Why do compounds having similar geometry have different magnetic moment ?

Explanation

This is because of the nature of the ligands present in the complex. If the crystal field stabilising energy (CFSE) i.e. `Delta_(o)` for the complex is high, the complex will have less number of unpaired electrons and less value of magnetic moment. In case `Delta_(o)` is low, the complex will have more number of unpaired electrons and high value of magnetic moment. For example, both `[Fe(H_2O)_(6)]^(3+)` and `[Fe(CN)_(6)]^(3-)` are octahedral complexes. Whereas former is paramagnetic, the latter is of diamagnetic nature. This is due to the reason the `H_2O` ligand has lesser `Delta_(o)` value as compared to `CN` ligand.