Chemical bonding model
Molecular bonding models are theoretical models used to explain molecular geometry, properties, and reactivity.
Overview
The models can be divided into two categories the repulsive and the attractive. The repulsive category which include VSEPR Theory and CFT assumes that electrons repulse each, accept in bonding pairs for VSEPR Theory. This repulsion results in the observed molecular geometries and properties. The attractive category which includes VBT, LFT, and MO Theory assumes that bonds are formed by atoms sharing electrons. While the attractive theories have proven to be better representations of reality the repulsive categories still have their uses although very limited. VSEPR Theory and CFT are widely considered the simplest and historic ways to introduce molecular structure and d-orbital splitting to students. As a result, material based on these theories is still included in most courses and most standardized tests. This practice is questionable, since these theories have extremely limited application in current research which generally applies variations of MO theory.
The Major Models
Valence Bond Theory (VBT) the earliest bonding theory still applied and taught. Components of the theory have been incorporated into all subsequent theories. VBT views bonds as weakly coupled orbitals with each atom sharing a valence electron in a manner governed by the octet or 18 electron rule. Lewis structures are a representation of VBT's most basic bonding while molecular geometry is derived from orbital hybridization. Orbital hybridization is often taught with VESPR Theory despite significant differences in the underlying theory.
Valence shell electron pair repulsion (VSEPR) Theory The simplest and most primitive of the theories currently taught. Describes geometry through the repulsion of electron fields which include bonds and lone pairs. It does not require any understanding of orbital shape. MO theory in contrast requires an understanding of orbital symmetry and the associated group theory needed to find linear combinations of orbitals.
Crystal Field Theory (CFT) This approximation begins with the geometries of the d orbitals derived from quantum mechanics. Ligands with their electron density are assumed to destabilize the metal d orbitals they interact with raising their energy while the remain d-orbitals drop in energy to balance the overall change in energy.
Ligand Field Theory (LFT) Considered a hybrid of CFT and MO Theory or simple an approximate application of MO Theory to transition metal complexes.
Molecular Orbital (MO) Theory The current and most often applied model of molecular bonding. MO Theory assumes that bonds are derived from a linear combination of atomic orbitals. In this linear combination each pair of atomic orbitals involved in bonding results in a bonding and anti-bonding orbital. The destabilized of orbitals of CFT are now seen as anti-bonding component of orbitals that have overall been stabilized through bonding interactions.
Computational Chemistry
Modern computational chemistry applies components of molecular models, most often MO Theory to simulate various chemical phenomenon associated with bonding. Computational chemistry also extends beyond covalent bonding to investigate the interactions of groups of molecules and higher order structure. In these systems the chemical bond is often simplified and approximated to reduce computing time.