Werner’s coordination theory

ALFRED WERNER

Alfred Werner was a Swiss chemist (1866-1919). Though metal complexes were known to chemists even before his time, very little was known about the bonding and structure of such compounds. Werner was the first to explain these aspects of complexes successfully. He proposed his theory on coordination compounds, now called Werner’s theory, by applying the theory of electrolytic dissociation and the principles of structural chemistry ingeniously to these compounds.

Werner’s theory triggered the imagination of the other chemists after his time in proposing advanced theories on metal complexes. His theory and pioneering experimental work on metal complexes won him the Nobel Prize for Chemistry in 1913.

In 1893 Alfred Werner, at the age of 26, proposed a theory which is now referred to as Werner’s coordination theory. His theory is a guiding principle in the concept of valence in inorganic chemistry. The well-known concept of ‘primary’ and ‘secondary valencies in metal complexes has been developed by him.

Postulates of Werner's Theory

Generally, elements exhibit two types of valencies, namely primary valency and secondary valency. The primary valency is also known as ionisable valency, and the secondary valency is otherwise known as nonionisable valency. Anions can satisfy the primary valency, whereas anions or neutral molecules can satisfy the secondary valency. In modern terms, the primary valency corresponds to the oxidation number, and the secondary valency corresponds to the coordination number.

Generally, all elements tend to satisfy both primary and secondary valencies. But in every case, the fulfilment of secondary valency appears to be more essential. For example, in CoCl₃.4NH₃, two of the three chloride ions are attached by secondary valency; hence, it is represented as [Co(NH₃)₄Cl₂)Cl.

The secondary valencies are directed towards some fixed positions in space. For example, In 4-coordinated complexes, the four valencies are arranged in either a planar or a tetrahedral manner, and in 6-coordinated complexes, the six valencies are directed toward the six corners of an octahedron.

Werner studied the amminechlorocobalt(III) complexes to substantiate his theory.

According to Werner’s theory, the first member of the series (A) is formulated as [Co(NH₃)₆]Cl₃. In this complex, the primary valencies (longer dotted lines) are satisfied by the three chloride ions.

The six secondary valencies (shorter solid lines) are satisfied by the six ammonia molecules. These ammonia molecules are very tightly bound to cobalt; hence, they do not dissociate in solution. But the three chloride ions are far away from the central cobalt, and hence they are less firmly held by the metal. Therefore, all three chloride ions dissociate in solution, giving [Co(NH₃)₆]³⁺and 3Cl^– ions, with a total of four ions. Thus, Ag⁺ precipitates all three chloride ions as AgCl, and the molar conductivity of the complex corresponds to that of a ter-univalent electrolyte (~404 ohm^-1).

Complex	No. of ions precipitated as AgCl	Formula	No. of Cl ions in solution	Molar conductivity (Ohm^-1)
CoCl₃.6NH₃	3	[Co(NH₃)₆]Cl₃	3	404
CoCl₃.5NH₃	2	[Co(NH₃)₅Cl]Cl₂	2	229
CoCl₃.4NH₃	1	[Co(NH₃)₄Cl₂]Cl	1	97
CoCl₃.3NH₃	0	[Co(NH₃)₆Cl₃]	0	0

The metal and ligand bond is called a coordinate covalent or dative (donating) bond. It involves sharing a pair of electrons between two atoms, the electrons originating from one of the atoms. A covalent bond involves sharing two electrons, usually an electron originating from each bonded atom. A coordinate covalent bond is a type of covalent bond.

The second member (B) of the series, CoCl₃.5NH_3, is formulated as [Co(NH₃)₅Cl]Cl₂. Since only five ammonia molecules satisfy the secondary valences, one chloride ion must play the double role of satisfying both a primary and a secondary valency. This is because the fulfillment of all secondary valencies is essential, according to one of Werner’s postulates.

Werner represented the bond between such a ligand and the central metal by a combined solid and dotted line, as shown in (B). The chloride, playing the double role, is very firmly held by the central metal and hence is not precipitated as AgCl by Ag⁺ ions in the solution. Two-thirds of the chloride content of the complex is present in the second coordination sphere; therefore, it is formulated as [Co(NH₃)₅Cl]Cl₂. The solution gives a total of 3 ions, [Co(NH₃)₅Cl]²⁺ and 2Cl^–. Its molar conductivity corresponds to that of a bi-univalent electrolyte (~229 ohm-¹). The extension of this theory to the next member of the series CoCl₃.4NH₃ leads to the structure (C) with the Werner formulation [Co(NH₃)₄Cl₂|Cl.

The two chloride ions in this complex satisfy both the primary and secondary valencies. Hence, both of them are tightly bound to the central metal. In solution, the complex dissociates into two ions, [Co(NH₃)₄Cl₂]⁺ and Cl^–. Only 1/3 of the chloride content of the complex gets precipitated as AgCl, and its molar conductivity corresponds to that of a uni-univalent electrolyte (~97 ohms).

Werner’s theory predicts structure (D) for the next member of the series, CoCl3.3NH3 and it is formulated as [Co(NH₃)₃C1₃].

This theory also predicts that this complex will not yield any chloride ions in solution. Actually, no chloride ion is precipitated as AgCl on treatment of D with AgNO₃ solution. It is a non-electrolyte in solution because no ions are produced in solution.

Defects of Werner's Theory

Werner’s theory describes the structures of many coordination compounds successfully; however, it does not explain the nature of bonding within the coordination sphere.

More than 90% of the known complexes at Werner’s time were either 4-coordinated or 6-coordinated. Werner’s theory is unable to account for the preference between 4- and 6-coordination among complexes.

Werner’s theory fails to account for the fact that certain 4-coordinated complexes are square planar whereas some others are tetrahedral.

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