Metal coordination polyhedron, inversion

Inversion of the Metal Coordination Polyhedron along the O-M-O Axis... 

The inversion of the N-atoms in the diethylenetriamine backbone is precluded. The central N-atom is always chiral, since the ethylene bridges around it have different conformations (<5 and A). Furthermore, the two other N-atoms are chiral as well in the bisamides and chirality of the metal ion should be taken into account. DTPA derivatives with three chiral N-atoms can be wrapped around the Ln3 + ion in 16 different ways (8 diastereomeric pairs, see Fig. 1). In crystal structures of these compounds the Ln3+ coordination polyhedron can generally be best described as a tricapped trigonal prism (TTP). For [Ln(DTPA)]2-A1=A2=A3 = B1=B2, and, consequently, only two enantiomers remain (1/1 ). Analogously, it can be seen that the DTPA-bisamides can have 4 diastereomeric pairs. 

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