Class 12 Chapter 2 Shapes of Complexes Trick to memorize

Coordination Numbers & Geometries

The coordination number (CN) of a metal center determines its geometry:

CN Geometry
2 Linear (180°)
4 Tetrahedral (109.5°) or Square Planar (90°)
5 Trigonal Bipyramidal (90°, 120°) or Square Pyramidal
6 Octahedral (90°)
7 Pentagonal Bipyramidal
8 Square Antiprismatic or Dodecahedral

Common Shapes of Complexes

  • Linear: [Ag(NH₃)₂]⁺, [Cu(CN)₂]⁻
  • Tetrahedral: [TiCl₄], [CoCl₄]²⁻
  • Square Planar: [PtCl₄]²⁻, [Ni(CN)₄]²⁻
  • Trigonal Bipyramidal: [Fe(CO)₅]
  • Square Pyramidal: [VO(acac)₂]
  • Octahedral: [Co(NH₃)₆]³⁺, [Cr(H₂O)₆]³⁺

Memorization Tricks & Mnemonics

“Lonely Tetrah, Square Plan!”
Linear goes first (CN 2).
Tetra reminds you of tetrahedral (CN 4).
– If the complex is flat with CN 4, think Square Planar (Pt & Ni).

“Three Bees in a Trigonal Hive”
– CN 5 splits into Trigonal Bipyramidal (like a three-sided hive with two extra slots above and below).

“Octa at 90°”
– CN 6 → Octahedral, all angles are 90°.

Remember: “Shapes increase with slots” — as coordination number increases, imagine adding “slots” for ligands around the central metal.

Practice Examples

  1. Predict the shape of [Ni(CN)₄]²⁻ and recall the mnemonic for CN 4 flat vs. tetra.
  2. For [Fe(CO)₅], CN 5: which geometry and what “hive” mnemonic applies?
  3. Decide the shape of [Co(NH₃)₆]³⁺ and use “Octa at 90°”.

Conclusion

By linking coordination numbers to simple catchphrases and visual images—“Lonely Tetrah,” “Trigonal Hive,” and “Octa at 90°”—you can quickly recall complex geometries under exam pressure. Practice with real complexes to cement these tricks!

 

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