Answer: [CrF6]3- (d 3 ).
- A [Co(NH<sub>3</sub>)<sub>6</sub>]3+ (d<sup>6</sup>, low spin, t<sub>2g</sub><sup>6</sup> = symmetric)
- B [Cu(H<sub>2</sub>O)6]2+ (d<sup>9</sup>)
- C [Mn(H<sub>2</sub>O)6]3+ (d<sup>4</sup> high spin)
- D [CrF6]3- (d<sup>3</sup>)
Correct answer: D. [CrF6]3- (d<sup>3</sup>)
Explanation: [CrF6]3- has d<sup>3</sup> configuration: t<sub>2g</sub><sup>3</sup> (one electron in each t<sub>2g</sub> orbital); this is symmetric and shows no Jahn-Teller distortion. [Co(NH<sub>3</sub>)<sub>6</sub>]3+ d<sup>6</sup> low spin is also symmetric but for different reasons. d<sup>3</sup> and high-spin d<sup>5</sup> are always symmetric.
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.