New pharmaceuticals must be enantiomerically pure for use in human medicine. If you work for a large biotechnology company, you will need to be able to recognize chiral sites in complex molecules. Which of the following complexes are chiral, and which form enantiomeric pairs?

PLAN Draw the mirror image of each complex and mentally rotate it; judge whether any rotation will allow the mirror image to be superimposed on the original molecule. If not, then the complex is chiral. Determine which complexes form enantiomeric pairs by finding pairs in which the two complexes are the nonsuperimposable mirror images of each other. If imagining the three-dimensional structure is difficult, build simple paper models of the complexes.

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(a) (b) (c) (d) = A = B = C = D