Paramagnetic Complexes and Unpaired Electrons Count

 

❓ Question

The number of paramagnetic metal complex species among the following — that have the same number of unpaired electrons — is asked for:

$$[{\text{Co(NH}}_3{)}_6{]}^{3+},[{\text{Co(C}}_2{\text{O}}_4{)}_3{]}^{3-},[{\text{MnCl}}_6{]}^{3-},[{\text{Mn(CN)}}_6{]}^{3-},[{\text{CoF}}_6{]}^{3-},[{\text{Fe(CN)}}_6{]}^{3-},[{\text{FeF}}_6{]}^{3-}$$

We must find which of these are paramagnetic and then see how many of them share the same number of unpaired electrons.

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🖼️ Question Image

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✍️ Short Explanation

This problem is based on:

👉 Crystal Field Theory
👉 Strong & weak field ligands
👉 Unpaired electrons.

Main idea:

Find number of unpaired electrons in each complex and count paramagnetic complexes having same value.

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🔷 Step 1 — $[Co(NH_3)_6]^{3+}$ 💯

Oxidation state:

$$Co^{3+}\Rightarrow 3d^6$$

$NH_3$ with $Co^{3+}$ gives low-spin complex.

Configuration:

$$t_{2g}^6e_g^0$$

Unpaired electrons:

$$\boxed{0}$$

Diamagnetic.

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🔷 Step 2 — $[Co(C_2O_4)_3]^{3-}$

Again:

$$Co^{3+}\Rightarrow d^6$$

Oxalate generally gives low-spin with $Co^{3+}$.

Configuration:

$$t_{2g}^6$$

Unpaired electrons:

$$\boxed{0}$$

Diamagnetic.

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🔷 Step 3 — $[MnCl_6]^{3-}$

Oxidation state:

$$Mn^{3+}\Rightarrow d^4$$

$Cl^-$ is weak field ligand → high spin.

Configuration:

$$t_{2g}^3e_g^1$$

Unpaired electrons:

$$\boxed{4}$$

Paramagnetic.

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🔷 Step 4 — $[Mn(CN)_6]^{3-}$

$$Mn^{3+}\Rightarrow d^4$$

$CN^-$ strong field → low spin.

Configuration:

$$t_{2g}^4$$

Unpaired electrons:

$$\boxed{2}$$

Paramagnetic.

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🔷 Step 5 — $[Co{F}_6{]}^{3-}$

$$Co^{3+}\Rightarrow d^6$$

$F^-$ weak field → high spin.

Configuration:

$$t_{2g}^4e_g^2$$

Unpaired electrons:

$$\boxed{4}$$

Paramagnetic.

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🔷 Step 6 — $[Fe(CN)_6]^{3-}$

$$Fe^{3+}\Rightarrow d^5$$

$CN^-$strong field → low spin.

Configuration:

$$t_{2g}^5$$

Unpaired electrons:

$$\boxed{1}$$

Paramagnetic.

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🔷 Step 7 — $[FeF_6]^{3-}$

$$Fe^{3+}\Rightarrow d^5$$

$F^-$ weak field → high spin.

Configuration:

$$t_{2g}^3e_g^2$$

Unpaired electrons:

$$\boxed{5}$$

Paramagnetic.

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🔷 Step 8 — Compare Paramagnetic Complexes

Paramagnetic complexes and unpaired electrons:

Complex	Unpaired Electrons
$[MnCl_6]^{3-}$	4
$[Mn(CN)_6]^{3-}$	2
$[Co{F}_6{]}^{3-}$	4
$[Fe(CN)_6]^{3-}$	1
$[FeF_6]^{3-}$	5

Same number occurs for:

$$[MnCl_6]^{3-}$$

and

$$[CoF_6]^{3-}$$

Both have:

$$\boxed{4}$$

Thus number of paramagnetic complexes having same number of unpaired electrons:

$$\boxed{2}$$

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🔷 Step 9 — JEE Trap Alert 🚨

❌ Strong vs weak field ligand confuse kar dena

❌ $Co^{3+}$ low-spin tendency ignore kar dena

Remember:

$CN^-$

$$\boxed{ \text{Strong field} }$$

$F^-,Cl^-$

$$\boxed{ \text{Weak field} }$$

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✅ Final Answer

$$\boxed{ 2 }$$

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📚 Related Topics

• Coordination Isomerism via AgNO₃ & BaCl₂ Tests
• Unpaired Electrons in Coordination Complexes
• Coordination Compounds Hybridization and Magnetism