Compare the acidity of [Co(H₂O)₆]²⁺ and [Co(H₂O)₆]³⁺

Co2+ (having electronic configuration 3d7) has a higher Crystal Field Stabilisation Energy than Co3+ (having electronic configuration d6) .
So, in the aqueous solution , due to the weak octahedral field , Co2+(aq) has higher stability than Co3+(aq) .
Due to the presence of weak field ligament H2O,  the Co2+(aq) has higher stability than Co3+(aq) .
Due to the presence of weak field ligament H2O, there are half field shells with all the spins parallel to each other , provides the extra stability .
Thus, [co(H2O)6]+2 ion is more stable and acisic than [co(H2O)6]+3 ion .