Question 20.6: Platinum(II) forms square planar complexes, and platinum(IV)......
Platinum(II) forms square planar complexes, and platinum(IV) gives octahedral complexes. How many diastereoisomers are possible for each of the following complexes? Draw their structures and identify cis and trans isomers.
(a) [Pt(NH_{3})_{3}Cl]^{+} (b) [Pt(NH_{3})Cl_{5}]^{-} (c) Pt(NH_{3})_{2}Cl(NO_{2})\\
(d) [Pt(NH_{3})_{4}ClBr]^{2+} (e) Co(NH_{3})_{3}Cl_{3}
STRATEGY
Diastereomers are isomers with the same connectivity of atoms, but different spatial arrangements. To identify diastereomers, draw a representation of the molecule and change positions of atoms to create a different spatial arrangement. Two structures may appear different, but they are not isomers if one can be merely rotated to obtain the conformation of the other one. In square planar and octahedral complexes, cis and trans isomerism occurs when two ligands can be arranged with a bond angle of 90° (cis) or 180° (trans). Cis and trans isomers are not possible when only one ligand differs from the others, as in complexes of the type MA_{3}B and MA_{5}B.
(a) No isomers are possible for [Pt(NH_{3})_{3}Cl]^{+}, a square planar complex of the type MA_{3}B.
(b) No isomers are possible for [Pt(NH_{3})Cl_{5}]^{-}, an octahedral complex of the type MA_{5}B.
(c) Cis and trans isomers are possible for Pt(NH_{3})_{2}Cl(NO_{2}), a square planar complex of the type MA_{2}BC. The Cl^{-} and NO_{2} ^{-} ligands can be on either adjacent (90° bond angle) or opposite corners of the square (180° bond angle).
(d) Cis and trans isomers are possible for [Pt(NH_{3})_{4}ClBr]^{2+}, an octahedral complex of the type MA_{4}BC. The Cl^{-} and Br^{-} ligands can be on either adjacent (90° bond angle) or opposite corners of the octahedron (180° bond angle).
(e) Two diastereoisomers are possible for an octahedral complex of the type MA_{3}B_{3}. Isomer (1) shown below has the three Cl^{-} ligands in adjacent positions on one triangular face of the octahedron; isomer (2) has all three Cl^{-} ligands in a plane that contains the Co(III) ion. In isomer (1), all three Cl — Co — Cl bond angles are 90°, whereas in isomer (2) two Cl — Co — Cl angles are 90° and the third is 180°.
