Bonding in Coordination Compounds

Bonding in Coordination Compounds

 

Valence Bond Theory

Valence bond theory states that the metal atom or ion under the influence of ligands can use its (n-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. These hybridised orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding.

The following table shows the combination of different numbers of orbitals to give different types of hybridization:

Thus the basic principles involved in the valence bond theory are:

•    Hybridisation of orbitals

•    Bonding between the ligands and the metal atoms/ion

•    Relation between the observed magnetic moment and the bond type

Limitation of the VB theory

•    It does not tell anything about the spectral properties of the complexes.

•    It does not give a quantitative interpretation of magnetic data.

•    It does not distinguish between strong and weak ligands.

•    It does not explain the colour exhibited by coordination compounds.

•    It does not give a quantitative interpretation of the thermodynamic or kinetic stabilities of coordination compounds.

Magnetic Properties of Coordination Compounds

A coordination compound is paramagnetic in nature if it has unpaired electrons and diamagnetic if all the electrons in the coordination compound are paired.

Crystal Field Theory

In crystal field theory (CFT), ligands are considered as point charges and the interaction between the ligands and the metal ion is purely electrostatic in nature. The five d-orbitals in an isolated gaseous metal atom/ion have same energy, i.e., they are degenerate. The degeneracy is lost in the presence of the ligand field.

The five d-orbitals are classified as:

Three d-orbitals, dxydyz and dzx are oriented in between the coordinate axes and are called t2g -orbitals.

Two d-orbitals, dx2 ‒ y2 and dz2 that are oriented along the x - y axes and are called eg - orbitals.

Factors affecting the splitting of d-orbitals

•    Nature of the ligand

•    Nature of the metal ions

•    Geometry of complex whether it is octahedral or tetrahedral

•    Oxidation state of the metal ion

 

·         Crystal field splitting in octahedral coordination complexes:

·         Crystal field splitting in tetrahedral coordination complexes:

·         For the same metal, the same ligands and metal-ligand distances, the difference in energy between eg and t2g level is

Colour in Coordination Compounds

The crystal field theory attributes the colour of the coordination compounds to d-d transition of the electron, i.e., the transition of an electron from t2g level to the higher eg level which accompanies the absorption of light in the visible spectrum.
In the absence of ligands, crystal field splitting does not occur and hence the substance is colourless.

Stability of coordination compounds

The stability of the coordination compound depends on

Nature of the ligand

Chelating ligands form strong and more stable complexes than the monodentate ligands. The π-  bond ligands forms more stable complexes than the σ- bonded complex.

Nature of the metal atom/ion

Small, highly charged metal ions form more stable complexes than large size.