Why FeCl3 is(III) and FeI (II) ?

Edited October 8, 201411 yr by Romix

Quick note: When writing out the names of these compounds, the (III) should go after the metal and it should be FeI2, not FeI.

 

The III and II denotes the oxidation state of the iron. Iron (III) chloride contains Fe3+, making the molecular formula FeCl3, whereas iron (II) iodide had Fe2+. As I understand it, you can get the iodide and the chloride as both Fe3+ And Fe2+ complexes. Why you would have one over the other is a bit of a null question since it really depends on the context.

You can't get FeI₃ but you can get FeCl₃ which is, I think, what the OP is asking about.

The reason is that Fe(III) is a strong enough oxidant to oxidise (some of) the iodide ions to iodine.

So if you could get FeI₃ it would decompose like this.

2 FeI₃ ---> 2 FeI₂ +I₂

At least this website suggests FeI₃:

http://www.webelements.com/iron/compounds.html

but (1) I understand too little of chemistry (2) The compound may be listed though it's very uncommon or unstable (3) Some websites, I believe not this one, create compounds automatically, whether they exist or not (4) I believe John Cuthber usually.

It also lists thallium triiodide as a Tl(III) species which is wrong.

That website isn't reliable.