JEE Main 2026 — Chemistry Inorganic Chemistry

2026integerhard

Number of paramagnetic complexes among the following is ________.

$[\text{MnBr}_4]^{2-}$ , $[\text{NiCl}_4]^{2-}$ , $[\text{Ni(CN)}_4]^{2-}$ , $[\text{Ni(CO)}_4]$ , $[\text{CoF}_6]^{3-}$ , $[\text{Fe(CN)}_6]^{4-}$ , $[\text{Mn(CN)}_6]^{3-}$ , $[\text{Ti(CN)}_6]^{3-}$ , $[\text{Cu(H}_2\text{O)}_6]^{2+}$ , $[\text{Co(C}_2\text{O}_4\text{)}_3]^{3-}$

Official previous-year question

Held on 8 Apr 2026 · Verified 6 Jul 2026.

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Solution

To determine the magnetic nature of the given complexes, we analyze the oxidation state, electronic configuration, and the nature of the ligands for each central metal atom/ion:

$[\text{MnBr}_4]^{2-}$ : Mn $^{2+}$ has a $3d^5$ configuration. Br $^-$ is a weak field ligand. The complex is tetrahedral with $5$ unpaired electrons. (Paramagnetic)
$[\text{NiCl}_4]^{2-}$ : Ni $^{2+}$ has a $3d^8$ configuration. Cl $^-$ is a weak field ligand. The complex is tetrahedral with $2$ unpaired electrons. (Paramagnetic)
$[\text{Ni(CN)}_4]^{2-}$ : Ni $^{2+}$ has a $3d^8$ configuration. CN $^-$ is a strong field ligand. The complex is square planar ( $dsp^2$ ) with all electrons paired. (Diamagnetic)
$[\text{Ni(CO)}_4]$ : Ni $^0$ has a $3d^8 4s^2$ configuration, which becomes $3d^{10}$ in the presence of the strong field ligand CO. The complex is tetrahedral with all electrons paired. (Diamagnetic)
$[\text{CoF}_6]^{3-}$ : Co $^{3+}$ has a $3d^6$ configuration. F $^-$ is a weak field ligand. The complex is octahedral and high spin ( $t_{2g}^4 e_g^2$ ) with $4$ unpaired electrons. (Paramagnetic)
$[\text{Fe(CN)}_6]^{4-}$ : Fe $^{2+}$ has a $3d^6$ configuration. CN $^-$ is a strong field ligand. The complex is octahedral and low spin ( $t_{2g}^6 e_g^0$ ) with all electrons paired. (Diamagnetic)
$[\text{Mn(CN)}_6]^{3-}$ : Mn $^{3+}$ has a $3d^4$ configuration. CN $^-$ is a strong field ligand. The complex is octahedral and low spin ( $t_{2g}^4 e_g^0$ ) with $2$ unpaired electrons. (Paramagnetic)
$[\text{Ti(CN)}_6]^{3-}$ : Ti $^{3+}$ has a $3d^1$ configuration. The complex is octahedral ( $t_{2g}^1 e_g^0$ ) with $1$ unpaired electron. (Paramagnetic)
$[\text{Cu(H}_2\text{O)}_6]^{2+}$ : Cu $^{2+}$ has a $3d^9$ configuration. The complex is octahedral ( $t_{2g}^6 e_g^3$ ) with $1$ unpaired electron. (Paramagnetic)
$[\text{Co(C}_2\text{O}_4\text{)}_3]^{3-}$ : Co $^{3+}$ has a $3d^6$ configuration. Oxalate ( $\text{C}_2\text{O}_4^{2-}$ ) acts as a strong field ligand for Co $^{3+}$ . The complex is octahedral and low spin ( $t_{2g}^6 e_g^0$ ) with all electrons paired. (Diamagnetic)

Therefore, the paramagnetic complexes are $[\text{MnBr}_4]^{2-}$ , $[\text{NiCl}_4]^{2-}$ , $[\text{CoF}_6]^{3-}$ , $[\text{Mn(CN)}_6]^{3-}$ , $[\text{Ti(CN)}_6]^{3-}$ , and $[\text{Cu(H}_2\text{O)}_6]^{2+}$ .

The total number of paramagnetic complexes is $6$ .

Answer: $6$

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Solution

To determine the magnetic nature of the given complexes, we analyze the oxidation state, electronic configuration, and the nature of the ligands for each central metal atom/ion:

$[\text{MnBr}_4]^{2-}$ : Mn $^{2+}$ has a $3d^5$ configuration. Br $^-$ is a weak field ligand. The complex is tetrahedral with $5$ unpaired electrons. (Paramagnetic)
$[\text{NiCl}_4]^{2-}$ : Ni $^{2+}$ has a $3d^8$ configuration. Cl $^-$ is a weak field ligand. The complex is tetrahedral with $2$ unpaired electrons. (Paramagnetic)
$[\text{Ni(CN)}_4]^{2-}$ : Ni $^{2+}$ has a $3d^8$ configuration. CN $^-$ is a strong field ligand. The complex is square planar ( $dsp^2$ ) with all electrons paired. (Diamagnetic)
$[\text{Ni(CO)}_4]$ : Ni $^0$ has a $3d^8 4s^2$ configuration, which becomes $3d^{10}$ in the presence of the strong field ligand CO. The complex is tetrahedral with all electrons paired. (Diamagnetic)
$[\text{CoF}_6]^{3-}$ : Co $^{3+}$ has a $3d^6$ configuration. F $^-$ is a weak field ligand. The complex is octahedral and high spin ( $t_{2g}^4 e_g^2$ ) with $4$ unpaired electrons. (Paramagnetic)
$[\text{Fe(CN)}_6]^{4-}$ : Fe $^{2+}$ has a $3d^6$ configuration. CN $^-$ is a strong field ligand. The complex is octahedral and low spin ( $t_{2g}^6 e_g^0$ ) with all electrons paired. (Diamagnetic)
$[\text{Mn(CN)}_6]^{3-}$ : Mn $^{3+}$ has a $3d^4$ configuration. CN $^-$ is a strong field ligand. The complex is octahedral and low spin ( $t_{2g}^4 e_g^0$ ) with $2$ unpaired electrons. (Paramagnetic)
$[\text{Ti(CN)}_6]^{3-}$ : Ti $^{3+}$ has a $3d^1$ configuration. The complex is octahedral ( $t_{2g}^1 e_g^0$ ) with $1$ unpaired electron. (Paramagnetic)
$[\text{Cu(H}_2\text{O)}_6]^{2+}$ : Cu $^{2+}$ has a $3d^9$ configuration. The complex is octahedral ( $t_{2g}^6 e_g^3$ ) with $1$ unpaired electron. (Paramagnetic)
$[\text{Co(C}_2\text{O}_4\text{)}_3]^{3-}$ : Co $^{3+}$ has a $3d^6$ configuration. Oxalate ( $\text{C}_2\text{O}_4^{2-}$ ) acts as a strong field ligand for Co $^{3+}$ . The complex is octahedral and low spin ( $t_{2g}^6 e_g^0$ ) with all electrons paired. (Diamagnetic)

The total number of paramagnetic complexes is $6$ .

Answer: $6$

JEE Main 2026 — Chemistry Inorganic Chemistry

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JEE Main 2026 — Chemistry Inorganic Chemistry

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Solution

Your note

JEE Main Inorganic Chemistry in other years

More JEE Main Inorganic Chemistry questions

Other Chemistry topics for JEE Main

Physical Chemistry

Organic Chemistry

Keep practicing Inorganic Chemistry