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5.2.7Unit 5. Transition Metals and Organic Nitrogen Chemistry

Square Planar Complexes — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 19 of 43 Unit quiz

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Part 1 of 6Square Planar Shape

5.2.7 Square Planar Complexes
Coordination number
  • Four ligands lie in the same plane around the metal ion, with bond angles of 90°.
Typical metals
  • This shape is typical for d⁸ metal ions such as Pt²⁺, Pd²⁺, and Ni²⁺ with strong-field ligands.
Example
  • [Pt(NH₃)₂Cl₂] is a neutral complex because the two chloride ligands balance the +2 charge on platinum.
Comparison
  • Unlike a tetrahedral complex, the four ligands lie in a flat arrangement rather than in three dimensions.

Part 2 of 6Isomerism

Isomerism
Cis form
  • In cis-[Pt(NH₃)₂Cl₂] the two chloride ligands are adjacent, at 90° to each other.
Trans form
  • In trans-[Pt(NH₃)₂Cl₂] the chloride ligands are on opposite sides, 180° apart.
Different properties
  • The isomers have different physical properties and, importantly, different biological activity.

Part 3 of 6Cisplatin as a Drug

1
Cisplatin is the cis isomer and is used to treat several types of cancer.
2
Inside a cell, chloride ligands are replaced by water or by nitrogen atoms of guanine bases on DNA in a ligand exchange reaction.
3
The complex binds to two sites on the same DNA strand, bending the strand and stopping it being copied.
4
Cells that divide rapidly cannot replicate their DNA and so die, which stops the tumour growing.
5
The trans isomer does not work in the same way because its ligands cannot bind to the two DNA sites.

Part 4 of 6Side Effects

Side Effects
Healthy cells
  • Rapidly dividing healthy cells, such as those in hair follicles and the gut, are also affected.
Symptoms
  • Patients may experience hair loss, nausea and kidney damage.
Management
  • Doses are carefully controlled, and the benefit of treating the cancer is weighed against the side effects.

Part 5 of 6Worked Example

Question Show that platinum has oxidation number +2 in [Pt(NH₃)₂Cl₂].

  1. The complex is neutral, so the total of all oxidation numbers is 0.
  2. NH₃ is a neutral ligand (0) and each Cl⁻ is -1, giving 2 x (-1) = -2.
  3. Oxidation number of Pt = 0 - (-2) = +2.

Answer Pt is +2

Part 6 of 6Exam Checklist & Tips

5.2.7 – Key Points for the Exam

  • Square planar A four-coordinate shape with all ligands in one plane at 90°.
  • Cisplatin cis-[Pt(NH₃)₂Cl₂], a platinum-based anticancer drug.
  • Guanine A DNA base whose nitrogen atom can bind to platinum.
  • Ligand exchange Replacement of ligands in a complex by others.

Common Mistakes

  • Describing cisplatin as a mixture of both isomers.
  • Saying cisplatin attacks cancer cells only.
  • Stating that the oxidation number of platinum in cisplatin is zero.
  • Drawing a square planar complex with a 109.5° angle.

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