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🧪 Chemistry  ·  d- and f-Block Elements  ·  NEET & JEE

Using crystal field theory, predict whether [Co(NH3)6]3+ is high spin or low spin:

Answer: Low spin: NH3 is a strong field ligand, large Delta causes electron pairing (t2g6, 0 unpaired).

  • A Low spin: NH3 is a strong field ligand, large Delta causes electron pairing (t2g6, 0 unpaired)
  • B High spin, since NH3 behaves as a weak field ligand toward cobalt(III) under most conditions encountered
  • C Paramagnetic with four unpaired electrons distributed across t2g and eg as frequently observed in practice
  • D An outcome that cannot be predicted from crystal field theory in many documented cases according to conventional understanding

Correct answer: A. Low spin: NH3 is a strong field ligand, large Delta causes electron pairing (t2g6, 0 unpaired)

Explanation: Co3+ has d6 configuration; NH3 is a strong field ligand causing large Delta, making electron pairing energetically favourable: t2g6 eg0 (0 unpaired), diamagnetic.

Colours of Common Transition Metal Ions (Aqueous)Cu²⁺ blueFe²⁺ pale greenFe³⁺ brown-yellowMn²⁺ pale pinkNi²⁺/Cr³⁺ greenColour arises from d-d electron transitions - d⁰ ions like Sc³⁺, Ti⁴⁺ are colourless

Aqueous solutions of transition metal ions show characteristic colours caused by electrons absorbing visible light to jump between split d-orbitals; the exact colour depends on the metal, its oxidation state, and surrounding ligands.

Concept context

Transition metals (d-block) and lanthanides/actinides (f-block). Known for coloured compounds, variable oxidation states, complex formation, and catalytic properties. Frequently tested in JEE and NEET.

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