Chemistry Q71 – JEE-MAIN 2026

Home Chemistry Coordination CompoundsQuestion #329

Number of paramagnetic complexes among the following is _______.

[M n B r_{4}]^{2 -}

[N i C l_{4}]^{2 -}

[N i (C N)_{4}]^{2 -}

[N i (C O)_{4}]

[C o F_{6}]^{3 -}

[F e (C N)_{6}]^{4 -}

[M n (C N)_{6}]^{3 -}

[T i (C N)_{6}]^{3 -}

[C u (H_{2} O)_{6}]^{2 +}

[C o (C_{2} O_{4})_{3}]^{3 -}

Enter Calculated Numerical Value:

Clue 1 of 3: Paramagnetism Definition

A complex is paramagnetic if it contains one or more unpaired electrons.

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Step 1: Determine Oxidation State and Electronic Configuration✦ Active

For each complex, calculate the oxidation state of the central metal ion and write its $d$ -electron configuration. Then, identify the nature of the ligands (strong field or weak field) and the coordination number.

\begin{array}{ll} Complex & Metal Ion (d-config) \\ 1. [M n B r_{4}]^{2 -} & M n^{2 +} (3 d^{5}) \\ 2. [N i C l_{4}]^{2 -} & N i^{2 +} (3 d^{8}) \\ 3. [N i (C N)_{4}]^{2 -} & N i^{2 +} (3 d^{8}) \\ 4. [N i (C O)_{4}] & N i^{0} (3 d^{10}) \\ 5. [C o F_{6}]^{3 -} & C o^{3 +} (3 d^{6}) \\ 6. [F e (C N)_{6}]^{4 -} & F e^{2 +} (3 d^{6}) \\ 7. [M n (C N)_{6}]^{3 -} & M n^{3 +} (3 d^{4}) \\ 8. [T i (C N)_{6}]^{3 -} & T i^{3 +} (3 d^{1}) \\ 9. [C u (H_{2} O)_{6}]^{2 +} & C u^{2 +} (3 d^{9}) \\ 10. [C o (C_{2} O_{4})_{3}]^{3 -} & C o^{3 +} (3 d^{6}) \end{array}

Step 2: Apply Crystal Field Theory to Determine Unpaired Electrons○ Expand

Based on the ligand field strength and coordination geometry, determine the number of unpaired electrons for each complex. A complex is paramagnetic if it has one or more unpaired electrons.

\begin{array}{llll} Complex & Metal Ion (d-config) & Ligand Type & Unpaired Electrons \\ 1. [M n B r_{4}]^{2 -} & M n^{2 +} (3 d^{5}) & W F L (t e t r a h e d r a l) & 5 (Paramagnetic) \\ 2. [N i C l_{4}]^{2 -} & N i^{2 +} (3 d^{8}) & W F L (t e t r a h e d r a l) & 2 (Paramagnetic) \\ 3. [N i (C N)_{4}]^{2 -} & N i^{2 +} (3 d^{8}) & S F L (s q u a r e p l a n a r) & 0 (Diamagnetic) \\ 4. [N i (C O)_{4}] & N i^{0} (3 d^{10}) & S F L (t e t r a h e d r a l) & 0 (Diamagnetic) \\ 5. [C o F_{6}]^{3 -} & C o^{3 +} (3 d^{6}) & W F L (o c t a h e d r a l) & 4 (Paramagnetic) \\ 6. [F e (C N)_{6}]^{4 -} & F e^{2 +} (3 d^{6}) & S F L (o c t a h e d r a l) & 0 (Diamagnetic) \\ 7. [M n (C N)_{6}]^{3 -} & M n^{3 +} (3 d^{4}) & S F L (o c t a h e d r a l) & 2 (Paramagnetic) \\ 8. [T i (C N)_{6}]^{3 -} & T i^{3 +} (3 d^{1}) & S F L (o c t a h e d r a l) & 1 (Paramagnetic) \\ 9. [C u (H_{2} O)_{6}]^{2 +} & C u^{2 +} (3 d^{9}) & W F L (o c t a h e d r a l) & 1 (Paramagnetic) \\ 10. [C o (C_{2} O_{4})_{3}]^{3 -} & C o^{3 +} (3 d^{6}) & S F L (o c t a h e d r a l) & 0 (Diamagnetic) \end{array}

💡 Teacher's Secret Hint

Remember that $d^{8}$ in square planar geometry with strong field ligands is always diamagnetic. For $d^{9}$ , there is always one unpaired electron regardless of ligand field strength.

Step 3: Count Paramagnetic Complexes○ Expand

Count the complexes identified as paramagnetic from the list. The paramagnetic complexes are: $[M n B r_{4}]^{2 -}$ , $[N i C l_{4}]^{2 -}$ , $[C o F_{6}]^{3 -}$ , $[M n (C N)_{6}]^{3 -}$ , $[T i (C N)_{6}]^{3 -}$ , and $[C u (H_{2} O)_{6}]^{2 +}$ . There are 6 paramagnetic complexes.

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Chemistry Question 71 – JEE-MAIN 2026

Clue 1 of 3: Paramagnetism Definition

🧠 Full Solution Path