Shapes of complex ions and isomerismAQA A-Level Chemistry: Revision notes
Section 1
Octahedral complexes
Transition metal ions commonly form octahedral complexes with small ligands such as H₂O and NH₃. Six ligands surround the central ion, with bond angles of 90°. [Cu(H₂O)₆]²⁺ and [Co(NH₃)₆]²⁺ are octahedral, and the co-ordination number is 6.
Bidentate ligands also give octahedral complexes, for example [Ni(en)₃]²⁺, where three bidentate ligands supply six bonds.
Section 2
Tetrahedral, square planar and linear complexes
Tetrahedral complexes, with bond angles of 109.5°, are commonly formed with larger ligands such as Cl⁻, because only four can fit around the central ion, e.g. [CoCl₄]²⁻ and [CuCl₄]²⁻.
Square planar complexes, with bond angles of 90°, are also formed, e.g. Pt(NH₃)₂Cl₂.
Ag⁺ forms the linear complex [Ag(NH₃)₂]⁺, with a bond angle of 180°, which is the species in Tollens' reagent.
Link shape to ligand size and number: six small ligands, octahedral; four large ligands, tetrahedral; two ligands, linear.
Section 3
Cis-trans isomerism
Complexes with the same formula can differ in how the ligands are arranged. Cis-trans isomerism is a special case of E-Z isomerism. In the cis isomer two identical ligands are next to each other (90°). In the trans isomer they are opposite (180°).
It occurs in octahedral complexes with monodentate ligands, such as [Co(NH₃)₄Cl₂]⁺ (the two Cl⁻ cis or trans), and in square planar complexes, such as Pt(NH₃)₂Cl₂.
Tetrahedral complexes show no cis-trans isomerism, because every position is adjacent to every other.
Section 4
Optical isomerism
Octahedral complexes with bidentate ligands can show optical isomerism. The two isomers are non-superimposable mirror images of each other, like left and right hands. Examples are [Ni(en)₃]²⁺ and [Cr(C₂O₄)₃]³⁻.
The two forms rotate the plane of plane-polarised light in opposite directions.
Optical isomerism in complexes needs bidentate ligands arranged octahedrally. Octahedral complexes with only monodentate ligands show cis-trans isomerism instead.
Section 5
Cisplatin
Cisplatin is the cis isomer of the square planar complex Pt(NH₃)₂Cl₂. The two Cl⁻ ligands are next to each other, and so are the two NH₃ ligands. In the trans isomer the Cl⁻ ligands are opposite each other. Cisplatin is used as an anticancer drug.
That's the notes covered.
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Exam questions on Shapes of complex ions and isomerism
- A student compares the shapes of four complex ions: [Ni(H₂O)₆]²⁺, [CoCl₄]²⁻, [Ag(NH₃)₂]⁺ and [Pt(NH₃)₂Cl₂].Explain why [Ni(H₂O)₆]²⁺ is octahedral but [CoCl₄]²⁻ is tetrahedral.2 marks
- The octahedral complex ion [Co(NH₃)₄Cl₂]⁺ exists in two forms, one green and one violet. The complex ion [Ni(en)₃]²⁺, where en is the bidentate ligand H₂NCH₂CH₂NH₂, also exists in two forms.State what is meant by optical isomers and explain why [Ni(en)₃]²⁺ shows optical isomerism.2 marks
- Platinum(II) forms the neutral complex Pt(NH₃)₂Cl₂, which has a square planar shape and exists as two isomers. One of them, cisplatin, is used as an anticancer drug.State the shape of the complex, its co-ordination number and the arrangement of the chloride ligands in cisplatin.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).