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
- Predict the shape of [Ni(CN)₄]²⁻ and recall the mnemonic for CN 4 flat vs. tetra.
- For [Fe(CO)₅], CN 5: which geometry and what “hive” mnemonic applies?
- 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!
