[SS]

Hi, does anyone know why [CuCl4] has a flattened tetrahedral structure, not regular tetrahedron?

[Donaldson Tan]

I think it's because the electron pair geometry of the central copper ion is octahedral. 4 vertices are taken up by Cl atoms, and the other 2 vertices are taken up by lone pairs. This should explain why CuCl₄^2- is a distorted tetradedron.

[puru]

Guys Are Forgetting the crystal feild theory by Bethe and van Vlecke.This also has in connection the Jahn And Teller distortion of Octahedral Complexes.
the d orbitals split as you know into t2g and eg orbitals.
Due to octahedral feild splitting and weak ligand effect of Cl [CuCl4]2- is distorted octahedron or squashed tetrahedon.

[Donaldson Tan]

Al Basra: care to enlighten us on Crystal Field Theory?

[Grafter]

Crystal Field Theory concerns transition metal complexes, in which ligands (which are treated as point charges) approach at various coordinations (eg octahedral, tetrahedral, square planar), and as a result raise and lower the d orbitals in energy. This splitting of orbitals can be sufficient to pair electrons in some d orbitals before all are filled. This gives rise to 'low spin' and 'high spin' states (as observed in iron in haemoglobin).

The energy jumps betweeb the raised orbitals and lowered orbitals are in the region of visible light, giving the characteristic colours of transition metal complexes.

In answer to your question, copper is a d9 transition metal, which experiences the Jahn-Teller effect, which effectively distorts an octahedral complex so much that two ligands on the z axis are sp loosely held, that they depart, leaving a square planar complex.

Further reading: Greenwood and Earnshaw