Wernerstheoryofcoordinationcompoundspdf14
Werner's Theory of Coordination Compounds
Coordination compounds are substances that contain a central metal atom or ion bonded to one or more ligands, which are ions or molecules that can donate a pair of electrons to the metal. Coordination compounds have many applications in chemistry, biology, and industry, such as catalysts, pigments, and metalloproteins. However, the structure and formation of these compounds were not well understood until the late 19th century, when a Swiss chemist named Alfred Werner proposed a theory that explained their properties and behavior.
Postulates of Werner's Theory
Werner's theory, which was published in 1893, was based on the following postulates:
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Metal atoms or ions have two types of valence: primary and secondary.
Primary valence corresponds to the oxidation state of the metal and is satisfied by negative ions outside the coordination sphere.
Secondary valence corresponds to the coordination number of the metal and is satisfied by ligands inside the coordination sphere.
The secondary valence is always constant for a given metal ion, regardless of the nature of the ligands.
The ligands in the coordination sphere are arranged in a definite geometric pattern around the metal.
The coordination sphere is the group of atoms or ions that are directly bonded to the metal. It is enclosed by square brackets in the formula of a coordination compound. For example, in [Co(NH3)6]Cl3, the coordination sphere is [Co(NH3)6] and the primary valence of Co is 3, while the secondary valence (or coordination number) is 6. The ligands are NH3 molecules, which are arranged in an octahedral geometry around Co.
Examples of Werner's Theory
Werner's theory was able to explain many phenomena that were puzzling to chemists at that time. For instance, he correctly predicted the existence and properties of different isomers of coordination compounds, such as cis- and trans- [Co(NH3)4Cl2]Cl. He also explained why some compounds, such as [Co(NH3)5Cl]Cl2, exhibited optical activity, while others, such as [Co(NH3)6]Cl3, did not. He also clarified the difference between double salts and complex salts. Double salts are formed by the combination of two simple salts in a fixed ratio, such as KAl(SO4)2.12H2O (potassium alum). They dissociate completely into their constituent ions in water. Complex salts are formed by the combination of a metal ion and a complex ion, such as K[Fe(CN)6] (potassium ferrocyanide). They do not dissociate completely in water; only the counterions separate from the complex ion.
Evidences for Werner's Theory
Werner's theory was supported by various experimental evidences, such as conductivity measurements, magnetic susceptibility measurements, molecular weight determinations, and analytical methods. For example, he used conductivity measurements to distinguish between different types of complexes. He found that complexes with primary valence equal to zero, such as [Co(NH3)6]Cl3, had low conductivity in water, indicating that they did not dissociate into ions. On the other hand, complexes with primary valence greater than zero, such as [Co(NH3)5H2O]SO4, had high conductivity in water, indicating that they dissociated into ions. He also used magnetic susceptibility measurements to determine the number of unpaired electrons in the metal ion, which correlated with its oxidation state and coordination number. For example, he found that [Co(NH3)6]Cl3 was diamagnetic, meaning that it had no unpaired electrons, while [Co(NH3)5Cl]Cl2 was paramagnetic, meaning that it had one unpaired electron.
Limitations of Werner's Theory
Although Werner's theory was a major breakthrough in the field of coordination chemistry, it had some limitations and drawbacks. For example, it could not explain the following aspects:
The nature and origin of the bond between the metal and the ligand.
The factors affecting the stability and reactivity of coordination compounds.
The variation of color and magnetic properties of coordination compounds with different ligands.
The formation and properties of coordination compounds with variable coordination numbers and geometries.
These aspects were later explained by more advanced theories, such as valence bond theory, crystal field theory, and ligand field theory, which took into account the electronic structure of the metal and the ligand, as well as the spatial arrangement of the ligands around the metal.
Conclusion
Werner's theory of coordination compounds was a landmark in the history of chemistry. It provided a rational and systematic way of describing and classifying coordination compounds, which were previously considered as mysterious and arbitrary. It also paved the way for further research and development in the field of coordination chemistry, which has many applications in various domains of science and technology.
References
[24.1: Werners Theory of Coordination Compounds]
[Chapter 23 Chemistry of Coordination Compounds]
[Werners Theory - Introduction, Postulates, Examples, Evidences ...]
[Coordination Chemistry: Bonding Theories](
[Coordination Chemistry: Werner's Theory](