[-SERS-]

I've just handed in a lab report for my transition metal complex chemistry paper where we conducted various physical, spectroscopic and chemical tests on two copper complexes. One of these was pyridinium tetrachloro cuprate (II) which we needed to firstly identify then determine its stereochemistry.

Anywho, when I and countless people in our lab took the complex's UV-vis spectrum when it was dissolved in acetonitrile, we got what we thought was the perfect graph looking like half a hill at the wavelength of 900nm. However, when we calculated the extinction coefficient we obtained values ranging from 50-70mol^-1 L cm^-1. From the textbook and the lecturer's notes, the complex being tetrahedral should have an extinction coefficient of around 200-800mol^-1 L cm^-1. Since the complex is closer to the square planar's value of 50<, what's actually happening?

Thanks

[Rudi]

Do you think that the compound comprises two pyridinium cations and one tetrachloridocuprate(II) complex anion?

[-SERS-]

Yes, (pyrH)2[CuCl4].

[Rudi]

In general it depends on the chloride concentration, whether the tetrachloridocuprate(II) entity exhibits a Jahn-Teller distorted octahedral coordination (with participation of solvent molecules) or a tetrahedral coordination.

So, if you have encountered the octahedral (pseudo square-planar) species, go ahead and add some hydrochloric acid to the solution. The formation of the tetrahedral CuCl4- should be directly visible with a colour change.

Does the compound exist in the solid-state? Is a crystal structure available?