Bonding (Werner’s Theory, VBT, and CFT) 

1.0Introduction 

Bonding in coordination compounds explains how central metal ions interact with surrounding ligands. Over time, several theories such as Werner’s Theory, Valence Bond Theory (VBT), and Crystal Field Theory (CFT) were developed to explain the structure, stability, geometry, and magnetic properties of these compounds.

2.0Werner’s Theory of Coordination Compounds

Postulates of Werner’s Theory

• Metals exhibit two types of valencies: Primary (Ionizable) and Secondary (Non-ionizable).
• The primary valency corresponds to the oxidation state of the metal ion.
• The secondary valency corresponds to the coordination number, i.e., the number of ligands attached to the central atom.
• Ligands satisfying the secondary valency occupy fixed positions in space, leading to specific geometries.
• Compounds show isomerism based on the arrangement of ligands.

Primary and Secondary Valencies

• Primary Valency (Oxidation State): Satisfied by negative ions; ionizable in solution.
• Secondary Valency (Coordination Number): Occupied by ligands (molecules/ions); not ionizable.

Examples Based on Werner’s Theory

• [CoCl₃·6NH₃]: Co has a primary valency of 3 and a secondary valency of 6.
• [CoCl₂·5NH₃]: Co has a primary valency of 3 and secondary valency of 6, with two Cl⁻ inside the coordination sphere and one outside.

Learn More: Werner's Theory of Coordination Compounds

3.0Valence Bond Theory (VBT)

Basic Concept of VBT

• The Valence Bond Theory explains bonding through hybridization of metal orbitals and the overlap of ligand orbitals.
• The geometry of complexes depends on the type of hybrid orbitals formed.

Hybridization in Complexes

• Octahedral Complexes: sp³d² or d²sp³
• Tetrahedral Complexes: sp³
• Square Planar Complexes: dsp²

Types of Complexes

• Inner Orbital Complexes: Involve d²sp³ hybridization (low-spin).
• Outer Orbital Complexes: Involve sp³d² hybridization (high-spin).

Examples

• [Co(NH₃)₆]³⁺ → d²sp³ hybridization → Octahedral geometry.
• [Ni(CN)₄]²⁻ → dsp² hybridization → Square planar geometry.

Limitations of VBT

• Does not explain the colour of complexes.
• Fails to explain spectral and magnetic properties accurately.

4.0Crystal Field Theory (CFT)

In Crystal Field Theory (CFT), the interaction between the metal ion and ligands is considered purely electrostatic. When ligands approach the central metal ion, the degenerate (equal-energy) d-orbitals split into groups with different energy levels.

Crystal Field Splitting in Octahedral Complexes

• d-orbitals split into t₂g (lower energy) and e_g (higher energy) sets.
• Crystal field splitting energy = Δ₀.

Crystal Field Splitting in Tetrahedral Complexes

• Reverse splitting compared to octahedral complexes.
• e set (lower energy), t₂ set (higher energy).
• Splitting is smaller than in octahedral complexes (Δt ≈ 4/9 Δ₀).

Factors Affecting Crystal Field Splitting (Δ)

• Nature of the metal ion.
• Oxidation state of the metal.
• Nature of ligands (Spectrochemical Series).
• Geometry of the complex.

High Spin vs Low Spin Complexes

• High Spin Complex: Weak field ligands, small Δ, electrons remain unpaired.
• Low Spin Complex: Strong field ligands, large Δ, electrons pair up in lower orbitals.

Applications of CFT

• Explains color of transition metal complexes.
• Accounts for magnetic properties (paramagnetic/diamagnetic).
• Justifies stability of complexes.

Limitations of CFT

• Considers only electrostatic interactions, ignores covalent bonding.
• Cannot explain the spectra of all complexes.

5.0Comparison of Werner’s, Valence Bond and Crystal Field Theory