Question

When $CoCl_3 \cdot 6NH_3$ is mixed with excess $AgNO_3$ solution, 3 moles of $AgCl$ are precipitated per mole. Write its structural formula, IUPAC name, hybridisation and magnetic behaviour (VBT).

Hint:

Strong field ligands ($NH_3$, $CN^-$, CO) force pairing $\rightarrow$ low spin $\rightarrow$ inner orbital complex $\rightarrow$ $d^2sp^3$ for octahedral.

Answer 1

Step 1: Concept
The number of $AgCl$ precipitated tells us how many $Cl^-$ ions are outside the coordination sphere (ionizable).

Step 2: Analysis
3 moles of $AgCl$ precipitated means 3 ionizable $Cl^-$ ions per formula unit. This means all three chlorides are outside the coordination sphere as counter ions. The 6 $NH_3$ molecules are all inside the coordination sphere. Structural formula: $[Co(NH_3)_6]Cl_3$.

Step 3: IUPAC Name
Hexaamminecobalt(III) chloride. Co is in $+3$ oxidation state (since $3 \times (+1) = +3$). Six ammine ligands. Chloride counter ions.

Step 4: Hybridisation and Magnetic Behaviour
$Co^{3+}$ has configuration $3d^6$. $NH_3$ is a strong field ligand, forcing pairing of all 6 d-electrons into 3 $t_{2g}$ orbitals ($t_{2g}^6 e_g^0$). This vacates the $3d$ orbitals for hybridisation. The hybridisation is $d^2sp^3$ (inner orbital). All electrons are paired $\rightarrow$

diamagnetic.

Final Answer:
Structural Formula: $[Co(NH_3)_6]Cl_3$
IUPAC Name: Hexaamminecobalt(III) chloride
Hybridisation: $d^2sp^3$
Magnetic Behaviour: Diamagnetic (0 unpaired electrons)