Alfred Werner's inorganic counterparts of racemic and mesomeric tartaric acid: a milestone revisited.

Both isomers of [(en)(2)Co(micro-NO(2))(micro-NH(2))Co(en)(2)](4+) synthesized by Alfred Werner in 1913 have been optically resolved and are therefore the classic inorganic analogues of the sugars threose and erythrose, rather than the rac- and meso-tartaric acids, as Werner believed. The nitro bridge is unsymmetrically N-O bonded, and each dinuclear ion is therefore asymmetric, a fact also clearly apparent in the (1)H and (13)C NMR spectra.