Optical isomerism With asymmetric nitrogen

Iodinated contrast agents with polyhydroxylated carbon side-chains contain a number of asymmetric carbon atoms yielding numerous optical isomers which relate to each other as enantiomers or diastereoisomers. Sterically hindered non-asymmetric carbon or nitrogen atoms might result in additional asymmetry centres while the partial double bond character of the acyl-carbon-nitrogen bond of amide functions can lead to cisitrans isomerism. Such isomers are labelled rotamers when heating in solution is able to modify their ratio. Isomerism of iodixanol has been described by Priebe et al. [122], Fossheim et al. [123] and by Molander et al. [115]. 

Optical activity may be achieved in at least three ways (a) by the asymmetric arrangement of ligands about the central metal ion (b) by the use of a ligand which is itself optically active and (c) by the use of a ligand in which an atom (or atoms) becomes chiral upon attachment to the metal. The second of these is illustrated by propylenediamine and the tetradentate (7) and the last by (8) and by (9). Neither the nitrogen atoms nor the arsenic atoms in these last two ligand molecules are chiral, but the ones near the centers of their respective molecules become so when they are coordinated to a metal ion. In each case, they can be of the same or of opposite chirality. Thus complexes with a six-coordinate metal can theoretically exist in a large number of isomeric forms these differ in stability and not all of them have been isolated. 

---