Compare the splitting energy (CFSE) into the following compound and give appropritate reason ? [Co(NH3) 6] 3+ , [Rh(NH3) 6] 3+,[Ir(NH3) 6] 3+

In given compounds number of ligands, types of ligands and oxidation state is same for central atom belive to same group but different transition series 3d, 4d and 5d respectively. We know that as move top to bottom size of d-orbital(3d--> 4d -->5d) is increases so ligand approches to d-orbitals more closely so the repulsion between d-orbital of metal and ligand is high and splitting energy increases. 

Thus order of splitting energy 

[Co(NH3)6]3+ < [Rh(NH3)6]3+ < [Ir(NH3)6]3+

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