(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+
SOLUTION:
(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 belong 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+ .
