[effiechristina]

Hi everyone,

I'm trying to study for my bioinorganic class, and I'm struggling with an example we were given in class.

My prof was giving us an example, comparing reduction of Fe(H₂O)₆³⁺ with reduction of Fe(CN)₆^-3.

So I was looking at the first one, Fe(H₂O)₆⁺³ + e^-  Fe(H₂O)₆²⁺ with a E° of 0.77 V

He wrote that the +3 species is low spin (it's d⁵ with two of the t_2g orbitals spin paired, and the remaining one unpaired) and the +2 species is high spin.

So I'm confused. I thought that water was generally considered a weak ish field ligand, so therefore a smaller Δoct and it should be high spin instead, with one electron in each orbital (t_2g and e_g). Am I wrong in thinking this? Is it because by making it low spin, it avoids populating the higher energy orbitals?

The +2 species he says is high spin, which makes sense to me based on the ligand type. But since this is d⁶ if it was low spin, you would have a diamagnetic species with the t_2g level filled with spin-paired electrons, and very stable. I would think this would be more favorable, and so it would "want" to be low spin, not high spin as he says.

I've been over my inorganic text and I just can't figure out why it isn't obeying the "trends" I read. Thank you so much for any help in advance!

[unsu]

[Fe(H₂O)₆]³⁺ and [Fe(H₂O)₆]²⁺ are high-spin
[Fe(CN)₆]^3- is low-spin

[kriggy]

I find hard to belive that [Fe(H₂O)₆]³⁺ has the orbitals occupied as your professor describes since water is somewhere in the middle of spectrochemical series, maybe chech your text book but I would think that it has all T2g and Eg orbitals filled with one electron each.
The Fe²⁺ in octahedral complex will always be high spin.

The atom doesnt decide if it "wants to be" high spin or low spin. If the energy requirement for pairing two electrons is lower than the energy requirement needed to ocupy Eg orbital then the electrons will be paired, otherwise, Eg orbital will be occupied.

Im not sure if I follow your thoughts. You were talking about comparing reduction potential of cyano and aqua complex but you list only one reduction potential. I would think that the cyano complex will be easier to reduce than aqua but that is just a guess