Why are different colours observed in octahedral and tetrahedral complexesfor the same metal and same ligands?
The CFSE or crystal field spli4tting energy of the octahedral and tetrahedral field is related as: 
Where, ∆t = crystal field splitting energy in the tetrahedral field
ΔO= crystal field splitting energy in an octahedral field.
• Hence, the CFSE has a higher value in the octahedral field rather than in tetrahedral field which also accounts for greater splitting in the octahedral field for the same metal and the ligand. Higher the CFSE, higher will be the energy radiated in d-d transition which also means lower will be the wavelength of the light absorbed (as 
).
• Another reason which can be taken into account for the difference in colour is that, the pairing of the unpaired d electrons in the octahedral complexes in the presence of strong ligand field, which does not occur in case of tetrahedral complexes as they are formed by using outer orbital electrons (low spin complexes) so, there will be no pairing even in the presence of strong ligand field.