Valence Bond Theory

• Valence bond (VB) theory explains the bonding in coordination compounds through the overlap of metal and ligand orbitals to form covalent bonds
• It considers the metal ion and ligands separately and then brings them together to form a complex
• VB theory focuses on the hybridization of metal orbitals to accommodate ligand electrons

Key Concepts of Valence Bond Theory

• The central metal ion provides empty orbitals for bonding
• Ligands donate electron pairs to form coordinate covalent bonds
• Hybridization of metal orbitals occurs to create appropriate orbitals with specific directional properties
• The number of hybrid orbitals formed is equal to the coordination number of the metal ion
• The geometry of the complex is determined by the type of hybridization
• Inner and outer orbital complexes are formed based on whether inner (n-1)d orbitals or outer nd orbitals are used in hybridization

Hybridization Schemes and Geometry

• sp hybridization results in a linear geometry e.g., [Ag(NH3)2]+
• sp2 hybridization results in a trigonal planar geometry
• sp3 hybridization results in a tetrahedral geometry e.g., [NiCl4]2-
• dsp2 hybridization results in a square planar geometry e.g., [Pt(NH3)4]2+
• dsp3 hybridization results in a trigonal bipyramidal geometry e.g., [Fe(CO)5]
• d2sp3 hybridization results in an octahedral geometry e.g., [Cr(NH3)6]3+
• sp3d2 hybridization results in an octahedral geometry e.g., [FeF6]3-

Inner vs. Outer Orbital Complexes

• Inner orbital complexes use (n-1)d orbitals for hybridization, resulting in stronger metal-ligand interactions; these are typically formed by strong field ligands
• Outer orbital complexes use nd orbitals for hybridization and are typically formed by weak field ligands; these usually result in weaker metal-ligand interactions
• The use of inner or outer d orbitals can influence the magnetic properties of the complex

Magnetic Properties

• If the complex has unpaired electrons, it is paramagnetic and is attracted to a magnetic field
• If all electrons are paired, the complex is diamagnetic and is repelled by a magnetic field
• VB theory predicts the number of unpaired electrons based on the electronic configuration after hybridization, which helps determine the magnetic behavior of the complex

Limitations

• VB theory does not explain the color of coordination compounds
• It does not provide a quantitative interpretation of the thermodynamic or kinetic stabilities of complexes
• It often oversimplifies the nature of metal-ligand interactions by treating them as purely covalent
• VB theory does not accurately predict the magnetic properties of all complexes, especially those involving weak field ligands
• It does not account for the spectrochemical series or the relative strengths of ligands

Examples and Applications

• [Ni(CO)4]: Nickel has a 3d8 4s2 configuration; in Ni(CO)4, nickel undergoes sp3 hybridization to form a tetrahedral complex; CO is a strong field ligand that forces pairing of electrons, resulting in a diamagnetic complex
• [Fe(CN)6]4-: Iron has a 3d6 4s2 configuration; in [Fe(CN)6]4-, iron undergoes d2sp3 hybridization to form an inner orbital octahedral complex; CN- is a strong field ligand, causing pairing of electrons and resulting in a diamagnetic complex
• [FeF6]3-: Iron has a 3d5 4s2 configuration, and as Fe3+ it is a d5 system; in [FeF6]3-, iron undergoes sp3d2 hybridization to form an outer orbital octahedral complex; F- is a weak field ligand, so no electron pairing occurs, resulting in a paramagnetic complex with five unpaired electrons
• [CoF6]3-: Cobalt has a 3d7 4s2 configuration, and as Co3+ it is a d6 system; in [CoF6]3-, cobalt undergoes sp3d2 hybridization to form an outer orbital octahedral complex; F- is a weak field ligand, so the complex is paramagnetic with four unpaired electrons
• [Co(NH3)6]3+: In [Co(NH3)6]3+, cobalt undergoes d2sp3 hybridization to form an inner orbital octahedral complex; NH3 is a strong field ligand, causing pairing of electrons and resulting in a diamagnetic complex
• [Cu(NH3)4]2+: Copper as Cu2+ is a d9 system; forming a square planar complex through dsp2 hybridization requires the promotion of one electron, leading to one unpaired electron and a paramagnetic complex