Hybridization of Fe in [Fe(H₂O)₅NO]SO₄ (brown ring complex) is

Hybridization of Fe in [Fe(H₂O)₅NO]SO₄ (brown ring complex) is

The hybridization of iron in the brown ring complex [Fe(H₂O)₅NO]SO₄ is sp³d². This is determined by the following structural and electronic characteristics:

Key Factors:

• Oxidation State of Iron:
  • Iron exhibits a +1 oxidation state in this complex.
  • The electronic configuration of Fe(I) is 3d7 (after rearrangement from 3d⁶4s¹ to 3d⁷4s⁰).
• Unpaired Electrons and Magnetism:
  • The complex has three unpaired electrons (d⁷ configuration).
  • This results in a magnetic moment of √15 BM (spin-only value), confirming paramagnetism.
• Ligand Field Effects:
  • NO⁺ (nitrosonium ion) acts as a strong-field ligand, while H₂O is weak-field.
  • Despite NO⁺'s strong-field nature, the odd number of d-electrons (d⁷) prevents full pairing, leading to high-spin behavior.
• Hybridization and Geometry:
  • The presence of six ligands (five H₂O and one NO) dictates an octahedral geometry.
  • The outer orbital hybridization (sp³d²) occurs because the 4d orbitals (not inner 3d) are involved.
  • This is consistent with the complex's paramagnetism and high-spin state.

Why Not Inner Orbital Hybridization?

• Inner orbital hybridization (d²sp³) would require paired electrons and low-spin complexes (e.g., Fe²⁺ with strong-field ligands). Here, the d⁷ configuration and unpaired electrons necessitate outer orbital hybridization.

Thus, the hybridization of Fe in [Fe(H₂O)₅NO]SO₄ is unequivocally sp³d².