Number of paramagnetic complexes among…

Numerical answer+4 / -1Answer matches NTA’s final key

Number of paramagnetic complexes among the following is ______.

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

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Solution

Find the metal's d count and whether the ligand field pairs the electrons.

Complex	Metal ion	Unpaired electrons
[MnBr $_4$ ]²⁻	Mn²⁺, d⁵, tetrahedral	5
[NiCl $_4$ ]²⁻	Ni²⁺, d⁸, tetrahedral (weak field)	2
[Ni(CN) $_4$ ]²⁻	Ni²⁺, d⁸, square planar	0
[Ni(CO) $_4$ ]	Ni(0), d¹⁰	0
[CoF $_6$ ]³⁻	Co³⁺, d⁶, high spin (F⁻ is weak)	4
[Fe(CN) $_6$ ]⁴⁻	Fe²⁺, d⁶, low spin	0
[Mn(CN) $_6$ ]³⁻	Mn³⁺, d⁴, low spin	2
[Ti(CN) $_6$ ]³⁻	Ti³⁺, d¹	1
[Cu(H $_2$ O) $_6$ ]²⁺	Cu²⁺, d⁹	1
[Co(C $_2$ O $_4$ ) $_3$ ]³⁻	Co³⁺, d⁶, low spin	0

Six complexes have unpaired electrons.

Answer: 6.

Answer checked against the official answer key. The solution is our own working.

JEE MAIN 2026 · Coordination Compounds

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