Question 20.5: Consider the following structural representations of the [Co......
Consider the following structural representations of the [Co(NH_{3})_{4}(H_{2}O)Cl]^{2+} ion:
(a) Label the isomers as cis or trans.
(b) Which isomers are identical, and which are different?
STRATEGY
Diastereomers are compounds with the same chemical formula and connectivity of atoms, but different spatial arrangements. To identify diastereiomers evaluate structures to determine if the atoms have different spatial arrangements. Two structures may appear different, but they are not isomers if one can be rotated to turn it into to the other one. In octahedral complexes, cis and trans isomerism occurs when two ligands can be arranged with a bond angle of 90° (cis) or 180° (trans).
(a) In the complex ion [Co(NH_{3})_{4}(H_{2}O)Cl]^{2+} look at the location of the H_{2}O and Cl^{-} ligands. Isomers that have H_{2}O and Cl^{-} on adjacent corners of the octahedron with a bond angle of 90° are cis, whereas those that have H_{2}O and Cl^{-} on opposite corners with a bond angle of 180° are trans. Isomers (1) and (4) have H_{2}O and Cl^{-} on opposite corners of the octahedron and are therefore trans isomers. Isomers (2) and (3) have H_{2}O and Cl^{-} on adjacent corners of the octahedron and are therefore cis isomers.
(b) Because only two isomers are possible for complexes of the type MA_{4}BC, all the cis isomers are identical and all the trans isomers are identical. Structures (1) and (4) are identical, as can be seen by rotating (1) counterclockwise by 90° about the vertical (z) axis:
Structures (2) and (3) are identical, as can be seen by rotating (2) clockwise by 90° about the x axis and then counterclockwise by 90° about the y axis:
