Question 20.7: Drawing the Structures of Diastereoisomers Draw the structur......
Drawing the Structures of Diastereoisomers
Draw the structures of all possible diastereoisomers of triamminetrichlorocobalt(III), Co(NH_3)_3Cl_3.
Two diastereoisomers are possible for an octahedral complex of the type MA_3B_3. Isomer (a) shown below has the three Cl^- ligands in adjacent positions on one triangular face of the octahedron; isomer (b) has all three Cl^- ligands in a plane that contains the Co(III) ion. In isomer (a), all three Cl-Co-Cl bond angles are 90°, whereas in isomer (b) two Cl-Co-Cl angles are 90° and the third is 180°. (second photo below)
To convince yourself that only two diastereoisomers of Co(NH_3)_3Cl_3 exist, look back at Figure 20.19 and consider the products obtained from cis- and trans- [Co(NH_3)_4Cl_2]^+ when one of the four NH_3 ligands is replaced by a Cl^-. There are two kinds of NH_3 ligands in cis- [Co(NH_3)_4Cl_2]^+: two NH_3 ligands that are trans to each other and two NH_3 ligands that are trans to a Cl^-. Replacing an NH_3 trans to another NH_3 with Cl^- gives Co(NH_3)_3Cl_3 isomer (a), whereas replacing an NH_3 trans to a Cl^- with another Cl^- gives isomer (b). All four NH_3 ligands in trans- [Co(NH_3)_4Cl_2]^+ are equivalent, and replacing any one of them with a Cl^- gives isomer (b). [If you rotate the resulting Co(NH_3)_3Cl_3 complex, you can see that it is identical to the previous drawing of isomer (b).] Thus, there are only two diastereoisomers for a complex of the type MA_3B_3. (the third photo below)
