Werner’s Theory
Werners theory of coordination compounds, isomers in coordination compounds, primary valence, secondary valence, etc.
Werner, a well-known scientist, proposed his theory of coordination compounds in 1823, which describes the formation and structure of complex compounds and was later named Werner’s Theory of Coordinate Compounds.
He was given the Nobel Prize for this theory, and he is also known as the “Father of Coordination Chemistry.”
Werner’s theory’s important postulates are as follows:
- The central metal or metal atoms in coordination compounds have two forms of valency which are primary valency and secondary valency. The oxidation state corresponds to the primary valency, while the coordinate number corresponds to the secondary valency.
- Each metal atom has a fixed number of secondary valencies, like it has a fixed coordinate number.
- The metal atom satisfies both its primary and secondary valencies. Negative ions satisfy primary valencies, but neutral molecules or negative ions satisfy secondary valencies.
- The secondary valencies are always directed towards a particular place in space, resulting in determination of geometry of the coordinate compound’s. As an example: A metal ion’s secondary valencies are grouped octahedrally around the central metal ion if it has six of them. If the metal ion contains four secondary valencies, they are organized in a tetrahedral or square planar pattern around the center metal ion. The stereochemistry of the complex ion is thus determined by the secondary valency while the primary valency is non-directional.
Example
CoCl3.6NH3 Complex: The coordination number of Co in this compound is 6, and all six secondary valencies are met by ammonia molecules . Cl- ions satisfy the three primary valencies . These have a non-directional character. When silver nitrate is added, these chloride ions precipitate instantly. In this scenario, there are four ions: three chloride ions and one complex ion. While writing the compound’s formula, the central ion and neutral molecules or ions satisfying secondary valencies are enclosed in square brackets. Therefore, the complex can be written as [Co(NH3 )6]Cl3 and is depicted in fig.
CoCl3.5NH3 complex: The coordination number of cobalt in this compound is also 6, but the number of NH3 molecules is reduced to 5 from 6 and one remaining slot is now occupied by chloride ions. Because it has both primary and secondary valency, this chloride ion exhibits dual behavior. In the figure, the secondary valency is represented by a full line, while the main valency is represented by a dotted line.
This structure satisfies cobalt’s three primary and six secondary valencies. As a result, the complex formed can be written as [CoCl(NH3)5]Cl2 with five ammonia molecules and one chloride ion inside the square brackets and two chloride ions outside the brackets.
CoCl3.4NH3 complex: Two chloride ions in this compound exhibit dual behavior, satisfying both Primary and Secondary Valencies. This compound will precipitate AgNO3, which corresponds to one Cl- ion, and the total number of ions, in this case is two. As a result, it can be written as [CoCl2(NH3)4]Cl.
CoCl3.3NH3 complex: In this molecule, three chloride ions satisfy primary and also secondary valency. At room temperature, silver nitrate does not precipitate Cl-. Hence, the complex compound behaves as a neutral non-conducting molecule. It may be written as [CoCl3(NH3)3].
Isomers in coordination compounds and Werner’s Theory
Werner shifted his attention to the geometrical configurations of the coordinated groups around the central cation and effectively explained the origin of these compounds’ optical and geometrical isomerism. The following are some examples:
[CoCl2(NH3)4]Cl: Werner stated that there are three theoretical structures for this complex. Planar, trigonal prisms and octahedral are examples. There are three possible isomers for a planar structure, three for a trigonal prism, and two for an octahedral structure.
PRESENTED BY :- Miss Priyanka Dash (Lect. in Chemistry)
