Iron Oxidation States: Fe+2, Fe+3 & Fe- Explained

[ProjectFringe]

I saw on Wikipedia that Fe has both positive and negative oxidation states.
I know that Fe will willingly give up its 2 electrons to form an ionic bond with O for example, making it Fe⁺².

1. But how can Fe⁺³ exist? This means it gives up three electrons right? Does this mean the Fe atoms electron configuration would resemble something like vanadiums?

2. And in what situation would Fe gain an electron to form Fe^-? This means it has three electrons instead of two correct?

 

[mjc123]

1. No. Vanadium is 4s² 3d³; Fe³⁺ is 3d⁵.

2. The Wikipedia article on iron gives some examples of negative oxidation state compounds. Why not look them up?

 

[ProjectFringe]

1. No. Vanadium is 4s² 3d³; Fe³⁺ is 3d⁵.

2. The Wikipedia article on iron gives some examples of negative oxidation state compounds. Why not look them up?

I guess what was (is) confusing me is the different electron configurations for Fe ions.

I watched a video about it and my confusion is why Fe³⁺ is able to donate 3 electrons to form a compound like FeCl₃ when it has 5 electrons in its outer shell (d)?

 

[mjc123]

Fe³⁺ doesn't donate any electrons. Fe gives up 3 electrons to become Fe³⁺

 

[ProjectFringe]

Fe³⁺ doesn't donate any electrons. Fe gives up 3 electrons to become Fe³⁺

Sorry for my ignorance. What is the difference between 'donate' and 'give up'?

I still can't understand why Fe would want to give up 3 electrons. So far I have only studied smaller elements (Ca and smaller) and everything made sense because, with the exception of hydrogen, all the elements wanted 8 electrons in their outer shell. So they would give up or gain electrons to reach this state.

But now for some reason (if I'm understanding correctly, which I assume I'm not), Fe would rather have 5 electrons in its outer shell (as Fe³⁺) rather than 2 electrons (as Fe). Why would Fe do this?

And is Fe becoming Fe³⁺ as a result of its interaction with Cl₃, or is Fe³⁺ being used as a reactant to create FeCl₃? In either situation, I'm still confused because I don't see the 3 electrons needed to bond with Cl₃ in the outer shells of either Fe or Fe³⁺!

 

[mjc123]

The octet (or noble-gas-configuration) model works reasonably well for the s and p block elements (though not perfectly, what about e.g. SF₆ or ClF₃?), but you can't apply it to the transition metals. When you increase the oxidation state, you have to balance the energy input of removing an electron with the energy output in bonding or lattice energy (mutatis mutandis for reducing the oxidation state). For s and p block elements, the noble gas configuration often constitutes a natural stopping point - going beyond this is energetically unfavourable, as the large increase in ionisation energy is not fully compensated by increased bonding energy. For d block elements, which are generally far from a noble gas configuration, the difference between successive oxidation states is not as great, and these elements often have multiple stable oxidation states. As a beginner, you are better off just learning what these are for each element, and rationalising them afterwards as you learn more about the electronic structures.

PS There is no such thing as Cl₃. There is a Cl₃^- ion, but it is not involved here. FeCl₃ is Fe³⁺(Cl^-)₃ not Fe⁺Cl₃^-.

 

[ProjectFringe]

Okay, thanks for your help!