Stereoisomerism in complexes
Stereoisomerism in complexes
- Complexes can show stereoisomerism — same bonds, different 3-D arrangement.
- Two kinds: geometrical and optical.
- It can even change the complex's polarity.
Practice
Transition-metal complexes can show both geometrical and optical isomerism.
Square planar/octahedral complexes show cis/trans; octahedral ones with bidentate ligands show optical isomerism.
Geometrical (cis/trans)
- Appears in square planar complexes (e.g. $[\text{Pt}(\text{NH}_3)_2\text{Cl}_2]$) and octahedral ones.
- The two identical ligands are adjacent (cis) or opposite (trans).
Practice
Geometrical (cis/trans) isomerism in [Pt(NH₃)₂Cl₂] means the two identical ligands are:
In a square planar complex the matching ligands sit next to each other (cis) or across (trans).
Optical isomerism
- Appears in octahedral complexes with bidentate ligands, e.g. $[\text{Ni}(\text{en})_3]^{2+}$.
- It has two non-superimposable mirror images.
- Polarity: a cis form may be polar, while the trans form is often non-polar (its dipoles cancel).
Practice
Optical isomerism appears in octahedral complexes that have:
Octahedral complexes with bidentate ligands can exist as two non-superimposable mirror images.
Practice
Compared with a cis complex, the matching trans complex is often:
In the trans form the opposite ligands' dipoles cancel, so it is often non-polar.
You've got it
Key idea
- geometrical (cis/trans): in square planar ([Pt(NH₃)₂Cl₂]) and octahedral complexes — ligands adjacent (cis) or opposite (trans)
- optical: in octahedral complexes with bidentate ligands (e.g. [Ni(en)₃]²⁺) — mirror images
- a cis complex may be polar; the trans form is often non-polar