Your question is too general. You must specify what is "cobalt complex"

This wording "the Electrochemical performance of the crystal" does not make any sense. 

Thanks for your response,,, Sir

Actually... I have been tried to synthesize  Cobalt  trinuclear molecules ....... and it formed crystals.......so now I want to check the electrochemical properties like CV,  

and problem is that this molecule is not dissolving .........

Is there any other process to check the Electrochemical properties .....Please suggest me........

To look at the redox chemistry of Co complexes usually requires dissolving it in an organic solvent - MeCN, CH2Cl2 are the most common. The electrolyte that is used is [NBu4][PF6] as normal ionic compounds don't dissolve in organic solvents. If however the compound doesn't dissolve at all you can try solid state electrochemistry. 

You may try to dissolve in polar solvent like acetonitrile or DMF or in DCM. 

 Thanks to all of you for your precious time, spent in adding answer.

DMSO, DMF, DCM are the most useful

thank you.... Hichem Haffar