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Topic: Cu(II) co-ordinate complexes with halide ligands  (Read 3476 times)

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Offline zeoblade

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Cu(II) co-ordinate complexes with halide ligands
« on: March 06, 2010, 12:48:53 AM »
When I dissolved NaCl in CuSO4 solution, the solution changed from bright blue to a slight green tinge and then back to bright blue after adding excess water

I'm thinking that water ligands bite Cu to make [Cu(OH2)4]2+(aq) in a square planar arrangement and the sulphate oxygen atoms are biting the remaining sites. So when I dissolve NaCl, the Cl anions displaced the water ligands causing a colour change to a green tinge. [CuCl4]2-(aq) has a yellow colour and combined with the blue [Cu(OH2)4]2+ will turn the solution a greenish tinge. Is this wrong to say?

So when excess water is added, the Cl anions are solvated by water preventing them from biting Cu(II) so that [Cu(OH2)4]2+ will reform?

When I use KBr, the solution became bright green and returned to bright blue after excess water addition

What is the difference between KBr and NaCl that would cause such a difference in colour of the copper co-ordinate complex? I feel the Br anions are displacing the water ligands as Cl anions displaced water ligands but why the difference in colour? I always assumed that the transition metal was the one responsible for the colour but in this case my assumption proved wrong. Is it the larger electron shell of Br and lower electronegativity that would make the Cu-Br bond longer and cause the difference in colour?

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