3._Valence_Bond_Theory_-_Filled_in

Valence Bond Theory (VBT)

Introduction

Valence Bond Theory, developed by George E.P. Box, is a model used to explain how atoms bond together in molecules. Box famously stated, 'All models are wrong, but some are useful,' highlighting the limitations and applicability of theoretical frameworks in chemistry.

Applicability

• Organic Molecules: VBT proves particularly useful in the modeling of organic compounds, where the bonding characteristics can often be predicted accurately.

• Inorganic Bonding: The theory is partially effective for certain inorganic bonding situations but is not the preferred model due to its limitations.

Core Concept

Valence Bond Theory is a localized quantum-mechanical model that explains atomic bonding through two main processes:

1. Hybridization of Atomic Orbitals: This involves the mixing of different types of atomic orbitals to create new hybrid orbitals that are better suited for bonding.

2. Promotion of Electrons: To accommodate bonding preferences, some electrons are promoted to higher energy levels, which aligns with experimental observations and allows for the formation of bonds.

Bonding in H2

• Description: In hydrogen (H2), two hydrogen atomic orbitals overlap, resulting in the formation of a single bond.

• Orbital Characteristics:

  • s-bond (Sigma Bond): This is formed by the head-on overlap of atomic orbitals, creating a symmetric bonding interaction about the bonding axis.

Bonding in N2 (Uncorrected)

• Electron Configuration: For nitrogen (N), the electron configuration is [He] 2s², 2p³.

• Bonding Characteristics: The formation of a triple bond includes:

  • π-bond: Created through the side-on overlap of atomic orbitals, resulting in an asymmetrical bonding interaction about the bonding axis.

Problems with Valence Bond Theory

Experimental Observations

• When comparing predictions made by uncorrected VBT with experimental data for molecules like nitrogen (N2) and methane (CH4), significant discrepancies arise.

• Issues in Bond Directionality: For instance, the C-H bonds in methane (CH4) should exhibit directional properties, yet VBT predicts non-directional bonding, which contradicts observable outcomes.

Corrections to Valence Bond Theory: Hybridization and Promotion

Hybridization Concepts

• In the case of methane (CH4):

  • Steric Number (SN): The steric number is 4, indicating the presence of four bonded pairs of electrons, leading to a tetrahedral molecular geometry.

  • Atomic Orbital (AO) Hybridization: This involves the mixing of one 2s and three 2p atomic orbitals to form four equivalent sp³ hybrid orbitals.

Hybridization of AOs Depends on Geometry

Here is a summary of hybridization types and their associated geometries:

• Linear: sp (2 hybrid orbitals)

• Trigonal Planar: sp² (3 hybrid orbitals)

• Tetrahedral: sp³ (4 hybrid orbitals)

• Square Planar: sp²d (4 hybrid orbitals)

• Trigonal Bipyramidal: sp³d (5 hybrid orbitals)

• Square Pyramidal: sp³d (5 hybrid orbitals)

• Octahedral: sp³d² (6 hybrid orbitals)

Note on d Orbitals

• It is important to note that d orbitals are generally less likely to participate in bonding for main group elements, yet they are crucial in the formation of bonds with transition metal complexes.

Orbital Directionality and Hybridization

• Concept: Bonding involves the constructive and destructive interference of wave functions from overlapping atomic orbitals, which ultimately leads to bond formation.

• Molecular Geometries: The mixing of orbitals contributes to the unique geometries and properties of the resulting molecules.

Mixing Orbitals to Make Bonds

• Mechanism: Hybridization involves the combination of atomic orbitals of one atom with those of another to create bonding orbitals.

  • For instance, in methane (CH4), four C-H bonds are formed through hybridized orbitals, ensuring optimal geometry and overlap for strong bonding.