Q1(b). Fifth ammonia ligand on copper(II)

Answer in brief

The fifth NH₃ must occupy a weak, elongated axial site of Jahn-Teller-distorted Cu(II); K₅ ≈ 0.302 is consequently much smaller than K₄ ≈ 135.

Key points

• Cu²⁺ is d⁹, with uneven occupation of the octahedral eg set. Axial elongation stabilises the complex.
• The first four NH₃ ligands favour the four short equatorial positions, giving [Cu(NH₃)₄(H₂O)₂]²⁺ with long axial Cu-OH₂ bonds.
• The fifth NH₃ replaces water at a long, weak axial position; its binding gains much less energy than equatorial binding.
• K₄ = 10^2.13 = 134.9; K₅ = 10^−0.52 = 0.302; K₄/K₅ ≈ 447. Ordinary statistical decreases alone do not explain this abrupt fall.