The Student Room Group

Ligand substitution question

Hey, I was just wondering why, when [Cu(H2O)6]2+ reacts with ammonia only 4 put of the 6 water molecules get substituted with ammonia to make [Cu(H20)2(NH3)4]2+ ?
Thank you!
Hi, what level is this question being asked at? if this is below degree level then you don't really need to worry because the answer is quite complicated. Simply the complex of the lowest energy state will form, as (almost) everything in chemistry follows.

If you're looking for a more detailed answer, it comes down to crystal field theory, probability and rate constants. Briefly, ammonia will replace water molecules in the complex because they have are stronger field ligands in the spectrochemical series. Cu2+ complexes are thought to prefer a square planar formation, hence why all the ammonia ligands are in one plane and replace the water. The two water ligands that aren't replaced are in the z direction and aren't as tightly bound to the complex due to Jahn-Teller distortions. As a result, thermodynamics play less of a role and more of a calculation of solvation probability. Because the solution is aqueous there are a lot more water molecules so is more likely to be solvated by this than ammonia molecules.

This page helps:
https://chemistry.stackexchange.com/questions/29126/why-is-ligand-substitution-only-partial-with-copperii-ions-and-ammonia
I hope this helps :smile: If you need any further explanation feel free to ask.

Rob
PhD Chemistry Student
Student Ambassador
(edited 1 year ago)
Original post by Uni of Southampton Students
Hi, what level is this question being asked at? if this is below degree level then you don't really need to worry because the answer is quite complicated. Simply the complex of the lowest energy state will form, as (almost) everything in chemistry follows.

If you're looking for a more detailed answer, it comes down to crystal field theory, probability and rate constants. Briefly, ammonia will replace water molecules in the complex because they have are stronger field ligands in the spectrochemical series. Cu2+ complexes are thought to prefer a square planar formation, hence why all the ammonia ligands are in one plane and replace the water. The two water ligands that aren't replaced are in the z direction and aren't as tightly bound to the complex due to Jahn-Teller distortions. As a result, thermodynamics play less of a role and more of a calculation of solvation probability. Because the solution is aqueous there are a lot more water molecules so is more likely to be solvated by this than ammonia molecules.

This page helps:
https://chemistry.stackexchange.com/questions/29126/why-is-ligand-substitution-only-partial-with-copperii-ions-and-ammonia
I hope this helps :smile: If you need any further explanation feel free to ask.

Rob
PhD Chemistry Student
Student Ambassador


Yep, I understood about 40% of this but I really appreciate you answering! Thanks!

Quick Reply

Latest