What is the chemical composition of Ferrous Ferrocyanide?

To this day, iron-cyanide complexes confuses me. So, I want to settle this once and for all. After scavenging the internet, I gathered the following result:


$$
\ce{Ferr}
\color{blue}{\ce{ous}}~\ce{Ferr}\color{blue}{\ce{i}}\ce{cyanide -> Fe3^{II}[Fe^{III}(CN)6]2}$$


$$
\ce{Ferr}
\color{blue}{\ce{ous}}~\ce{Ferr}\color{blue}{\ce{o}}\ce{cyanide ->} \color{reD}{\ce{???}}$$


$$
\ce{Ferr}
\color{blue}{\ce{ic}}~\ce{Ferr}\color{blue}{\ce{i}}\ce{cyanide (Prussian}~\color{green}{\ce{Green}} \ce{)-> Fe^{III}[Fe^{III}(CN)6]}$$


$$
\ce{Ferr}
\color{blue}{\ce{ic}}~\ce{Ferr}\color{blue}{\ce{i}}\ce{cyanide (Prussian}~\color{brown}{\ce{Brown}} \ce{)-> Fe^{III}Fe^{III}(CN)6.2H2O}$$


$$
\ce{Ferr}
\color{blue}{\ce{ic}}~\ce{Ferr}\color{blue}{\ce{o}}\ce{cyanide (Prussian}~\color{blue}{\ce{Blue}} \ce{)-> Fe4^{III}[Fe^{II}(CN)6]3}$$


There are some mentions of "ferrous ferrocyanide" on the internet but I couldn't find its formula.


1. > 
> Prussian blue (also known as Berlin blue or, in painting, Parisian or Paris blue) is a dark blue pigment produced by oxidation of **ferrous ferrocyanide salts** ([Prussian Blue - Wikipedia](https://en.wikipedia.org/wiki/Prussian_blue))
> 
> 
>
2. Comment section of this question "[What's the chemical formula of "Everitt's salt"?](https://chemistry.stackexchange.com/questions/87000/whats-the-chemical-formula-of-everitts-salt)"
3. > 
> Evidence in the literature on the synthesis of ferrous ferricyanide is critically discussed. Pyrolysis and pressure effects on Prussian
> Blue lead to **ferrous ferrocyanide** together with decomposition
> by-products, and not to ferrous ferricyanide. The latter compound
> could be a precursor in the formation of Turnbull's Blue or an
> excited state of Prussian Blue, but it is not a stable chemical
> species.
> 
> 
> Reguera, E., Fernández-Bertrán, J. & Balmaseda, J. The existence of ferrous ferricyanide. Transition Metal Chemistry 24,
> 648–654 (1999). <https://doi.org/10.1023/A:1006942415737>
> 
> 
>
4. > 
> [![enter image description here](https://i.stack.imgur.com/fGEDB.png)](https://i.stack.imgur.com/fGEDB.png)
> 
> 
> Memoirs and Proceedings of the Manchester Literary & Philosophical Society: (Manchester Memoirs.)., Manchester Literary
> and Philosophical Society, **1909**
> 
> 
>


**Question**: What is the chemical formula for "Ferrous Ferrocyanide"?




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 To be honest, I don't even know the correct formula for Prussian blue. There are so many formula for PB, some of which has potassium included. Moreover, "[Turnbull Blue" has similar kind of formula adding more confusion](https://chemistry.stackexchange.com/questions/43816/what-is-the-chemical-composition-of-turnbulls-blue).




It is not common to find the ferrous ferrocyanide because it oxidizes rapidly. As you can see the remaining salts or pigments are pretty stable in air. This one is white. I quote from a very old Analytical Chemistry book by Treadwell, pg. 155, this should address your query. It states:



> 
> Potassium Ferrocyanide, $\ce{K4[Fe(CN)6]}$, produces in solutions of ferrous salts, with complete exclusion of air, a white precipitate of potassium ferrous ferrocyanide or of ferrous ferrocyanide, depending upon whether one or two molecules of ferrous salt react with one molecule of potassium ferrocyanide:
> 
> 
> $$\ce{K4[Fe(CN)6] + FeSO4 <=> K2SO4 + K2Fe[Fe(CN)6]}$$
> 
> 
> $$\ce{K4[Fe(CN)6] + 2 FeSO4 <=> 2 K2SO4 + Fe2[Fe(CN)6]}$$
> 
> 
> Although both of the above salts are white, a light-blue color is almost always obtained, because the precipitate is immediately oxidized somewhat by the air, forming the ferric salt of hydroferrocyanic acid (Prussian blue):
> 
> 
> $$\ce{6 Fe2[Fe(CN)6] + 6 H2O + 3 O2 <=> $\underset{\text{Prussian blue}}{\ce{2 Fe4[Fe(CN)6]3}}$ + 4 Fe(OH)3}$$
> 
> 
> 


 6

And then don't forget we have nitroprusside, still used as a short-term, if fairly rough way to reduce peripheral hypertension in a hypertensive crisis (and on the WHO list of essential medications). I'm not sure of its Fe oxidation state, but I'm sure it's one of the above. A remarkable drug, still in use, if even for dramatic (and short term) purposes.


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