Antiprisms twisted capped

Upon complexation with a lanthanide ion, these complexes may form square antiprism or twisted square antiprism (TS APR) structures with a vacant coordination site in the cap position, which is assumed to be occupied by a solvent molecule. Just as in the chelated complexes described previously, two distinct types of chiral stereochemistry are present. In analogy with OC-6 species, the sense of rotation of the pendant arms is denoted as A or A depending upon if the arms rotate clockwise (A) or counterclockwise (A) as one proceeds down the direction of the C4 axis. There is also chirality (or helicity) associated with the nonplanar 12-membered ring. If one looks along the skew-line connecting the coordinated nitrogens, the carbon atoms... 

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