Stereoisomerism in Transition Element Complexes
Quick Notes
- Geometrical isomerism (cis/trans) occurs in square planar and octahedral complexes.
- Square planar:
cis = same side, trans = opposite.
- Octahedral:
cis = H2O ligands next to each other, trans = opposite.
- Square planar:
- Optical isomerism occurs in complexes with bidentate ligands forming non-superimposable mirror images.
- Examples: [Ni(en)3]2+ and [Ni(en)2(H2O)2]2+
- Polarity of a complex depends on ligand arrangement:
- Symmetrical (trans) complexes are often non-polar.
- Asymmetrical (cis) complexes may be polar due to uneven distribution of dipoles.
Full Notes
Stereoisomers have the same formula and connectivity but differ in the spatial arrangement of their atoms.
Transition metal complexes can show two main types, Geometrical Isomerism and Optical Isomerism.
Geometrical (Cis-Trans) Isomerism
Geometrical isomerism occurs when two or more ligands can occupy different positions around a central metal. This can happen in both octahedral and square planar complexes.
Square Planar Complexes
Common with Pt(II) and Ni(II) complexes (coordination number 4):
Example [Pt(NH3)2Cl2]
![CIE A-Level Chemistry cis and trans geometrical isomers of [Pt(NH3)2Cl2] square planar complex.](images/transcis.png)
- Cis-isomer: NH3 ligands or Cl⁻ ligands next to each other.
- Trans-isomer: NH3 and Cl⁻ ligands opposite each other.
Octahedral Complexes
Common with coordination number 6:
Example [Co(NH3)4(H2O)2]2+
![CIE A-Level Chemistry cis and trans geometrical isomers of [Co(NH3)4(H2O)2]2+ octahedral complex.](images/octahedralcomplexcopy.png)
- Cis-isomer: Two H2O molecules adjacent (90° apart).
- Trans-isomer: Two H2O molecules opposite (180° apart).
Example [Ni(H2NCH2CH2NH2)2(H2O)2]2+
![CIE A-Level Chemistry cis and trans geometrical isomers of [Ni(en)2(H2O)2]2+ octahedral complex.](images/nickelisomers.png)
- Cis = two H2O on same side
- Trans = two H2O on opposite sides
Optical Isomerism
Optical isomerism occurs when a complex can exist as non-superimposable mirror images (called enantiomers).
This type of isomerism is most common in octahedral complexes with bidentate ligands.
Example [Ni(H2NCH2CH2NH2)3]2+
![CIE A-Level Chemistry optical isomers of [Ni(en)3]2+ complex.](images/nickelopticals.png)
- Contains three bidentate ligands (en), forming a 3D structure.
- Two optical isomers: mirror images that cannot be superimposed.
- They rotate plane-polarised light in opposite directions.
Example [Ni(H2NCH2CH2NH2)2(H2O)2]2+
![CIE A-Level Chemistry optical isomers of [Ni(en)2(H2O)2]2+ octahedral complex.](images/bidentateisomers.png)
- Two bidentate ligands and two monodentate ligands.
- Still capable of forming optical isomers depending on spatial arrangement.
Predicting Polarity of Complexes
The overall polarity of a complex ion depends on the arrangement of ligands:
- Trans-isomers are symmetrical and any dipoles cancel out = non-polar complex.
- Cis-isomers aren’t symmetrical, dipoles don’t cancel out = polar complex.
Polarity affects properties such as solubility and dipole interactions (this is especially important in biological systems).
Summary
- Transition metal complexes can show geometrical and optical isomerism.
- Cis/trans isomerism is based on ligand positioning.
- Optical isomerism arises with asymmetrical 3D structures using bidentate ligands.
- Polarity depends on symmetry where trans is usually non-polar and cis is often polar.
- These forms of isomerism are not just theoretical, they have real-world uses, like drug design and catalysis.