Question 20.7: Draw the structures of all possible diastereoisomers and ena......
Draw the structures of all possible diastereoisomers and enantiomers of [Co(en)_{2}Cl_{2}]^{+}.
STRATEGY
Because of the relatively short distance between the two N atoms, ethylenediamine always spans adjacent corners of an octahedron (see Figure 20.13). The Cl^{-} ligands, however, can be on either adjacent or opposite corners. Therefore, there are two diastereoisomers, cis and trans. Because the trans isomer has several symmetry planes—one cuts through the Co and the en ligands—it is achiral and has no enantiomers. The cis isomer, however, is chiral and exists as a pair of enantiomers that are nonidentical mirror images.
You can see that the cis enantiomer on the right is not the same as the one on the left if you rotate it by 180° about the vertical N — Co — N axis.
