Introduction

In coordination compounds, there are two sorts of valencies, according to Werner’s ionisation of coordination compounds theory:

• The charge on complex ions is influenced by primary valencies, ionisable valencies that anions may meet.
• The quantity of secondary valencies dictates the coordination number of the metal atom. Secondary valencies are non-ionisable valencies that are filled by ligands.

Primary valency: Werner’s Theory of Coordination Compounds

Primary valencies are the valencies that metals show when they produce simple salts like cobalt chloride, sodium chloride, copper chloride, etc.

In current terminology, the symbol O represents a metal’s oxidation number.

The main valencies of Co are 3 in CoCl3, and the oxidation state of Co is +3.

Furthermore, the oxidation state of Na in NaCl is +1, while CuSO4 has an oxidation state of +2.

• Because the main valencies are ionisable, they may be combined.
• These are not written as part of the coordination process.
• In complex compounds, they are non-directional and provide no geometrical information.
• A good example is [Co(NH3)6]Cl3 , there are three essential valencies. The third state of oxidation (plus three)

Secondary valency: Werner’s theory of coordination compounds

• Negative ions, neutral molecules, or a combination of the two determine the secondary valency of metals.
• In current terminology, the metal’s coordination number is referred to as a whole number.
• The secondary valencies are expressed inside the coordination sphere of the coordinate system.
• They are directed in character and provide the complex with a certain shape.
• These are incapable of being ionised in any way.
• For example, the coordination number of [Co(NH3)6]Cl3 is 6.

The Valence bond Theory 

A metal atom or ion may use its (n-1)d, ns, np or ns, np, nd orbitals for hybridisation, resulting in a set of equivalent orbitals with specified geometry. This could be octahedral, tetrahedral, square planar, and so on, according to Valence bond theory. Valence bond theory is a branch of chemistry that studies molecular interactions. These hybridised orbitals may overlap with ligand orbitals, forming electron pairs for bond formation.

As a result, the following are the core concepts of valence bond theory:

• Hybridisation is the concept of intermixing of orbitals to form new hybrid orbitals having the same energy.
• A bond is created between the ligands and the metal atoms/ions.
• There are certain flaws with the Valence Bond idea.

It gives no details on the spectral features of the compounds in question.

• This program does not quantitatively analyse magnetic data.
• It doesn’t distinguish between strong ligands and those that are weak.
• However, it doesn’t explain why coordination molecules are coloured the way they are.
• The thermodynamic and kinetic stabilities of coordination molecules have never been quantified.

Crystal Field Theory

Ligands are treated as point charges in crystal field theory (CFT), and their interactions with metal ions are electrostatic. The energy of the five degenerate-orbitals in a gaseous metal atom/ion is the same, showing that they are degenerate, but the energies of the other four orbitals are different. In the presence of the ligand field, the degeneration is no longer visible.

The five d-orbitals may be divided into five categories:

The following are the definitions of three d-orbitals, dxy, dyz, and dzx, which are referred to as t2g –orbitals and are positioned between the coordinate axes:

There are two d-orbitals:  dx2 – y2 and dz2, which are perpendicular to the x-y axis and z -axis, called e.g. orbitals.

The following factors influence the splitting of d-orbitals:

• The ligand’s chemical structure
• The chemical composition of metal ions
• There are two types of complex geometry: octahedral and tetrahedral.
• The oxidation state of a metal ion is defined as
• The crystal field is split up into octahedral compounds.

In this situation, the energy of the set of orbitals exceeds that of the t2g set of orbitals. Energy separation, or P, is larger than one for ligands with a large spin complex (the pairing energy, i.e., the energy required for electron pairing in a single orbital).

Ligands form low spin complexes with a substantial energy separation, O > P. where O is splitting energy and P is pairing energy.

The Most Significant Difference Between VBT and CFT

“Valence bond theory” is the name given to the theory of valence bonding. It’s a chemical bonding theory that explains how atoms create various chemical connections. This theory describes how chemical bonds are formed by overlapping or mixing atomic orbitals. The term “crystal field theory” is used to describe this concept (CFT). It’s a theory that explains how electron orbitals (usually d or f orbitals) break degeneracies (equal energy electron shells) as a result of a static electric field created by an anion or anions near the electron orbitals (or ligands).

Conclusion

This theory describes how chemical bonds are formed by overlapping or mixing atomic orbitals. The term “crystal field theory” is used to describe this concept (CFT). It’s a theory that explains how electron orbitals (usually d or f orbitals) break degeneracies (equal energy electron shells) as a result of a static electric field created by an anion or anions near the electron orbitals (or ligands).After learning about Werner’s theory VBT, CFT, you must immediately prepare for your exams. As soon as possible, begin planning for the event. Werner’s explanation for the coordinate compound’s colour was insufficient. Coordination compounds’ magnetic and optical characteristics could not be fully explained.