expand valence bond theory - Chemistry - Coordination Compounds

Question

expand valence bond theory?

Kirti Gogia · Accepted Answer

The Valence Bond Theory was introduced by Heitler and London
(1927); developed further by Pauling and others This theory is
basically concerned with the energetics of covalent bond formation
about which lewis and VSEPR models are silent
1) The atomic orbitals of two atoms overlap with each other to
form a bond Greater is the overlapping, stronger is the bond formed
The potential energy of the system gets lowered as the two atoms
come close to each other
3) At the equilibrium inter nuclear distance (bond distance) the
energy touches a minimum Any attempt to bring the nuclei still
closer results in a sudden increase in energy and consequently
destabilisation of molecule
4) Because of orbital overlap electron density increases between
the nuclei, which brings them together
5) The direction of covalent bond is along the region of overlap
of atomic orbitals i e covalent bond is directional
6) There are two types of covalent bond, depending on the nature
of overlap - Ïƒ bond and Ï€ bond
7) Ïƒ bond is formed by the head-to-head overlap of
orbitals along the inter-nuclear axis These are of three types s-s,
s-p and p-p
8) Ï€ bond formed by the sideways overlap of orbitals
P, d and f orbitals form Ï€ bonds by sideways overlap
9) It also takes into account the concept of Hybridisation It
states that atomic orbitals of slightly different energies combine
to form a new set of equivalent orbitals (having the same energy
and shape) known as hybrid orbitals
These hybrid orbitals are more stable than atomic orbitals, and
participate in bond formation
The VB theory and concept of Hybridisation can also be applied
to co-ordination compounds The basic principles are
1) The metal atom or ion under the influence of ligands
can use its (nâˆ’1)d,
ns, np or ns, np, nd
orbitals for hybridisation, to yield a set of equivalent orbitals
of definite geometry such as octahedral, tetrahedral, square
planar, and so on
2) These hybridised orbitals are allowed to overlap with
ligand orbitals that can donate electron pairs for
bonding