One mole of the octahedral complex compound Co(NH3)5Cl3 gives 3 moles of ions on dissolution in water. One mole of the…

🧠 Werner Test: Inner vs Outer Cl⁻

For $\mathrm{Co(NH_3)_5 Cl_3}$ (empirical):

• Total ions in solution: 3 → 1 cation + 2 anions (or vice versa).
• AgCl precipitate: 2 mol per 1 mol complex → 2 outer-sphere Cl⁻.

🗺️ Build Inner Sphere

• 5 NH₃ + 1 Cl⁻ inner = 6 (octahedral). Outer = 2 Cl⁻.
• Formula: $[\mathrm{Co(NH_3)_5 Cl}]\mathrm{Cl_2}$.
• Co OS: $x + 5(0) + (-1) = +2$ (cation charge) → $x = +3$. ✓ Co(III).

🗺️ Verify

• $[\mathrm{Co(NH_3)_5 Cl}]^{2+} + 2 \mathrm{Cl^-}$ → 3 ions total ✓
• 2 outer Cl⁻ → 2 mol AgCl on AgNO₃ addition ✓

🗺️ Eliminate Other Options

• (b) $[\mathrm{Co(NH_3)_4 Cl}]\mathrm{Cl_2 \cdot NH_3}$: Outer-sphere NH₃ is unusual; non-standard.
• (c) $[\mathrm{Co(NH_3)_4 Cl_2}]\mathrm{Cl \cdot NH_3}$: Gives 1 mol AgCl, not 2.
• (d) $[\mathrm{Co(NH_3)_3 Cl_3}] \cdot 2\mathrm{NH_3}$: Neutral complex → 0 ions in solution; inner 3 Cl⁻ won't precipitate.

⚡ Werner Empirical → Coordination Formula

| Empirical | Coordination | AgCl moles | |---|---|---| | $\mathrm{CoCl_3 \cdot 6NH_3}$ | $[\mathrm{Co(NH_3)_6}]\mathrm{Cl_3}$ | 3 | | $\mathrm{CoCl_3 \cdot 5NH_3}$ | $[\mathrm{Co(NH_3)_5 Cl}]\mathrm{Cl_2}$ | 2 | | $\mathrm{CoCl_3 \cdot 4NH_3}$ | $[\mathrm{Co(NH_3)_4 Cl_2}]\mathrm{Cl}$ | 1 | | $\mathrm{CoCl_3 \cdot 3NH_3}$ | $[\mathrm{Co(NH_3)_3 Cl_3}]$ | 0 |

⚠️ Inner-Sphere Cl⁻ Doesn't Precipitate with AgNO₃

Coordinate-bonded Cl⁻ is held tightly by the metal and doesn't dissociate freely. Only ionic (outer-sphere) Cl⁻ precipitates with AgNO₃.

$\boxed{\text{Answer: (a) } [\mathrm{Co(NH_3)_5 Cl}]\mathrm{Cl_2}}$