Ethane-1,2-diamine (en) is added to an aqueous solution of \( \ce{[Ni(H2O)6]^2+} \) until the molar ratio en : Ni is 3 : 1. Which of the following correctly gives the net reaction and the colour of the final solution?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( \ce{[Ni(H2O)6]^2+ + 3en -> [Ni(en)3]^2+ + 6H2O} \); violet
Ethane-1,2-diamine is a didentate ligand: it binds Ni2+ through both of its nitrogen atoms, so each en molecule takes the place of two water molecules. The water ligands are replaced one en at a time, and the colour changes at every step:
\( \displaystyle \ce{[Ni(H2O)6]^2+(aq) + en(aq) -> [Ni(H2O)4(en)]^2+(aq) + 2H2O} \) (green → pale blue)
\( \displaystyle \ce{[Ni(H2O)4(en)]^2+(aq) + en(aq) -> [Ni(H2O)2(en)2]^2+(aq) + 2H2O} \) (pale blue → blue/purple)
\( \displaystyle \ce{[Ni(H2O)2(en)2]^2+(aq) + en(aq) -> [Ni(en)3]^2+(aq) + 2H2O} \) (blue/purple → violet)
Adding the three steps, three en molecules replace all six water molecules, and the final solution of [Ni(en)3]2+ is violet:
\( \displaystyle \ce{[Ni(H2O)6]^2+(aq) + 3en(aq) -> [Ni(en)3]^2+(aq) + 6H2O} \)
The coordination number of nickel stays 6 throughout; only the ligands, and so the colour, change.
Why the other options are incorrect
Option (2) \( \ce{[Ni(H2O)6]^2+ + 3en -> [Ni(H2O)3(en)3]^2+ + 3H2O} \); violet — Incorrect. This treats en as a unidentate ligand. Three didentate en and three H2O would need nine positions around Ni2+, which has a coordination number of 6.
Option (3) \( \ce{[Ni(H2O)6]^2+ + 3en -> [Ni(en)3]^2+ + 6H2O} \); green — Incorrect. The equation is right, but green is the colour of the starting [Ni(H2O)6]2+ solution, not of [Ni(en)3]2+.
Option (4) \( \ce{[Ni(H2O)6]^2+ + 3en -> [Ni(H2O)3(en)3]^2+ + 3H2O} \); pale blue — Incorrect. The equation again treats en as unidentate, and pale blue is the colour after only one en has been added (ratio 1 : 1), from [Ni(H2O)4(en)]2+.
NCERT (2026-27 edition):
5.5.5 Colour in Coordination Compounds – “The influence of the ligand on the colour of a complex may be illustrated by considering the [Ni(H2O)6]2+ complex, which forms when nickel(II) chloride is dissolved in water. If the didentate ligand, ethane-1,2-diamine(en) is progressively added in the molar ratios en:Ni, 1:1, 2:1, 3:1, the following series of reactions and their associated colour changes occur:”
[Ni(H2O)6]2+ (aq) (green) + en (aq) = [Ni(H2O)4(en)]2+(aq) (pale blue) + 2H2O
[Ni(H2O)4(en)]2+(aq) + en (aq) = [Ni(H2O)2(en)2]2+(aq) (blue/purple) + 2H2O
[Ni(H2O)2(en)2]2+(aq) + en (aq) = [Ni(en)3]2+(aq) (violet) + 2H2O