594. 

Bailar, ed., Chemistry of the Coordination Compounds, Reinhold Publishing, New York, 1956, pp. 334-335, cites several instances, specifically [Feiphen) ] (Dwyer), [Cr(C204)3] (King), and [Co(en)3] (Jonassen, Bailar, and Huffmann). 

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