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JEE Advanced 2018 Paper 1 · Chemistry · Coordination Compounds · medium

(s) about the following coordination compounds. Fe = 26, Co = 27, Ni = 28)

One or more options correct

Select the correct statement(s) about the following coordination compounds. (Atomic numbers: Fe = 26, Co = 27, Ni = 28)
  1. A. [Ni(CN)4]2−\mathrm{[Ni(CN)_4]^{2-}} is diamagnetic and square planar, whereas [NiCl4]2−\mathrm{[NiCl_4]^{2-}} is tetrahedral.
  2. B. [NiCl4]2−\mathrm{[NiCl_4]^{2-}} has a spin-only magnetic moment of about 2.832.83 BM.
  3. C. [Co(en)2Cl2]+\mathrm{[Co(en)_2Cl_2]^+} (en = ethane-1,2-diamine) exhibits three geometrical isomers.
  4. D. The crystal field stabilisation energy of high-spin [Fe(H2O)6]2+\mathrm{[Fe(H_2O)_6]^{2+}} is −0.4 Δo-0.4\,\Delta_o (pairing energy ignored).
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Answer: A, B, D

Ni2+^{2+} is d8d^8. With the strong-field CN−\mathrm{CN^-} ligand the electrons pair and the ion adopts dsp2dsp^2 square planar geometry, which is diamagnetic; with the weak-field Cl−\mathrm{Cl^-} the ion is sp3sp^3 tetrahedral with two unpaired electrons, so μ=2(2+2)=2.83\mu=\sqrt{2(2+2)}=2.83 BM. Options A and B are therefore correct. In [Co(en)2Cl2]+\mathrm{[Co(en)_2Cl_2]^+} the chelate rings fix only two arrangements of the chlorides: cis and trans. That is two geometrical isomers, and the cis form is chiral, giving three stereoisomers in total, so option C (three geometrical isomers) is wrong. High-spin Fe2+^{2+} is d6d^6, t2g4eg2t_{2g}^4e_g^2, so CFSE =4(−0.4)+2(+0.6)=−0.4 Δo=4(-0.4)+2(+0.6)=-0.4\,\Delta_o, making D correct.
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