In non-coordinating solvents the reaction would be

2 Co(OAc)2 + H2O2 = 2 Co(III)(OAc)2(OH)

In coordinating (thus solvating) solvents such as water, methanol, DMSO some extent of solvent coordination can occur. Of course the extent of dissociation and solvation depends on the polarity of the coordinating solvent. Furthermore,  the acetate group behaves accordingly (can be anionic or covalent  with one bound or two bounds to the metal. I would expect rather covalent behaviour  for the acetate group in  your case in MeOH.  For the formation hexa-solvated complexes (below) strong (non-coordinating) anions like triflate, sulfate etc. are required. 

2 [CoL6]2+(OTf)2- + H2O2 = 2 [CoL6]3+(OTf)2-(OH)-  where L=H2O, MeOH, DMSO

Imre correctly described reaction but this process does not stop there.

The Co2+ electron goes H2O2 to form OH- and OH(dot) equals to OH-radical.

OH-radical can react with CH3OH to produce .CH2OH which decays to CH2O

or it can react with H2O2. in 30% H2O2 it produces superoxide radical: HOO.

My major point is dealing with multiple radical production and potential singlet oxygen formation which can result in some polymerization reactions of organic solvent. 

Please see our publications with Dr. Mason: http://www.researchgate.net/profile/Sergey_Dikalov/publications/

Sergey is right about that the reaction with H2O2 begins with a homolytic cleavage resulting in the formation of hydroxyl radicals, which are extremely reactive with any CH-containing compounds. In this sense MeOH (especially DMSO) are considered as hydroxyl radical scavengers and thus not (so) appropriate as media for this reaction. Nevetheless, the OH radical is not so selective. At this H2O2 concentration level I think it will also oxidize Co(II) as shown above.

I am highly grateful to both of you for your valuable expert comments. A few more question in my mind are:

1- As Mr. Toth indicated the oxidation of Co(II) with hydrogen peroxide takes place in MeOH at room temperature as is apparent from an instantaneous colour change from pink to dark green. If the resulting dark green crystals are dissolved in glacial acetic acid, will it convert to Co(III) acetate?

2- Practical observation is, when these crystals are redissolved in acetic acid, and the resulting solution is left for evaporation, a sticky mass results which on complete drying changes into a sort of amorphous solid. Is this Co(III) acetate?  

3- IF MeOH is replaced with glacial acetic acid as reaction medium, can we obtain Co(III) acetate directly?

4- Will the hydroxyl radical, as result of homolytic cleavage of hydrogen peroxide, also attack the methyl group of acetic acid?

5- Can the possibility of attack by hydroxyl radical be minimized with the very slow drop wise addition of hydrogen peroxide?

6- Will temperature play any role in suppressing the attack by hydroxyl radical?  

Mumtaz

OH-radical can attack the methyl group of acetic acid but this reaction is (A) slower than reaction with Methanol by several fold; (B) Methanol can generate much more byproducts compared with acetic acid oxidation. Oxidation of acetic acid is much cleaner and result in CO2 and methanol release which will not participate in further reactions due to their very low concentrations.  

The possibility of secondary reactions of OH-radicals will not be minimized by slower reactions however the yield of byproducts can be reduced. For example, excess of H2O2 can work as a reducing agents to CoIII or even a source of other byproducts such as superoxide due to one-electron oxidation of H2O2 to HO2. So, it is worth to try. 

Most reactions of hydroxyl radicals are very fast and close to the diffusion-control processes. Temperature will not likely affect these much more then any other reactions.

An alternative strategy is to use a different mild oxidant, for example, perchloride such as KClO4.

Thank you very much Mr. Dikalov. Your suggestions are highly valuable. Based on your suggestions it could be said that glacial acetic acid would be a better solvent for synthesizing Co(III) acetate, as it is quite stable towards oxidative attack. The switching of hydrogen peroxide with another oxidant also sounds good.

Mr. Dikalov there is one doubt, as the reduction potential of Co(III) is very high, will a mild oxidant be able to oxidise Co(II) in glacial acetic acid? 

Co(III) acetate exhibits the so-called basic carboxylate structure and consists of an oxygen centered triangle containing a {M3O} unit with the general formula [M3O(O2CR)6L3]+ (M= metal; R= CH3 for acetate; L= neutral ligand like water). Other than Co, the basic carboxylate  structure containing a mu-3 bridging oxygen bonded to three M atoms  (namely {M3O} unit) and bridging carboxylate ligand  is formed by many metals like Mn, Fe, Cr, Ru, Rh, etc.

