Question

The initial steps in polymerization to form Prussian Blue is a photo-initiated reduction of Fe(III) to Fe(II).

i. This is a charge-transfer transition – what kind of charge transfer transition is it, and what is oxidized?  

ii. A gas is released as the photo-sensitized mixture is irradiated. What is it?

iii. Why does reduction of the Fe(III) center make the complex more labile? Explain in terms of CFSE.

Answer 1

i) This is a Metal to Ligand Charge Transfer Reaction, in which Fe(II) is Oxidized, i.e. electron density is transferred from Fe(II) to Cyanide (CN^{​​​​​-}​​​​) Ligand.

ii) The Gas Released during Photosensitization is Cyanogen (CN)₂

_{iii) This question is a bit wrong. Reduction of Fe(III) center doesn't make the complex more Labile, instead it makes the Complex more Inert. I'll explain this using the Crystal Field Theory.}

Thus, due to the greater CFSE value for Fe(II), it will be more Inert. It can also be explained by the Charge-Transfer concept. A Metal to Ligand Charge Transfer always favours the low oxidation state of the Metal, thus the Fe(II) Complex is more stable and in turn, more Inert.