Nitrite Chemistry (4): K2A[B(NO2)6] (A=Ba/Pb, B=Fe/Co)
You know, even the Second Hospital cannot contain me...
We have gained much knowledge about these compounds so now we just need to deal with specific properties of these elements.
Co2+ is very air-stable in weak field ligand environment, and [Co(H2O)6]3+ lies much higher than O2 so air would never oxidize it. However when the field strength increases(even to NH3) Co3+ quickly dominates since d6 is much more preferred than d7 in a low-spin configuration, the latter has an extra electron.
Fe2+, on the other hand, is very air-sensitive unless in strongly acidic solution, or with strong field ligands like CN-. Although NO2- is also a strong field ligand, it cannot stop oxidation to hydrated Fe2O3 at all as shown below.
Of course, besides oxygen, HNO2 can also oxidize these low-valent complexes, and is much more effective as shown in previous experiments.
K2Ba[Co(NO2)6]
Mixed acetate solution prepared as the Ni one mentioned before(in fact Co dissolves in H2SO4 even at RT so this is quite easy, but heating is better), but no excess of AcOH was added. Also after reaction some BaCO3 was shaken with it to remove any free AcOH. As mentioned above, this solution is stable and doesn't require special treatment. pH of such solution is 6, which might be due to the acidity of Co2+. Its color was purple-red which indicates formation of complex.
The NaNO2 was dissolved into 40mL water and covered with large volume of paraffin oil. These solutions are not deoxygenated as boiling can cause decomposition of something and it's simply hard to remove oxygen at all. Then the acetate solution was poured into it very carefully with the help of glass rod.
After settling solution was removed as possible with dropper then poured into another container(to recycle paraffin), and then EtOH was injected onto the solid to remove some solution while diluting other. Repeat until the solution that cannot be removed became almost colorless.
With some sample, we found that in air the solid seems to be stable while solution quickly turned pale and yellow solid was produced.
The paraffin oil was poured into the recycle container and the solid was quickly shaken with EtOH and after decanting EtOH it was transferred into a funnel(EtOH can help) and washed with EtOH to remove paraffin. Then solid was dried in a sealed box with CaCl2, measured and added into bottle, then dried for a further period before sealing.
It's hard to prove its identity: while addition of EtOH into solution precipitated more, the solid can hardly dissolve back into water and forms a pale yellow instead of red solution. This is very possibly due to the ease of oxidation of solution, and precipitated Co(III) salt may stop further dissolution. Addition of H2SO4 into solid released some gas and turned the solid into, at least after dilution, a somewhat red solution(Co2+) and yellow solid([Co(NO2)6]3- salts). Due to the insolubility and inevitable formation of Co(III) species that can precipitate Ba2+ and K+, the Ba2+ part cannot be detected. However it cannot be anything but the product and the matcha color matches the paper, so we can go.
Yield: 4.73g(17.2%). Loss is, obviously, due to the high solubility, and addition of EtOH may help, as long as the density does not become too low and float above paraffin.
K2Pb[Co(NO2)6]
Similar to the corresponding Ni complex, but AcOH was reduced to 16.5g since we have known that PbCO3 cannot dissolve into AcOH and we can only use a deficiency of acid. The final pH was ~5, which is mainly due to the lower basicity of CoCO3 compared to BaCO3. Also we found that the reason that Pb cannot stoichiometrically reduce Cu2+ is that it became chunks after activation by AcOH.
The acetate solution was treated the same as the Ba salt. Solution was found to be a much lighter red color, and the solid was shiny brownish-black just like the Cu complex. Like the later complexes, its solubility is obviously much lower than the Ba one and the grains are much larger. However its solubility is much higher than the Pb salts of them since it's soluble even when K and Pb are in excess. Also it is somewhat sticky even when dry.
Yield: 17.37g(56%).
K2Ba[Fe(NO2)6]
The original paper used 170g(2mol) KNO2 in 80mL water, and this is in fact possible, but not possible here due to the low solubility of NaNO2 so we have to use half only. Hope we can make it...
Attempt 1
As Fe2+ would be oxidized very quickly in alkaline media, we decided to use alternative pathway.
