[scissors99]

Hey guys,

So I did an experiment on reversible and irreversible reactions via cyclic Voltammetry. And my results show that Co3+ as Co(diNOsar)]Br3 is a reversible (Co3+ can be reduced to Co2+ AND oxidised back) process and [Co(en)3]I3 is irreversible (Co3+ can be reduced to Co2+ but not oxidised back) (both in pH 1 electrolyte solution). And im not quite sure why? I know it has something to do with lability of the ligands and maybe crystal field splitting. But for the most part Im lost.

Thank you in advance

[Midnightduck]

Cool question Scissors99, the short answer is the chelate effect is more pronounced in the diNOsar ligand. I break it down a bit more in the post below, and have a video response at around 18 minutes in for my stream (midnightduck on twitch). It can really help to draw this out.

first lets go over why Co³⁺ does not lose its ligands before the reduction. Co³⁺ is "low spin", which means that all of the electrons are in the dxy, dxz, and dyz orbitals are much lower in energy than the dx²-y² and dz² Orbitals. This is what causes the Co³⁺ center to never give up its ligands. However when it gains just 1 electron the ligands become labile again and it can give and take ligands. When those ligands are labile, the en ligand is less likely to stay on (exchanging for solvent), because it has less bonds to the metal. The diNosar ligand has 6 bonds to the metal center whereas the en ligands have only 2.

Hope this helps!

[scissors99]

Hi,

Thanks for the reply, makes a lot of sense. So when the en is exchanged for the solvent, I’m guessing Cl-, would that be enough to stop the oxidation of Co2+?

We also conducted Cyclic voltamettry with Coen but this time in pH6-7 and this gave a quasi reversible reaction. So at pH1 (0.1M HCl) it was irreversible and pH6-7 (KCl/H2O) it was reversible. does that have anything to with H+ concentration effect on the ethylenediamine ligand?

Again thank you in advance

[Midnightduck]

Honestly it can be that the Cl^- binds, but the major issue may come from the en ligand being protinated by the acid. If it makes NH₃⁺, that would make it a much worse ligand so it would not be able to rebind to the Co³⁺

[scissors99]

Well that makes more sense. Thank you for your time