Account for the following: [Fe(H2O)6]3− is strongly paramagnetic whereas [Ni(NH3)6]2+ weakly paramagnetic.
The ligand NH3NH3 is neither a strong field ligand nor a weak field ligand in the [Ni(NH3)6]2+[Ni(NH3)6]2+ complex ion. It is, in fact, a weak strong field ligand. However, because the crystal field stabilisation energy is smaller than the pairing energy, the ligand NH3NH3 acts as a weak field ligand in the [Ni(NH3)6]2+[Ni(NH3)6]2+ ion. As a result, the electronic configuration under the effect of an octahedral crystal field is t2 g6eg2t2 g6eg2. The complex has two unpaired electrons, according to the aforementioned electrical structure. As a result, the [Ni(NH3)6]2+[Ni(NH3)6]2+ complex is weakly paramagnetic. In the complex [Fe(H2O)6]3+[Fe(H2O)6]3+ the oxidation state of FeFe is +3+3, having the structure 3 d53 d5. Water (H2O)(H2O) is a weak ligand, and 3 d3 d electrons do not couple up in its presence. The sp3d2sp3d2 hybridization results in an outer orbital octahedral complex with 5 unpaired electrons. As a result, it is very paramagnetic.
