Answer: Low spin: NH 3 is a strong field ligand, large Delta causes electron pairing (t 2g 6 , 0 unpaired).
- A Low spin: NH<sub>3</sub> is a strong field ligand, large Delta causes electron pairing (t<sub>2g</sub><sup>6</sup>, 0 unpaired)
- B High spin, since NH<sub>3</sub> behaves as a weak field ligand toward cobalt(III) under most conditions encountered
- C Paramagnetic with four unpaired electrons distributed across t<sub>2g</sub> 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: NH<sub>3</sub> is a strong field ligand, large Delta causes electron pairing (t<sub>2g</sub><sup>6</sup>, 0 unpaired)
Explanation: Co3+ has d<sup>6</sup> configuration; NH<sub>3</sub> is a strong field ligand causing large Delta, making electron pairing energetically favourable: t<sub>2g</sub><sup>6</sup> e<sub>g</sub><sup>0</sup> (0 unpaired), diamagnetic.
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.