Theory of bonding

Theory Of Bonding


An explanation for observed chemical and spectroscopic behavior.
Theory Of Bonding
Valence-bond theory
Molecular orbital theory
Atomic orbital theory
Crystal field theory



Sometimes, simple overlap between s and/or p orbitals can’t explain the actual shape or properties of compounds

Theory Of Bonding
Theory Of Bonding

How do bonds form?
The valence bond model or atomic orbital model explains how atoms come together and form molecules.
The model theorizes that a covalent bond forms when two orbitals overlap to produce a new combined orbital containing two electrons of opposite spin.
This overlapping results in a decrease in the energy of the atoms forming the bond.
The shared electron pair is most likely to be found in the space between the two nuclei of the atoms forming the bonds.

Theory Of Bonding
Valence-bond theory
• Three main points:
1. Covalent bonding is a result of overlap of atomic orbitals present in each of the components of the bond.
2. These overlapping orbitals must contain two e-
of opposite spin.
3. As a result of overlap, the probability of finding electrons in the space between the nucleii is increased.


Theory Of Bonding
Example H2

The newly combined orbital will contain an electron pair with opposite spin just like a filled atomic orbital.
Theory Of Bonding
Other Points on the Valence Bond Theory
This theory can also be applied to molecules with more than two atoms such as water.
Each covalent bond results in a new combined orbital with two oppositely spinning electrons.
In order for atoms to bond according to the valence bond model, the orbitals must have an unpaired electron.

Theory Of Bonding
Example HF
In hydrogen fluoride the 1s orbital of the H will overlap with the half-filled 2p orbital of the F forming a covalent bond.

Theory Of Bonding
Hybrid Electron Domain
Orbital Set Geometry

sp linear
sp2 trigonal planar
sp3 tetrahedral
sp3d trigonal bipyramidal
sp3d2 octahedral

Theory Of Bonding
Advantages of Valence Bond Theory
Describes correctly a process of bond forming / bond breaking with a very compact wave function, providing a very simple and intuitive tool for the understandings of chemical bonds.
Calculates the energy of individual VB structures (diabatic states), which provides chemical insights into the origin of reaction barriers.
Calculates the resonance energy, which is an important concept in electronic delocalization effects.

Theory Of Bonding
Summary of Valence Bond Theory

Write an acceptable Lewis structure for the molecule.
Determine the number of VSEPR objects around all central atoms and determine the geometry around the atom.
Construct hybrid orbitals suitable for the predicted bonding.
Link orbitals together to make bonds.
Describe the bonding. Include the names of the orbitals involved in each bond. Draw pictures of the bonds formed by the overlap of these orbitals.
Two objects around Be, so AX2 (linear)
Two orbitals pointing 180° from each other needed, so use two sp hybrids
Theory Of Bonding
Crystal field theory



The relationship between colors and complex metal ions
(CFT) is a model that describes the electronic structure of transition metal compounds, all of which can be considered coordination complexes
Theory Of Bonding
Concentrates on the splitting of the d orbitals of the transition metal atom into groups as a result of electrostatic interactions between the ligands and the electrons in the unhybridized orbitals of the transition metal atom.
Because CFT bases all interactions on electrostatics, each ligand is represented by a negative point charge. The way these point charges interact with the d-orbitals of a metal atom explains the geometries adopted by various transition metal complexes


Theory Of Bonding
Crystal Field Theory - Describes bonding in Metal Complexes
Basic Assumption in CFT:
Electrostatic interaction between ligand and metal

d-orbitals align along the octahedral axis will be affected the most.
More directly the ligand attacks the metal orbital, the higher the the energy of the d-orbital.
In an octahedral field the degeneracy of the five d-orbitals is lifted
Theory Of Bonding
d-Orbitals and Ligand Interaction
(Octahedral Field)

Ligands approach metal
d-orbitals pointing directly at axis are affected most by electrostatic interaction
d-orbitals not pointing directly at axis are least affected (stabilized) by electrostatic interaction
Theory Of Bonding
Electron Configuration in Octahedral Field
Electron configuration of metal ion:
s-electrons are lost first.
Ti3+ is a d1, V3+ is d2 , and Cr3+ is d3
Hund`s rule:
First three electrons are in separate d orbitals with their spins parallel.
Fourth e- has choice:
Higher orbital if  is small; High spin
Lower orbital if  is large: Low spin.
Weak field ligands
Small  , High spin complex
Strong field Ligands
Large  , Low spin complex
Theory Of Bonding
Color Absorption of Co3+ Complexes
The Colors of Some Complexes of the Co3+ Ion

Theory Of Bonding
Crystal Field Theory provides a basis for explaining many features of transition-metal complexes. Examples include why transition metal complexes are highly colored, and why some are paramagnetic while others are diamagnetic. The spectrochemical series for ligands explains nicely the origin of color and magnetism for these compounds. There is evidence to suggest that the metal-ligand bond has covalent character which explains why these complexes are very stable.
Theory Of Bonding

Molecular orbital theory $ Atomic orbital theory

The goal of molecular orbital theory is to describe molecules in a similar way to how we describe atoms, that is, in terms of orbitals, orbital diagrams, and electron configurations.

Theory Of Bonding

In atoms, electrons occupy atomic orbitals, but in molecules they occupy similar molecular orbitals which surround the molecule.
The two 1s atomic orbitals combine to form two molecular orbitals, one bonding (s) and one antibonding (s*).
Theory Of Bonding
This is an illustration of molecular orbital diagram of H2.
Notice that one electron from each atom is being “shared” to form a covalent bond. This is an example of orbital mixing.
Theory Of Bonding

The molecular orbital volume encompasses the whole molecule.
The electrons Each line in the diagram represents an orbital.
fill the molecular orbitals of molecules like electrons fill atomic orbitals in atoms

Theory Of Bonding

Electrons go into the lowest energy orbital available to form lowest potential energy for the molecule.
The maximum number of electrons in each molecular orbital is two. (Pauli exclusion principle)
One electron goes into orbitals of equal energy, with parallel spin, before they begin to pair up. (Hund`s Rule.)

Theory Of Bonding
Conclusions
Bonding electrons are localized between atoms (or are lone pairs).
Atomic orbitals overlap to form bonds.
Two electrons of opposite spin can occupy the overlapping orbitals.
Bonding increases the probability of finding electrons in between atoms.
It is also possible for atoms to form ionic and metallic bonds.

Theory Of Bonding