Answer: [CrF6]3- (d3).
- A [Co(NH3)6]3+ (d6, low spin, t2g6 = symmetric)
- B [Cu(H2O)6]2+ (d9)
- C [Mn(H2O)6]3+ (d4 high spin)
- D [CrF6]3- (d3)
Correct answer: D. [CrF6]3- (d3)
Explanation: [CrF6]3- has d3 configuration: t2g3 (one electron in each t2g orbital); this is symmetric and shows no Jahn-Teller distortion. [Co(NH3)6]3+ d6 low spin is also symmetric but for different reasons. d3 and high-spin d5 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.