Werner's Theory
In 1893, Werner produced a theory to explain the structures, formation and nature of bonding in the coordination compounds. This theory is known as Werner's theory of coordination compounds.
Some important postulates of this theory are given below-
- The central metal atoms in coordination compounds show two types of valency. first one is the primary valency (Principal or Ionizable) and the second one is the secondary valency (Auxiliary or non-Ionizable).
- The primary valency relates to the oxidation state and the secondary valency relates to the coordinate number.
- The number of secondary valences is fixed for every metal atom. that means the coordination number is fixed.
- Central metal atom satisfy both its primary and secondary valencies. Primary valency is satisfied by negative ion whereas secondary valancies are satisfied by negative ion or by neutral molecules.
- The secondary valancies are always directed towards fixed position in space and this cause definite geometry of the coordinate compound. For examples: If a metal ion has six secondary valencies, these are arranged octahedrally around the central metal ion. If the metal ion has four secondary valencies, these are arranged in either tetrahedral or square planar arrangement around the central metal ion. The secondary valency thus determines the stereochemistry of the complex ion. While the primary valency is non-directional.
- The secondary valencies are generally represented by solid lines while the primary valencies are represented by dashed lines and the ions which satisfy both primary and secondary valencies will be drawn with both solid and dashed lines.
Limitations of Werne'rs Theory
- It could not explain the inability of all elements to form coordination compounds.
- The Werners theory could not explain the directional properties of bonds in various coordination compounds.
- It does not explain the colour, the magnetic and optical properties shown by coordination compounds.
Test Your Knowledge: Werner's Theory
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The primary valency corresponds to the oxidation state of the central metal ion. It is satisfied by negative ions (anions) that readily dissociate or ionize when the complex dissolves in a solution.
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This statement is incorrect because the secondary valency is fixed for a given metal ion and does not depend on the nature of the ligand. It defines the coordination number and spatial arrangement of ligands around the central atom.
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Since only two chloride ions are ionisable, they must be outside the coordination sphere (primary valency). The third chloride is coordinated to the metal through secondary valency and is not ionisable.
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The geometry of a coordination complex is dictated by the secondary valencies, which correspond to the coordination number and spatial arrangement of ligands around the central metal ion.
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Primary valencies correspond to oxidation states and are satisfied by anions that ionize in solution. Secondary valencies correspond to coordination number and are satisfied by ligands bound directly to the metal, hence non-ionizable.
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Werner’s predictions about spatial arrangements of ligands were confirmed by X-ray crystallography, which revealed octahedral and square planar geometries consistent with his theory.
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The coordination number is defined by the number of ligands directly attached to the central metal ion via secondary valencies, not by the oxidation state or ionic radius alone.
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In this complex, three chloride ions are outside the coordination sphere, satisfying primary valency. They are ionizable and precipitate as AgCl when treated with AgNO3.
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Ionization isomerism arises when ligands exchange between the coordination sphere and the ionizable sphere, a concept directly predicted by Werner’s distinction between primary and secondary valencies.
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Secondary valencies correspond to the coordination number and dictate the geometry of the complex (octahedral, tetrahedral, square planar, etc.), while primary valencies only determine ionization.
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Ionization isomerism occurs when ligands exchange between the coordination sphere and the ionizable sphere, leading to different ions in solution.
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Coordination number 6 typically corresponds to an octahedral arrangement of ligands around the central metal ion, as predicted by Werner’s theory.
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Werner’s theory explained bonding and geometry but could not account for electronic properties such as magnetism and color. Crystal Field Theory later addressed these limitations.
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This square planar complex can exist in cis and trans forms, demonstrating geometrical isomerism.
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Werner’s theory was pioneering because it established the distinction between ionizable and non-ionizable valencies, predicting coordination numbers and geometries that remain central to coordination chemistry today.