6.90g(0.05mol) K2CO3, slightly more than 19.7g(0.1mol) BaCO3 and 18g(0.3mol, excess is strictly forbidden) AcOH was mixed in ~50mL water and boiled. When reaction is over this is cooled to RT, pH verified to be neutral or slightly basic, and a freshly prepared solution of 13.9g(0.05mol) FeSO4.7H2O in minimal(~50mL) water was added to it, stirred and quickly filtered. But this is useless, during the long filtration it still oxidized.
So we followed the old way and found that FeCO3 is not that air-sensitive, it just became dark green. The final acetate solution was treated with some Fe powder to reduce the oxidized part. Then 69g(1mol) NaNO2 was put into the filtration bottle and minimal water was added to simply cover it. The solution together with Fe was directly filtered onto it to minimalize oxidation. Although the solution was the damn brown color, the very characteristic orange color of the product can be clearly seen. Foam also appeared due to the combination of vacuum and decomposition by residual AcOH. Gradually solution became colorless as the residual oxygen in the bottle was removed. At the end of filtration, before lifting vacuum, we shake the bottle to make sure they fully react and to make getting product easier(some stuck on the wall).
The mixture was filtered to form orange/brown solid and orange/brown solution. During this we removed the foam as possible, since it looks dirty. Solid was washed with EtOH.
BUT THAT SOLID WAS Fe(OH)3 WHAT THE HELL
Maybe the original way is also OK or even better(since no acid would remain)?
In order to find out the reason, we made another little experiment. Some FeSO4 was dissolved in saturated NaNO2 to form an orange solution. This was left there for a few minutes. It was peaceful for a short while but then quickly decomposed into a dark-colored stuff and NO was released very violently just like the paper said(and we observed during this experiment). So we repeated but used 10M KOAc as the solvent. No decomposition was observed, but in a few minutes it was oxidized into insoluble stuff that might be Fe(OH)3.
OK, so this complex is very air-sensitive at least in solution, and the Co complex might also be so. Clearly NO2- cannot stabilize Fe2+ that much as CN-. Due to the very similar color of the complex and Fe(OH)3, we drew a wrong conclusion which lead to a tragedy.
Attempt 2
...
If filtration is impossible...
We first prepared the K-Ba-H solution with no excess of acid, then covered it with paraffin oil, and added FeCO3 from 20.9g(0.075mol) FeSO4.7H2O and 13.8g(0.1mol) K2CO3 solutions. During preparation of FeCO3 the solutions were kept dilute, and stirred violently when mixing. This way minimal sulfate would remain. Due to difficulty in the later stage of washing we directly squeezed it between tissues and added. After full reaction this should be a K-Ba-Fe solution with minimal free acid, and the chunky FeCO3 can be easily avoided. Trace BaCO3 and BaSO4 shouldn't be a big deal. pH is unknown. Some Fe powder was added but this seems useless. It was completely opaque with brown stuff floating everywhere.
This was added to NaNO2 slurry, at first the characteristic orange appeared, but later very large amount of foam appeared, and product disappeared(maybe foam lead air into solution), only a very small amount remained and was hidden in solid NaNO2, impossible to collect.
Let's analyze possible reasons:
1. FeCO3 lost reactivity which caused very large excess of acid
2. Fe complex is even more unstable than Co (of course)
3. Acetate should be added beforehand to create buffer(?)
4.
[Fe(NO2)6]4- + [NO2]- + 2H+ + 3H2O = Fe(OH)3 + 6[NO2]- + NO + 3H+ + H2O
This is a damn autocatalytic reaction! (common ones not eliminated to show their role)
While the one for Co is:
[Co(NO2)6]4- + [NO2]- + 2H+ = [Co(NO2)6]3- + NO + H2O
We can also write formulas for air oxidation:
4[Fe(NO2)6]4- + O2 + 10H2O = 4Fe(OH)3 + 24[NO2]- + 8H+
4[Co(NO2)6]4- + O2 + 2H2O = 4[Co(NO2)6]3- + 4OH-
Still the same, oxidation of Co complex is self-limiting while oxidation of Fe complex is self-promoting!
Also, we should add more water since solubility of NaNO2 was too low.
Attempt 3
Once you mix them together, the countdown begins... and when you see the foam it's already way too late.
We prepared a K-Ba-Cu solution with stoichiometric AcOH and treated it with excess of BaCO3 to remove acid, and filtered it, then covered with paraffin oil. The volume even at its minimal is ~150mL, due to the very low solubility of nonpolar Cu2(OAc)4(H2O)2, and even so something green remained, but might be simply dyed BaCO3. IDK why the solution became green while it was blue before filtration, probably the funnel is not very clean.
