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Palladium chloride

Dear Reader,

I have recently had occasion to do some palladium chemistry, now for those of you who are not in the ‘know’, metal chlorides tend to be soluble except for silver, copper(I) {In the cuprous state copper seems to be like silver(I)}, thallium(I), lead(II), anhydrous rhodium(III), anhydrous chromium(III), platinum(II) and palladium(II),

I needed to use palladium chloride as a starting material for some chemistry, we might want to ask the question of why is palladium(II) chloride so insoluble, the reason is the fact that palladium chloride is polymeric. In recent years a lot of people in chemistry have become excited about the idea of polymers which contain metals.

While a lot of very clever and difficult to make molecules can be made which link metals together, I want to show you that we can have a lot of fun with a very simple and cheap ligand. Which is (wait for it, wait for it, do not get too excited) chloride. The reason why chloride can link metals is that a chloride anion has lone pairs.

These four lone pairs are arranged in a tetrahedral manner around the centre of the chloride in the way we would expect from VSEPR, the lone pairs then bridge between palladium cations. What we finally get are infinite electrically neutral chains of palladium chloride which does not dissolve in water. The unit cell for PdCl2 (According to A.F. Wells, Phase Transition, 1992, 38, 127-220) is a box which is 3.82 Å by 3.35 Å by 11.02 Å, thankfully all the angles are 90 degrees so the box is
easy to use.

The palladium atoms are at the following locations

0.5 0.0 0.5

0.0 0.5 0.0

While the chlorine atoms are at

0.1730 0.0 0.1320

0.6730 0.5 0.3680

0.3270 0.5 0.6320

0.8270 0.0 0.8680

Always remember that if an atom is at a surface, edge or corner of the unit cell then by translational symmetry thing it will appear more than once in the unit cell.

So the cell has four quarters of a palladium atom and two halves of a palladium atom which makes two palladium atoms.

It has four halves of a chlorine atom and two whole chlorines which makes a total of four chlorines, so we can tell that the Pd to Cl ratio is 1:2. This confirms that we have the dichloride.

Here is a view of a unit cell for palladium(II) chloride.

Unit cell of PdCl2, note that the a, b, and c axes are in red, green and blue

Here is the spacefilling version of the unit cell.

Spacing filling version of the unit cell of PdCl2

If we extend the solid into other unit cells we get a nice chain of palladium chloride, nice to look at but a pain to try to react in aqueous media.

Chain of PdCl2 units showing how it is a 1D coordination polymer

Rather than just giving up on the palladium chloride there is a way forwards, if we add some sodium chloride to the slurry of palladium chloride in water then the new chlorides, these are palladium chlorides with anionic chloro complexes such as [PdCl4]2-. According to R.G. Dickinson, Journal of the American Chemical Society, 1922, 44, 2404-2411. This is a monoclinic solid which has a cell which is a 7.04 Å by 7.04 Å by 4.10 Å box. Again you will be glad to know that all the angles are 90 degrees.

Palladium atoms are at the corners

0.0  0.0 0.0

Potassium atoms are at the centre of the rectangular
faces

0.5 0.0 0.5

0.0 0.5 0.5

Chlorine atoms are at the following ‘funny’ sites

0.23 0.23 0.0

0.77 0.23 0.0

0.23 0.73 0.0

0.73 0.73 0.0

Those of you with super visual thinking should be able to see from a unit cell the square planar PdCl4 units. Here is a unit cell.

Unit cell of K2PdCl4

For the rest of the population here is a group of four unit cells which should allow you to see two of the square planar units.

Four unit cells of K2PdCl4 showing two PdCl4 dianions in full

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2 Responses

  1. I really enjoyed this article. Last year I had to teach unit cells to my AP chemists, it was quite an interesting experience as the class was pretty much split 50-50 between the ones who could ‘see’ the unit cell and the ones that couldn’t! There seemed to be a mix of boys and girls on both sides so I’m not sure it can be linked to spatial awareness or the age-old arguement about boys being better drivers! Thanks for sharing, and keep it up!!

    (I’ll bookmark this article for use next semester, if you don’t mind!)

  2. Feel free to use the PdCl2 cell for your teaching, it might be a bit of a change from cheap things like NaCl which are used for teaching crystallography and unit cells.

    If I feel in the mood I might do some more solids such as ZnS and PtS. If I can find a bottle of ZnS I might take a photo of the blue glow you see when it is irradated with UV.

    The other thing I am thinking of doing is giving LiCl, NaCl, KCl, RbCl and CsCl a heavy dose of X or gamma rays. When you irradate NaCl you form a particle in a box. Electrons tend to get displaced from their rightful sites and get trapped in the lattice. The unit cell defines the size of the box, which in turn defines the wavelength of light which the particle in the box absorbs at.

    I have dreamed of painting a sheet of card with PVA school glue with a picture, then dropping rock salt onto the picture. I would then repeat the process with other colourless group I halides. I would let the glue set. Then irradate the picture and then be able to photograph it.

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