A Demonstration of Stereoselectivity Good lecture demonstrations of stereoselective effects are scarce since, in nonhiological systems, such effects are usually small or, a t least, not easily observed. Diastereomeric vanadyl(IV) tartrate complexes, however, exhibit large and highly visible differences and the formation of these complexes provides a striking demonstration of stereoselective factors. Alkaline solutions of 1:l vanadyl(N) tartrates differ markedly in color according to whether dextrorotatory, racemic, or meso-tartrate is present. The complexes found in solutions prepared from dextrorotatory and raeemic tartaric acids are the tartrate-bridged binuelear anions [(VO)z(D- tart)^]' (violet) and [(VO)n(D-tart) (L-tart)]*- (red-brown), respectively.' In these formulas, "tart" designates a tetranegative tartrate ligand. The species present in alkaline vanadyl(N) meso-tartrate solution are uncertain but they are apparently not binuelear-bridged c o m p l e ~ e s The . ~ differences in stabilities among these diastereomers can be explained by arguments based on ligand conf~rmation.~, The formation of the vanadyl(1V) tartrate complexes is easily demonstrated. Solutions contamng 1 g VOSOI.2Hz0 (0.005 mole) and 0.75 d D-, DL-, or meso-tartaric acid (0.005 mole) in 800 ml water are prepared. The diasterwmeric complexes form when the solutions are made basic (pH 8-9) with aqueous ammonia. The acidic solutions are indefinitely stable; however, a t higher pH, air oxidation of V ( N ) to V(V) occurs in a few hours. An interesting reaction occurs when equimolar alkaline solutions of vanadyl(1V) D- and L-tartrate are mixed. The color of the mixture changes from violet t o red-brawn in about a minute as the ligand exchange reaction [(V0)2(Dtart)*]'- + [(VO)Z(L-tart)z] 2[(V0)2 (D-tart) (L-tart)]'. proceeds. This reaction provides a n interesting sample of a chemical conversion which is partially entropy driven. The mixed-ligand racemic complex has a statistical entropy contribution to its stahilitv of Rln 2.3
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'Tapseott, R. E., and Belford, R. L., Inorg. Chem., 6,735 (1967).
2Tapscott, R. E., Belford, R. L., and Paul, I. C., Coord. Chem. Reu., 4,323 (1969). Marcus, Y., and Eliezer, I., Coord. Chem. Reu., 4,273 (1969) Robert E. Tapscott
University of New Mexico Albuquerque, 87131
586
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Journal of Chemical Education
