Question:

The pair having the same magnetic moment is :
[At. No.: Cr = 24, Mn = 25, Fe = 26, Co = 27]

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Find the d-electron count and number of unpaired electrons (water is a weak-field ligand; tetrahedral chloro complex is also high spin).
Updated On: Nov 11, 2026
  • \(\mathrm{[Cr(H_2O)_6]^{2+}}\) and \(\mathrm{[Fe(H_2O)_6]^{2+}}\)
  • \(\mathrm{[Mn(H_2O)_6]^{2+}}\) and \(\mathrm{[Cr(H_2O)_6]^{2+}}\)
  • \(\mathrm{[CoCl_4]^{2-}}\) and \(\mathrm{[Fe(H_2O)_6]^{2+}}\)
  • \(\mathrm{[Cr(H_2O)_6]^{2+}}\) and \(\mathrm{[CoCl_4]^{2-}}\)
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The Correct Option is A

Solution and Explanation

\(\mathrm{Cr^{2+}}\): \(d^4\); with weak-field \(\mathrm{H_2O}\) it is high spin, 4 unpaired electrons.
\(\mathrm{Fe^{2+}}\): \(d^6\); high spin with \(\mathrm{H_2O}\), 4 unpaired electrons.
\(\mathrm{Mn^{2+}}\): \(d^5\), 5 unpaired electrons.
\(\mathrm{Co^{2+}}\) in \(\mathrm{[CoCl_4]^{2-}}\): \(d^7\), tetrahedral, 3 unpaired electrons.
\(\mu = \sqrt{n(n+2)}\): n = 4 gives 4.9 BM for the Cr and Fe complexes, 5.9 BM for Mn, 3.87 BM for Co.
So the pair with equal magnetic moment is \(\mathrm{[Cr(H_2O)_6]^{2+}}\) and \(\mathrm{[Fe(H_2O)_6]^{2+}}\).
Answer: (1) \(\mathrm{[Cr(H_2O)_6]^{2+}}\) and \(\mathrm{[Fe(H_2O)_6]^{2+}}\)
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