In [Fe(CN)6]3- iron is in +3 state and in [Fe(CN)6]4,iron is in +2 state. [Fe(CN)6]3- contains five electrons in d-level (3d5). In this complex iron undergoes d2sp3 hybridisation.
Due to the presence of one unpaired electron, [Fe(CN)6]4-, is paramagnetic. In [Fe(CN) ] iron contains six electrons in d-level (3d6). It undergoes d2sp3 hybridisation and has no unpaired electrons. Hence, [Fe(CN)6]4- is diamagnetic.
