Frequently Asked Question

The magnetic moments of these Fe(II) complexes can be explained by considering the number of unpaired electrons in the d orbitals of the iron ion, which is determined by the ligand field splitting and the type of ligands coordinating to the metal center. Here's a breakdown:
a. [Fe(NH3)6](NO3)2: 5.5 B.M.
Ligand Field: Ammonia (NH3) is a strong field ligand, causing a large splitting of the d orbitals.
Electronic Configuration: Fe(II) has 6 d electrons. In a strong field, the lower energy d orbitals (t2g) are completely filled (6 electrons), leaving 4 electrons in the higher energy d orbitals (eg). This results in 4 unpaired electrons.
Spin-Only Magnetic Moment: The spin-only magnetic moment (μs) can be calculated using the formula μs = √n(n+2) B.M., where n is the number of unpaired electrons. For 4 unpaired electrons, μs = √4(4+2) B.M. = 4.9 B.M.
Observed Moment: The observed moment (5.5 B.M.) is slightly higher than the spin-only moment due to orbital contribution.
b. K4[Fe(CN)6]: 0 B.M.
Ligand Field: Cyanide (CN-) is a very strong field ligand, resulting in even larger splitting of the d orbitals.
Electronic Configuration: In a very strong field, all 6 electrons are forced into the lower energy t2g orbitals, leaving 0 unpaired electrons.
Spin-Only Magnetic Moment: With 0 unpaired electrons, μs = √0(0+2) B.M. = 0 B.M.
Observed Moment: The observed moment (0 B.M.) aligns with the spin-only moment, indicating a diamagnetic complex.
c. (Et4N)2[FeCl4]: 5.0 B.M.
Ligand Field: Chloride (Cl-) is a weak field ligand, causing a smaller splitting of the d orbitals.
Electronic Configuration: In a weak field, the d orbitals are filled according to Hund's rule, resulting in 4 unpaired electrons.
Spin-Only Magnetic Moment: μs = √4(4+2) B.M. = 4.9 B.M.
Observed Moment: The observed moment (5.0 B.M.) is close to the spin-only moment, indicating a high-spin complex.
d. [Fe(phthalocyanine)]: 2.8 B.M.
Ligand Field: Phthalocyanine is a complex ligand with strong field effects. However, due to its planar structure, the Fe(II) ion is surrounded by 4 nitrogen atoms of the phthalocyanine ligand, leading to a square planar geometry.
Electronic Configuration: In a square planar complex, the d orbitals split into 2 sets. The Fe(II) ion, with its 6 d electrons, will have 2 unpaired electrons.
Spin-Only Magnetic Moment: μs = √2(2+2) B.M. = 2.8 B.M.
Observed Moment: The observed moment (2.8 B.M.) aligns with the spin-only moment, indicating a low-spin complex.
Remember, these are simplified explanations. Actual magnetic moments can be influenced by factors like spin-orbit coupling and zero-field splitting, which can make them deviate slightly from the spin-only values.