The equivalents of ethylene diamine required to replace the neutral ligands from the coordination sphere of the trans-complex of CoCl₃ . 4NH₃ is _________. (Round off to the Nearest Integer).

Solution

Chemistry ma’am · 4 steps

Find the four neutral NH₃ in the sphere, then remember each en grabs two sites at once.

1. Write the complex. Cobalt here is Co³⁺ with coordination number 6. The trans isomer of CoCl₃.4NH₃ is [Co(NH₃)₄Cl₂]Cl: four NH₃ and two Cl⁻ inside the sphere, with the two Cl⁻ opposite each other.
2. Neutral ligands. The neutral ligands are the four NH₃ molecules. They occupy four of the six coordination sites.
3. Denticity of en. Ethylene diamine, H₂N-CH₂-CH₂-NH₂, is bidentate. Each en binds through both nitrogens and fills 2 sites.

   en needed = (4 sites)/(2 sites per en) = 2

4. So. Two en replace the four NH₃, giving [Co(en)₂Cl₂]Cl. The answer is 2.

The common slip: 4. Replacing each NH₃ with one en molecule gives 4. That ignores that en is bidentate and fills two sites at once.

The idea to keep. When one ligand replaces another, count coordination sites, not molecules. A bidentate ligand like en takes the place of two monodentate ligands.

Answer from the NTA final answer key. Solution by PrepWiser.