The formation of dark green crystals indicates that a new product is formed. Oxidation of Co(II) by hydrogen peroxide to Co(III) is well known. Indeed this (H2O2 oxidation) of CoCo3 in the presence of acetylacetone (acacH) ligand is the standard procedure employed for synthesis of  mononuclear [Co(acaca)3] (acac= acetylacetonate) which is also dark green in colour.  

I am not sure if  the green crystal obtained by reaction of Cobaltous acetate in methanol with H2O2 in your case is the trinuclear Cobaltic acetate.   

Instead of KClO4  you could also use KMnO4 in acetic acid as a very strong oxidant for your goal. The disadvantage of both is that metals other than Co will be mixed into your mixture. For this reason I'd stay with H2O2 in acetic acid or use peracetic acid (which is essentially the same as H2O2 + CH3CO2H is in equilibrium with CH3CO3H + H2O). 

Mr. Toth I agree with you in using hydrogen peroxide + acetic acid for this purpose. By the way if we use potassium perchlorate, the product would contain potassium chloride in addition to cobalt(III) acetate. How could we effectively separate cobalt(III) acetate from the mixture? Or will the presence of potassium chloride make no interference with the visible spectrum of the product mixture?

I understand that being a colorless substance, potassium chloride will make no interference with the visible spectrum of cobalt(III) acetate that is dark green in color.

The following links can be helpful especially the first one which describes the synthesis of the oxo-centered trinuclear cobaltic acetate.

http://dx.doi.org/10.1134/S1070363208110042

http://dx.doi.org/10.1134/S1087659610010128

Thank you very much Mr. Srinivadan. Could you supply me with the full text? It will be highly appreciated.

Mr. M Danish I can send you the full texts  in email (I do not  know your email. Mine is [email protected]).  From your observation that you are getting green crystals on H2O2 treatment of the pink Co(II) acetate it is very obvious that you have got a new product.. Have you analysed the crystals by elemental analysis or by chemical analysis?  In my opinion, your green crystalline material can be of interest because your method of synthesis appears very simple.  You may like to consider optimising your synthetic protocol and try to prepare crystals suitable for single crystal study. 

Pl. check the reaction of  a small amount your dry green product  with a little dilute HCl acid. If Cl2 gas is evolved (you can get the characteristic smell) then it is confirmed that the green product contains Co(III) because Co(III) has oxidising properties. 

Dear Mumtaz Danish

The green color is of cobalt monoxide Co(II)O,

Co(II),2Acetate,4H2O + HOOH------> Co(II)O(cristal green Color) + 2acetate + 2H2O

N.B. The transformation of Co(II) to Co(III) need high temerature.

Dear  Mr. Srinivadan, we have not performed any characterization of the crystals yet but we are planing to do so. Actually the study is in the very early stage.

The presence of Co(III) may also be checked by adding a small amount of potassium iodide. the liberated iodine may be reconverted to iodide with sodium thiosulfate. This treatment should be associated with a color change from dark green to pink if the original substance was Co(III).

Dear Mr. Riri do you have any proof for your suggestion? Your equation doesn't appear balanced.  Normally Co(II)oxide which is also dark olive green is prepared through thermal dehydration of Co(OH)2, obtained through precipitation of Co(II) using sodium hydroxide.

With mild oxidizing agents, the conversion of Co(II) to Co(III) really requires a temperature higher than room temperature, but with an strong oxidizing agent like hydrogen peroxide this seems to be unlikely.

i send you Mr Danish a file in your mail.

best regards

Dear Mr. Riri, according to your suggested mechanism the final reaction mixture will be heterogeneous containing solid Co(II) oxide along with negatively charged acetate ions only. As a matter of fact this must not happen that a reaction results in an electrically polarized reaction mixture. Every reaction mixture is electrically neutral before and after reaction.

As I wrote you earlier I would not  add other metals and/or anions (with KClO4 or KMnO4) into your mixture.  Although the Cl- anion probably prefers K over Co, there is an equilibrium in solution and some Co3+(OAc)-2Cl- , Co3+(OAc)-Cl-2, K+OAc-  might also be present disturbing the isolation of clean Co(OAc)3. A moderate excess of H2O2 in acetic acid (in situ peracetic acid) or peracetic in water (in situ H2O2 + acetic acid) should be good enough for this purpose.