Binuclear (& energetic) complexes of cobalt
Binuclear complexes of cobalt are subject to many publications.
See, for example, "The Formation, Structure, and Reactions of Binuclear Complexes of Cobalt" by A.G. Sykes and J.A. Wail in "Progress in Inorganic
Chemistry, Vol. 13".
The most easily obtained are those having 1 bridge formed by 2 oxygen atoms - mu-peroxo and mu-superoxo.
The first one is easily formed when an ammoniacal solution of cobalt salt absorbs atmospheric oxygen, so it is often a side product during the
preparation of various cobalt ammines. The very interesting property of the ion is the insolubility of nitrate which allows interesting metathesis
reactions. The second one is the result of further oxidation by mu-perexo complex with some oxidizing agent (e.g., H2O2 or persulfate).
The mu-peroxo cation has the form of [A Co O2 Co A]4+ and mu-superoxo - [A Co O2 Co A]5+. A is a group of ligands, for example (NH3)5. Both cations
have cobalt in 3rd oxidation state. So, the one core is, for example, Co(NH3)5 O- and the second is connected with the first by -O-O- bridge. In the
superoxo complex, the -O-O- bridge loses 1 electron and the charge of cation is increased.
So, basically, there is a similarity between the structure of H2O2 (H-O-O-H) and these types of complexes which suggests some similarity in chemistry
caused by the properties of -O-O- bond.
For the preparation of [(NH3)5Co(O2)Co(NH3)5](NO3)4 I used the procedure published in "Inorganic Syntheses, Vol. 12" on 1/2 scale.
The substances I used:
- Co(NO3)2 * 6H2O "B&K" (chemik.aip.pl) pure grade
- NH3 25 % water solution (labshop.nl)
- KNO3 - recrystallized from a food grade E252 ("Natural Spices B.V.")
- ethanol 96% (labshop.nl, denaturated BITREX, redistilled over KOH)
- deionized water
Because the original procedure uses 15M NH3 solution and NaNO3 some obvious modifications to the procedure were applied.
25.27g Co(NO3)2 * 6H2O was dissolved in 35 ml H2O. The solution was filtered (no precipitate). 125ml 25% NH3 was added. At this stage, some
orange-yellow precipitate was formed but it will disappear in later stages.
The suspension was put into a freezer to cool it down because the oxidation is better to perform in 0-5C range (I think this is for the purity of the
precipitate).
For cooling Ice + salt + a bit of ethanol mixture was prepared (it had a temperature below 0C).
The oxidation was performed in the Duran bottle with the filter-bottle cap:
The air stream was created by applying suction from an aspirator. The magnetic stirrer was turned on.
After 45 min 15ml of 25% NH3 was added (to correct the NH3 amount based on 15M -> 13.4M NH3 strength recalculation).
After 2.5hr from the beginning, the air stream was stopped and the cooling bath was temporarily removed. 11.97g KNO3 was dissolved in 25ml warm H2O
and added to the solution.
The stream of air & stirring were restarted first without cooling. In 0.5hr the cooling was commenced again and the oxidation was continued for
0.5 hr more.
After that, the cold solution was vacuum-filtered through filter paper. The brown solid was washed with 25% NH3 (it dissolves the complex, so don't
use much; the filtrate was combined with the mother liquor) and 2 times with the ethanol.
At this stage, the solid had a very prominent luster.
It was put into a vacuum desiccator over 96% H2SO4.
It was kept for 2 days there. During the period, the color was changed to dark green (as a result of losing 2 molecules of H2O). The yield was 5.02g
which is only 1/4 compared to the published in the procedure.
Both wet and dry complex burns very energetically. When wrapped in a foil and put into a fire it explodes with a loud sound.
The solubility in 25% NH3 aq. is between 5 and 10%. It is insoluble in water and decomposes in acid solutions.
10% HCl has shown an excellent result for washing glassware after the preparation but you should keep it, let's say, for 1 day in the acid.
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