NELECT= [real] option will help you in this regard.. Suppose your cobaltocene have 97 electrons then its oxidised form will have 96 electrons...you have to mention in INCAR as NELECT = 96.....
I have done VASP calculation using NELECT but the energy difference is very huge....
actually i want to calculate the corresponding eq, i have done in Gaussian , i got exact energy but VASP didn't get the energy...please let me know how to get....