Both isomers of [(en)(2)Co(mu-NO2)(mu-NH2)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 H-1 and C-13 NMR spectra.