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JEE Main 2018 · Chemistry · Coordination Compounds · medium

Which pair of complex ions has the same spin-only magnetic moment?

Single correct option

Which pair of complex ions has the same spin-only magnetic moment?
  1. A. [Mn(CN)6]3−\mathrm{[Mn(CN)_6]^{3-}} and [Ni(H2O)6]2+\mathrm{[Ni(H_2O)_6]^{2+}}
  2. B. [Fe(H2O)6]2+\mathrm{[Fe(H_2O)_6]^{2+}} and [Co(NH3)6]3+\mathrm{[Co(NH_3)_6]^{3+}}
  3. C. [Cr(H2O)6]3+\mathrm{[Cr(H_2O)_6]^{3+}} and [Mn(H2O)6]2+\mathrm{[Mn(H_2O)_6]^{2+}}
  4. D. [Fe(CN)6]3−\mathrm{[Fe(CN)_6]^{3-}} and [CoF6]3−\mathrm{[CoF_6]^{3-}}
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Answer: (A) [Mn(CN)6]3−\mathrm{[Mn(CN)_6]^{3-}} and [Ni(H2O)6]2+\mathrm{[Ni(H_2O)_6]^{2+}}

[Mn(CN)6]3−\mathrm{[Mn(CN)_6]^{3-}}: Mn3+\mathrm{Mn^{3+}} is d4d^4; strong-field CN−\mathrm{CN^-} gives t2g4t_{2g}^4 with 22 unpaired electrons (μ=8=2.83\mu=\sqrt8=2.83 BM). [Ni(H2O)6]2+\mathrm{[Ni(H_2O)_6]^{2+}}: d8d^8, t2g6eg2t_{2g}^6e_g^2, 22 unpaired (2.832.83 BM). The others differ: Fe2+\mathrm{Fe^{2+}} (4) vs Co3+\mathrm{Co^{3+}} low-spin (0); Cr3+\mathrm{Cr^{3+}} (3) vs Mn2+\mathrm{Mn^{2+}} (5); Fe3+\mathrm{Fe^{3+}} low-spin (1) vs Co3+\mathrm{Co^{3+}} high-spin in F−\mathrm{F^-} (4).
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