28.4 Stereoisomerism in transition element complexes
- Syllabus
- 9701–2028–2029
- Topic
- 28.4
- Level
- A2
Learning objectives
28.4.1Types of stereoisomerism shown by complexes• Describe the types of stereoisomerism shown by complexes, including those associated with bidentate ligands:- (a) geometrical (cis/trans) isomerism, e.g. square planar such as [Pt(NH₃)₂Cl ₂] and octahedral such as [Co(NH3)4(H2O)2]2+ and [Ni(H2NCH2CH2NH2)2(H2O)2]2+- (b) optical isomerism, e.g. [Ni(H2NCH2CH2NH2)3]2+ and [Ni(H2NCH2CH2NH2)2(H2O)2]2+28.4.2The overall polarity of complexes such• Deduce the overall polarity of complexes such as those described in 28.4.1(a) and 28.4.1(b)
Complexes show cis/trans arrangements and non-superimposable optical pairs
Stereoisomers have the same metal–ligand connectivity but different three-dimensional arrangements. Geometrical isomers differ by whether matching ligands are adjacent (cis) or opposite (trans); optical isomers are non-superimposable mirror images called enantiomers.
| Complex and geometry | Geometrical isomerism | Optical isomerism | Structural test |
|---|---|---|---|
| square-planar [Pt(NH₃)₂Cl₂] | cis and trans | no | Cl ligands 90° apart or 180° apart |
| octahedral [Co(NH₃)₄(H₂O)₂]²⁺ | cis and trans | no | H₂O ligands 90° apart or 180° apart |
| octahedral [Ni(en)₃]²⁺ | no cis/trans pair | yes: two enantiomers | three en chelate rings form left- and right-handed arrangements |
| octahedral [Ni(en)₂(H₂O)₂]²⁺ | cis and trans | cis form has an optical pair | cis chelate arrangement lacks a mirror plane; trans form has symmetry |
Each en ligand, H₂NCH₂CH₂NH₂, uses two nitrogen donor atoms and therefore occupies two adjacent octahedral sites. Keep the two donor atoms joined as one chelate when constructing or comparing structures.
To test a proposed pair: keep the formula and metal–ligand bonds unchanged; place all six octahedral or four square-planar sites; mark cis/trans by 90°/180° separation; for optical isomers reflect the entire chelate arrangement and check that no rotation superimposes it.
A different viewpoint or a freely rotated drawing is not a new stereoisomer. Optical activity requires non-superimposable mirror images, not merely the presence of a bidentate ligand.
Overall polarity is the vector sum of all metal–ligand bond dipoles
Assign each metal–donor bond a dipole direction, place those vectors in the actual three-dimensional geometry, and add them. Equal opposite vectors cancel; any non-zero resultant means the complex has an overall dipole and is polar.
| Complex arrangement | Symmetry / vector result | Overall polarity |
|---|---|---|
| cis-[Pt(NH₃)₂Cl₂] | unlike bond-dipole pairs are adjacent, so they do not cancel | polar |
| trans-[Pt(NH₃)₂Cl₂] | each ligand type lies in an equal opposite pair | non-polar |
| cis-[Co(NH₃)₄(H₂O)₂]²⁺ | the two distinct H₂O directions are adjacent; resultant remains | polar |
| trans-[Co(NH₃)₄(H₂O)₂]²⁺ | opposite matching directions cancel | non-polar |
| cis-[Ni(en)₂(H₂O)₂]²⁺ optical pair | each mirror image has the same non-zero dipole magnitude, reflected in direction | polar |
| trans-[Ni(en)₂(H₂O)₂]²⁺ | symmetric opposite contributions cancel | non-polar |
| either enantiomer of [Ni(en)₃]²⁺ | three identical chelates retain a symmetric zero vector sum | non-polar |
Polarity, ionic charge and chirality answer different questions. Charge is the algebraic total on the complex; polarity is a vector resultant; chirality asks whether the mirror image is superimposable. One does not determine either of the others.
Do not apply the shortcut ‘cis polar, trans non-polar’ without inspecting all ligands and the full geometry. It works for the named matched-pair examples because of their symmetry, not as a universal naming rule.