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JEE Main 2023 · Chemistry · Coordination Compounds · medium

1 mol of CoCl3·5NH3 is dissolved in water and excess AgNO3 is added; 0.2 mol of AgCl precipitates. Which of the following is correct for the complex?

Single correct option

0.10.1 mol of CoCl3⋅5NH3\mathrm{CoCl_3\cdot5NH_3} is dissolved in water and excess AgNO3\mathrm{AgNO_3} is added; 0.20.2 mol of AgCl\mathrm{AgCl} precipitates. Which of the following is correct for the complex?
  1. A. Formula [Co(NH3)5Cl]Cl2\mathrm{[Co(NH_3)_5Cl]Cl_2}, d2sp3d^2sp^3, diamagnetic
  2. B. Formula [Co(NH3)5Cl]Cl2\mathrm{[Co(NH_3)_5Cl]Cl_2}, sp3d2sp^3d^2, paramagnetic
  3. C. Formula [Co(NH3)5]Cl3\mathrm{[Co(NH_3)_5]Cl_3}, d2sp3d^2sp^3, diamagnetic
  4. D. Formula [Co(NH3)4Cl2]Cl\mathrm{[Co(NH_3)_4Cl_2]Cl}, d2sp3d^2sp^3, diamagnetic
Show answer and solution

Answer: (A) Formula [Co(NH3)5Cl]Cl2\mathrm{[Co(NH_3)_5Cl]Cl_2}, d2sp3d^2sp^3, diamagnetic

Two of the three chlorides precipitate, so two are ionisable (counter ions) and one is bound to Co: [Co(NH3)5Cl]Cl2\mathrm{[Co(NH_3)_5Cl]Cl_2} (coordination number 6, so [Co(NH3)5]Cl3\mathrm{[Co(NH_3)_5]Cl_3} is ruled out, and the tetraammine option would precipitate one chloride only). Co3+\mathrm{Co^{3+}} is d6d^6; with NH3\mathrm{NH_3}, a strong-field ligand, pairing occurs to give t2g6t_{2g}^6: inner-orbital d2sp3d^2sp^3 hybridisation and zero unpaired electrons (diamagnetic).
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