Coordination Compounds Part 2

Crystal field splitting (∆0) for octahedral complexes depends on oxidation state of the metal as well as to which transition series the metal belongs. For the same oxidation state, the crystal field splitting (∆0) increases as we move from 3d→4d→5d
Cr3+ and Fe3+ belong to 3d series, Mo3+ belongs to 4d series and Os3+ belongs to 5d series.
Therefore crystal field splitting (∆0) is highest for [Os(H2O)6]3+.