Postulates of Werner’s Theory

Alfred Werner (1866-1919), a Swiss physicist, was the first to formulate the architecture of coordination compounds. He synthesised and characterised a large number of coordination molecules, then investigated their physical and chemical behaviour using basic experimental approaches. Werner proposed that a metal ion had a primary valence and a secondary valence.

The capacity of metallic elements to behave as Lewis acids and form complexes with a variety of Lewis bases is one of their most important features. A metal complex is made up of a core metal atom or ion that is bound to one or more ligands, which are ions or molecules that have one or more pairs of electrons that can be shared with the metal.

Main Postulates of Werner’s Theory

Werner proposed his theory of coordination compounds in 1898. The following are the main tenets:

• Metals have two types of links (valences) in coordination compounds: primary and secondary
• Negative ions satisfy the primary valances, which are generally ionisable
• Secondary valences cannot be ionised. Neutral molecules or negative ions satisfy these needs. For a metal, the secondary valence is equal to the coordination number and is constant
• Ions/groups coupled to the metal by secondary connections have distinct spatial configurations that correspond to different coordination numbers

Such spatial arrangements are known as coordination polyhedra in current formulations. The ions outside the square bracket are called counter ions, and the species inside the square bracket are coordination entities or complexes.

Furthermore, he postulated that metal that octahedral, tetrahedral, and square planar geometries are more common in transition metal coordination complexes.

Coordination Entity

A coordination entity is a metal atom or ion with a fixed number of ions or molecules linked to it. The cobalt ion, for example, is surrounded by three ammonia molecules and three chloride ions in a coordination entity [Cocl3(NH3)3]

Central Atom

The core atom or ion in a coordination entity is the atom/ion to which a fixed number of ions/groups are bonded in a definite geometrical arrangement surrounding it.

Ligands

Ligands are ions or molecules that are bound to the coordination entity’s core atom/ion. Simple ions like Cl–, small molecules like H2O or NH3, bigger compounds like H2NCH2CH2NH2 or N(CH2CH2NH2)3, or even macromolecules like proteins are examples.

Coordination Number

The number of ligand donor atoms to which a metal ion in a complex is directly bound is known as the coordination number (CN).

Coordination Sphere

The coordination sphere is made up of the centre atom/ion and the ligands connected to it, which are all surrounded in square brackets. The ionisable groups are referred to as counter ions because they are written outside the bracket.

Coordination Polyhedron

A coordination polyhedron is defined by the spatial arrangement of the ligand atoms that are directly linked to the centre atom/ion. Octahedral, square planar, and tetrahedral coordination polyhedra are the most prevalent.

Conclusion

Postulates of Werner’s theory proposed that a metal atom/ion in a coordination complex uses two types of linkages (primary and secondary). The ionisable (ionic) and non-ionisable (covalent) bonds are the terms used in modern chemistry to describe these couplings. Werner anticipated the geometrical shapes of a large number of coordination entities using the isomerism property.

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