Which of the Complex of the following pairs has the largest value of CFSE? (1) [Co(CN)6]3- and [Co(NH3)6]3+ (2) [Co(NH3)6]3+ and [CoF6]3- (3) [Co(H2O)6]3+ and [Rh(H2O)6]3+ (4) [Co(H2O)6]2+ and [Co(H2O)6]3+

(1)  CN is the stronger ligand than NH₃ therefore CFSE of [Co(CN)₆]^3-  will be more than  [Co(NH₃)₆]³⁺

(2) NH₃ is stronger ligand than F therefore CFSE of [Co(NH₃)₆]³⁺ will be more than [CoF₆]^3-.
(3) Co belongs to 3d series whereas The Rh belong to 4d series. More the value of n more is CFSE therefore CFSE of  [Rh(H₂O)₆]³⁺  is more than [Co(H₂O)₆]3+ .
(4) Oxidation number of Co  in [Co(H₂O)₆]³⁺ is more than the Oxidation number of [Co(H₂O)₆]²⁺  therefore, CFSE of [Co(H₂O)6]³⁺ is more than  [Co(H₂O)₆]²⁺.

Related Question:

Although both [Mn(H2O)6]2+ and [FeF6]3- have a d5 configuration and high-spin complexes. But the dilute solutions of Mn2+ and Fe +3 complexes are therefore colorless. Why?

Which of the Complex of the following pairs has the highest value of CFSE?
Colour of Complexes due to charge transfer:
Why violet colour of [Ti(H2O)6]Cl3 disapear (colourless) on heating heating ? 
Why [Ni(CN)4]-2 is colourless while [Ni(H2O)4]-2 although both have +2 oxidation state and 3d*8 configuration ?
Why [FeF6]3– is colourless whereas [CoF6]3– is coloured ? 
Why Fe(CO)5 is colourless while Fe(bipy)(CO)3 is intensely purple in colour ?
Why all the tetrahedral Complexes are high spin Complexes ?