WERNER’S THEORY OF COORDINATION COMPOUNDS

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Werner’s theory of coordination compounds, proposed by Swiss chemist Alfred Werner in 1893, for understanding of complex compounds and laid the foundation for modern coordination chemistry.
Werner’s theory explained the existence of coordination complexes, particularly their geometry, bonding, and coordination numbers, offering a coherent explanation for phenomena that had puzzled chemists for years. This theory was so significant that Werner was awarded the Nobel Prize in Chemistry in 1913.

Werner’s Propositions

Werner’s theory of coordination compounds introduced two fundamental ideas: the concept of primary and secondary valency, and the arrangement of ligands around the central metal ion.
1.Primary and Secondary Valency: Werner proposed that metal ions exhibit two types of valencies:
  • Primary Valency of oxidation state of the central metal ion. This valency is satisfied by negative ions, much like the traditional valency in ionic compounds.
  • Secondary Valency refers to the coordination number of the metal ion, which is the number of ligands attached to the central metal ion. Secondary valency is directional, and the ligands attached to the metal ion via secondary valency create a definite geometry around the metal center.
For example, in the complex [Co (NH3​)6​] Cl3​ the cobalt ion has a primary valency of +3 (satisfied by three chloride ions) and a secondary valency of 6 (satisfied by six ammonia molecules).
2. Coordination Number: Werner suggested that the central metal ion in a coordination compound binds a specific number of ligands, which he called the coordination number. Coordination number is typically 4 or 6 but can vary depending on the metal and the ligands involved. The coordination number directly influences the geometry of the complex. For example:
  • Coordination number 6 typically leads to an octahedral geometry.
  • Coordination number 4 can lead to either a square planar or tetrahedral geometry
3. Spatial Arrangement of Ligands: Another key feature of Werner’s theory is the spatial arrangement of ligands around the central metal ion. Werner assume that ligands form well-defined geometric structures. This explained certain coordination compounds exhibit isomerism, a concept that was previously not well understood. For example,
[Co (NH3​)4​] Cl2+ exists in both cis- and trans- isomers, where the arrangement of chloride ligands differs in space.

Types of Coordination Compounds

Werner’s theory is applicable to various types of coordination compounds, which are classified based on the metal ion, ligands, and overall structure:
  • Neutral complexes have no net charge. For example, [Ni (CO)4], where nickel has a coordination number of 4 and is bonded to four carbonyl ligands in a tetrahedral arrangement.
  • Cationic complexes have a positive charge. An example is  [Co(NH3)6]3+  where cobalt is in the +3 oxidation state and is coordinated to six ammonia ligand
  • Anionic complexes carry a negative charge. An example is  [Fe(CN)6​]4−, where iron is in the +2 oxidation state and is coordinated to six cyanide ligands in an octahedral arrangement.
Isomerism in Coordination Compounds
Werner’s theory also explains the phenomenon of isomerism in coordination compounds.
Two major types of isomerism are:
1.Geometrical Isomerism: This occurs when ligands can be arranged differently in space around the central metal ion. For example, in [Pt (NH3)2 Cl2], the two chloride ligands can either be adjacent (cis) or opposite each other (trans).
2. Optical Isomerism: Certain complexes, especially those with chiral centers, exhibit optical isomerism. These isomers are non-superimposable mirror images of each other, much like the relationship between left and right hands

Impact  and Legacy of Werner’s Theory

Werner’s theory changed the study of inorganic chemistry, providing a robust framework for understanding the bonding and structure of complex compounds. It was instrumental in the development of modern coordination chemistry and influenced the study of bioinorganic chemistry, organometallic chemistry, and catalytic processes.
Werner’s theory provided the basis for understanding how these complexes interact with other chemical species. His work smooth the way for the synthesis of new coordination compounds with applications in medicine (such as chemotherapy drugs like cisplatin), catalysis, and materials science.

Key Sentences

Alfred Werner’s theory of coordination compounds provided a explanation of the bonding and spatial arrangement of ligands around metal ions. By introducing the concepts of primary and secondary valency, coordination number, and isomerism, Werner’s theory remains one of the foundational principles of coordination chemistry.
Werner’s theory, proposed by Alfred Werner in 1893, explains the bonding, structure, and geometry of coordination compounds. It introduced the concepts of primary and secondary valency and established the idea of coordination numbers and spatial arrangement of ligands around a central metal ion.
Primary valency refers to the oxidation state of the central metal ion and is satisfied by anions or negatively charged ions.
Secondary valency corresponds to the coordination number of the metal ion and is satisfied by neutral molecules or anions, which form a specific geometric arrangement around the metal.
Coordination number is the number of ligands directly bonded to the central metal ion. It determines the geometry of the coordination complex, such as octahedral for a coordination number of 6 and tetrahedral or square planar for a coordination number of 4.
Werner’s theory explained that ligands are arranged in specific geometries around the metal ion, allowing for different spatial arrangements. This explained geometrical isomerism, where ligands occupy different positions, and optical isomerism, where compounds have non-superimposable mirror images.
Examples include [Co (N H3)6]Cl3, where cobalt is coordinated to six ammonia molecules, and  [Fe(C N)6]4−, where iron is coordinated to six cyanide ions in an octahedral arrangement.
Before Werner’s theory, the bonding and structure of coordination compounds were poorly understood. Werner’s theory provided a clear explanation for the bonding, spatial arrangement, and reactivity of these compounds, establishing modern coordination chemistry.

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