Q. 19 The correct statement is
(A) TlBr3 is less soluble in water than TlBr
(B) Ag2 S is more soluble in water than Ag2O
(C) LiF is less stable than CsF
(D) [Co(NH3)5I]2+ is less stable than [Co(NH3)5 F]2+

The correct statement is option (D) [Co(NH3)5I]2+ is less stable than [Co(NH3)5F]2+.​​

Correct Answer and Concept

The only correct statement is (D) [Co(NH3)5I]2+ is less stable than [Co(NH3)5F]2+.​

• Fluoride is a stronger field and harder ligand than iodide, giving stronger Co–F bonding and higher thermodynamic stability to the fluorido complex compared with the iodido complex.​

• According to the hard–soft acid–base (HSAB) concept, Co(III) is a hard acid and prefers hard bases like F− over softer bases like I−, so the Co–F complex is more stable.​

Option-wise Explanation

Option (A): TlBr3 vs TlBr Solubility

Statement: “TlBr3 is less soluble in water than TlBr.” – This is incorrect.​

• TlBr (thallium(I) bromide) is a largely ionic, sparingly soluble salt; the +1 state of thallium forms halides with limited solubility similar to many heavy-metal halides.​

• TlBr3 contains thallium(III) and often exists in solution as complex anions like [TlBr4]−, which gives it significantly higher solubility in water than TlBr, so the given statement reverses the trend.​

Option (B): Ag2S vs Ag2O Solubility

Statement: “Ag2S is more soluble in water than Ag2O.” – This is incorrect.​

• Ag2S is much less soluble in water than Ag2O; this is a standard qualitative solubility fact used in inorganic qualitative analysis.​

• The very low solubility of Ag2S arises from strong covalent character and very high lattice energy; Ag2O, though sparingly soluble, still dissolves more than Ag2S, so the direction of comparison in the option is wrong.​

Option (C): LiF vs CsF Stability

Statement: “LiF is less stable than CsF.” – This is incorrect.​

• In alkali metal fluorides, lattice enthalpy is highest for LiF due to the small sizes and strong electrostatic attraction between Li+ and F−, making LiF very stable.​

• Cs+ is much larger and forms a less strongly bound lattice with F−, so CsF is thermally and lattice‑wise less stable than LiF, opposite to what the option claims.​

Option (D): Cobalt Ammine Complex Stability

Statement: “[Co(NH3)5I]2+ is less stable than [Co(NH3)5F]2+.” – This is correct.​

• Co(III) in these complexes is a hard Lewis acid, and F− is a hard ligand, while I− is soft; hard–hard interaction (Co–F) is more favorable, leading to greater stability of [Co(NH3)5F]2+.​

• F− is also a stronger σ‑donor and higher in the spectrochemical series than I−, increasing crystal field stabilization and overall thermodynamic stability of the fluorido complex compared with the iodido complex.​

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