Valence Bond Theory in Coordination Chemistry

Introduction to Valence Bond Theory (VBT)

• VBT explains the formation of coordination compounds.
• Proposed by Linus Pauling.
• Focuses on covalent bonding between central metal atoms and ligands.

Key Concepts

Central Metal Atom

• The central metal atom forms bonds with ligands through vacant orbitals.
• Electrons can be shared or donated between the metal atom and ligands.

Ligands

• Ligands are electron pair donors.
• Each ligand has at least one filled orbital to donate a lone pair of electrons to the central metal atom.
• Coordination number: Number of ligand atoms bonded to the metal.

Hybridization

• Hybridization occurs when atomic orbitals of the same energy mix to form new hybrid orbitals.
• Types of Hybridization:
  • Inner Orbital Complexes: Involve (n-1)d orbitals.
  • Outer Orbital Complexes: Involve nd orbitals.
• Directionality: The geometry of hybridized orbitals affects molecular shape.

Bonding in Coordination Compounds

Formation of Sigma Bonds

• Sigma bonds form due to the overlap of hybridized orbitals with filled ligand orbitals.
• Stronger overlaps create stronger bonds.

Coordination Number and Geometry

• Coordination number determines the spatial arrangement of ligands.
• Common geometries include octahedral (coordination number = 6) and tetrahedral (coordination number = 4).

Spin States of Complexes

• Complexes can be high-spin or low-spin depending on the ligands' field strength.
• Weak field ligands promote unpaired electrons (high-spin).
• Strong field ligands can cause pairing (low-spin).

Example: Iron Complex

• Consider the complex [Fe(CN)6]3-.
• Central metal: Fe (Iron) with an electronic configuration of 3d6.
• In the presence of strong field ligand (CN-), electrons can pair up leading to a low-spin state.

Electronic Configuration of Iron

1. Fe3+ Configuration:
   • Loses 3 electrons: 3d5, 4s0 (from 3d6 4s2).
   • Undergoes hybridization to accommodate ligands.
2. Coordination Sphere:
   • Six cyanide ligands coordinate to the Fe3+, forming an octahedral shape.

Magnetic Properties

• Complexes can be paramagnetic (unpaired electrons) or diamagnetic (all paired).

• Calculating magnetic moments using the formula:

  [ \mu = \sqrt{n(n + 2)} ]

  where n = number of unpaired electrons.

Summary

• Valence bond theory is crucial for understanding the bonding and structure of coordination compounds.
• The interaction between the central metal atom and ligands, along with hybridization and spin states, dictates the properties of the complex.