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VSEPR

One of the best things since sliced bread in chemistry is Valence Shell Electron Pair Repulsion theory, it is something which can often be done on the back of an envelope but it can rival the most expensive and complex computer calculations. 

I was recently thinking about tellurium(IV) compounds, the reaction of tellurium tetrachloride with alkenes forms addition products. One which I liked was the reaction of norbornene with tellurium tetrachloride. When the tellurium tetrachloride reacts with the alkene it forms a cationic intermidate (a carbocation) which rearranges. I suspect that the chloride anion stays close to the tellurium (because of electrostatic attraction) which then delivers it on the same side of the carbocation as the tellurium. Here is the mechanism of the reaction. 

Mechanism of the reaction of norborene with tellurium tetrachloride

OK organic mechanistic bit is over, now we have to look at the product again. 

Two views of the product

Now we don our gay apperel” (diffractospecs) to allow us to look at the product. The carbon-tellurium bond is 2.144 Å while the tellurium chlorine bonds are 2.467, 2.299 and 2.462 Å. These three chlorines are arranged like a T shape. The long Te-Cl bonds are those which are trans to another chlorine. Here is the shape of the molecule. 

 

The product (ball and stick view)

Trans to the short Te-Cl bond is a very long Te-Cl bond (3.013 Å), I will leave the question of when is a bond a bond to another day. Here is the molecule with the extra bond added. 

The product with the extra bond

Now we look at the space filling model 

Spacefilling model

If we consider the tellurium as having one carbon and four chlorines then it has a total of five atoms. But the geometry is a square based pyramid where the chlorine atoms and tellurium atom are close to being in a perfect flat plane, we might think that a trigonal bipyramid would be a more logical shape for a five coordinate atom if we apply the ideas of VSEPR. 

Product tilted to show a different view

I will leave it to you to work out why this shape would be more logical for a five coordinate atom. But in our case the tellurium has a lone pair on it, so in real life it is a six coordinate atom. This explains why it has a square based pyramid shape (which is related to an octahedron). Now we can rest for a moment, but stay tuned, don’t touch that dial and be ready for the next installment. 

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