Meanwhile NaNO2 was dissolved in minimal(~90mL) water. Since this process is highly endothermic it takes time. Final volume was ~120mL.
I hope they can still crystallize...
The acetate solution was treated with excess of Fe powder. However nothing happened even when some AcOH was added and the Fe showed a "passivated" appearance. So we had to filter solution out, treated it with Pb until it became colorless, use BaCO3 to neutralize, then add Fe again. It should be mentioned that CuSO4 can react with Fe, so this might be condition-dependent. Well this time. Solution was almost colorless with only slight yellow color, but shows strong positive with bipy test.
A glass rod was inserted into this, and... take a deep breath!
However, nothing precipitated out. But we successfully observed the deep red color of the solution... maybe solution was too dilute or displacement was not complete. The first should be the main reason as if much Pb remains the (likely) insoluble Pb salt should precipitate, although another possibility exists that the displaced part is too small to even precipitate Pb salt, all in all next time we should use less water and find alternative way.
I run out of Pb, so let's just stop here and think carefully...
Attempt 4
We can't give up as such.
First K-Ba solution was prepared(BaCO3 in excess, and then filter) and added to NaNO2 and fixed volume to 150mL, cover with paraffin oil.
The excess FeSO4 was dissolved into large volume of water under paraffin oil, heated to nearly boiling and larger excess of K2CO3 solution added. After stirring and settling aqueous layer was removed with dropper and solid was washed with water three times. During this we found that Fe2+ hydrolyze to become brown even when no oxygen or base was there. Then 6g AcOH was added very carefully and this was heated(not to boiling) to promote removal of CO2. This still took a long time but just wait.
When we added this to NaNO2 solution still very violent decomposition happened especially after stirring. This means FeCO3 cannot be used as intermediate. Fe(OH)2 cannot settle and reduces water quickly.
Attempt 5
We need to be careful in all the stages.
K-Ba solution was prepared, but this time we boiled it until no gas was evolved. We used a freshly prepared FeSO4-NaNO2 solution to test it, and the yellow solution did not violently decompose, but simply became opaque due to presence of BaCO3 and BaSO4, even after settling for about a minute. It's impossible to see if any K2Ba[Fe(NO2)6] precipitated out.
After dissolution of NaNO2 we tested the incubation period, which is 78s. In comparison, without acetate it is 15s, and with 10M KOAc it is too long to measure but decomposition still happened slowly. However, it was found that if large volume of EtOH was added the decomposition completely stopped, only when AcOH was added did it decompose to release NO. Hmm...
Fe2+ solution was prepared as such:
Excess of BaCO3 was boiled with AcOH in some water under paraffin oil until no CO2 was evolved. Then solid FeSO4.7H2O was also added and stirred. After cooling it settled and some solution can be obtained with dropper.
Volume was ~50mL so one pump of this and 3 pumps of the NaNO2 solution should be used. After mixing this was filtered immediately(paraffin oil loaded in funnel beforehand) and the very wonderhoy orange color can be seen, though the amount was way too small.
EtOH of same volume was added to NaNO2 solution(some NaNO2 precipitated) and as such much more can be obtained. But it quickly oxidized into red stuff even when water was already removed. Probably, the highly oxophilic Fe3+ can allow oxygen to be reduced without water, to Fe2O3 and not OH-.
So, preparing this and the stuff below is simply impossible at home. A wonderhoy and very important experiment though.
K2Pb[Fe(NO2)6]
...
Conclusion
It's a real challenge to make these at home.
Unlike the Ni/Cu ones, here excess of acid is strictly forbidden as nitrite can quickly oxidize Co2+ in acidic solution, and Fe2+ even in neutral solution without KOAc as buffer. Paraffin oil can help you a lot, as oxygen can also quickly oxidize them and even free Fe2+. The Co complex is air-stable when dry. The Fe complex requires very large excess of NaNO2, and the original paper used 2mol KNO2 in 80mL which is impossible for NaNO2. This Fe complex should be prepared in small portions and filtered immediately as very quick autocatalytic decomposition happens after a minute. However it is still extremely air-sensitive when dry so preparation is impossible at home.
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