[Fe(H2O)6]3+ is strongly paramagnetic whereas [Fe(CN)6]3- is weakly paramagnetic. Explain.
In the presence of CN⁻, which acts as a strong ligand, the 3d electrons pair up, leaving only one unpaired electron. The hybridisation involved is d²sp³, which results in the formation of an inner orbital complex. Consequently, the complex exhibits weak paramagnetism due to the presence of a single unpaired electron.
On the other hand, in the presence of H₂O, which is a weak ligand, the 3d electrons do not pair up. The hybridisation involved is sp³d², forming an outer orbital complex. This complex contains five unpaired electrons, which makes it strongly paramagnetic in nature.
Explanation
The provided context explicitly answers this question in the section for Intext Question 5.7. It explains that the difference arises due to the nature of the ligands. CN⁻ is a strong ligand that forces electron pairing in the 3d orbitals, resulting in d²sp³ hybridisation and only one unpaired electron (weakly paramagnetic). Conversely, H₂O is a weak ligand that does not force pairing, resulting in sp³d² hybridisation and five unpaired electrons (strongly paramagnetic).