Werner’s Theory of Coordination Compounds: Learn All Concepts

Werner’s theory of coordination compounds is a significantly important chapter in Class 12 Chemistry. We know that the transition metals form a number of complex compounds where the metal atoms are bound to many anions or neutral molecules. In modern terminology, such compounds are commonly referred to as coordination compounds. The chemistry of coordination compounds is an important and challenging area in modern inorganic chemistry. However, this property is not restricted to transition metals but is also exhibited by certain other metals to a small extent (Example: Chlorophyll is a coordination compound of magnesium).

The Werner’s postulate states that the central metals of coordination compounds exhibit two types of valences. They are – Primary Valency and Secondary Valency. In this article, we have discussed important concepts on Werner’s theory of coordination compounds including the postulate, explanation, evidence, limitation, and heteroleptic complex. Read till the end!

What is Werner’s Theory of Coordination Compounds?

The coordination compounds were known since the \({\text{18th}}\) century. However, no satisfactory theory was available to explain the observed properties of these compounds. Alfred Werner, in \(1893\) put forward his concept of auxiliary (secondary) valency for advancing a correct explanation for the characteristics of the coordination compounds. 

Postulates of Werner’s Theory of Coordinating Compounds

The fundamental postulates of Werner’s theory, which was a result of a dream and made Werner a Nobel Prize winner in chemistry, are summarised below:

1. Metals possess two types of valencies: primary(principal) or ionisable valency and secondary (auxiliary) or non-ionisable valency.

Primary valencies are those which a metal usually exhibits in the formation of its simple salts. Thus, in the formation of \({\text{PtC}}{{\text{l}}_4},{\text{CuS}}{{\text{O}}_4}\), and \(\mathrm{AgCl}\), The primary valencies of \(\mathrm{Pt}, \mathrm{Cu}\), and \(\mathrm{Ag}\) are \(4, 2\) and \(1\), respectively. Primary valencies are satisfied by negative ions.

Secondary valencies are those which a metal cation exercises towards a neutral molecule or negative group in forming its complex ions. Thus, secondary valencies may be satisfied by negative ions, neutral molecules having lone electron pair (Example: \({{\text{H}}_2}{\text{O}},{\text{N}}{{\text{H}}_3}\), etc.) or even sometimes by some positive groups. In every case, the coordination number of the metal must be fulfilled.

In modern terminology, the primary valency corresponds to oxidation number, and the secondary valency corresponds to coordination number. Primary valencies are shown by dotted lines, while secondary by thick lines.

2. Every metal has a fixed number of secondary valencies, for example, \(\mathrm{Co}^{3+}\) and \(\mathrm{Pt}^{4+}\) were recognised to have six secondary valencies, and \(\mathrm{Cu}^{2+}\) have four secondary valencies. The total number of secondary valencies required by a metal is more commonly known as coordination number \(\left( {{\rm{C}}{\rm{.N}}} \right){\rm{.}}\)

3. The secondary valencies are always directed towards fixed positions in space about the central metal ion. Thus, the number and arrangement of ligands in space determines the stereochemistry of a complex. Thus, in the case of \(6\) secondary valencies, the arrangement of secondary valencies is directed to the apices of a regular octahedral circumscribed about the metal ion. Whereas in the case of \(4\) secondary valencies, arrangement might be either in a square planar or a tetrahedral manner. Thus, this postulate predicted the possibilities of a variety of isomerism in coordination compounds. Remember that primary valencies are non-directional.

Werner introduced the square brackets \(\left[ {} \right]\) to enclose atoms making up the coordination complex and are therefore not ionised to distinguish between the two types of valencies. The portion enclosed in the bracket is known as the coordination sphere, and the portion present outside the bracket is the ionisation sphere.

Thus, according to Werner theory, the compound \({\text{CoC}}{{\text{l}}_3}.6{\text{N}}{{\text{H}}_3}\) maybe formulated as \(\left. {\left[{{\text{Co}}{{\left({{\text{N}}{{\text{H}}_3}} \right)}_6}} \right)} \right]{\text{C}}{{\text{l}}_3}\), i.e. It is called hexammine cobalt (III) chloride.

Since there are six ammonia molecules in the compound, they alone satisfy the six secondary valencies of cobalt (\(\left( {{\rm{C}}{\rm{.N}}} \right){\rm{.}}\) of cobalt is \(6\)). They are directly attached to the cobalt atom and are shown by thick lines.

The oxidation state \((+3)\) of cobalt (or primary valencies) is satisfied by three chloride ions. These are shown by dotted lines and are kept outside the coordination sphere, i.e. these are present in the ionisation sphere. The three chloride ions present in the ionisation sphere are loosely bound and, thus, precipitated on the addition of silver nitrate. Therefore, the complex will ionise in solution as below:

\(\left[{{\text{Co}}{{\left( {{\text{N}}{{\text{H}}_3}} \right)}_6}} \right]{\text{C}}{{\text{l}}_3} \to {\left[{{\text{Co}}{{\left({{\text{N}}{{\text{H}}_3}}  \right)}_6}}  \right]^{3 + }} + 3{\text{C}}{{\text{l}}^ – }\)

Thus, the number of moles of ions produced per mole of the complex in a solution will be \(1+3=4\).

Now let us draw the structure of another complex of \(\mathrm{CoCl}_{3}\) with \({\text{N}}{{\text{H}}_3}\) i.e., \({\text{CoC}}{{\text{l}}_3}.5{\text{N}}{{\text{H}}_3}\). This complex has only five ammonium molecules. One chloride ion must be present inside the coordination sphere to satisfy the six secondary valencies of cobalt. The six secondary valencies (\(5\) by \({\text{N}}{{\text{H}}_{\text{3}}}\) and one satisfied by \(\mathrm{Cl}\)) are shown by thick lines. The three primary valencies of cobalt \(\left(\mathrm{Co}^{3+}\right)\) are satisfied by three chloride ions (shown by dotted lines). Note that one chloride ion assumes dual behaviour, i.e., it satisfies both the primary and secondary valency of cobalt. Hence, such chloride ion is shown by thick as well as by dotted lines in the structure. Remember that an ion having a dual behaviour is not an ionisable ion, i.e., it is present in the coordination sphere and hence not precipitated by the reagent (\({\text{AgN}}{{\text{O}}_3}\) in the present case). Thus, the complex \({\text{CoC}}{{\text{l}}_3}.5{\text{N}}{{\text{H}}_3}\) can be written as below:

\(\left[ {{\text{Co}}{{\left( {{\text{N}}{{\text{H}}_3}} \right)}_5}{\text{Cl}}} \right]{\text{C}}{{\text{l}}_2} \to {\left[ {{\text{Co}}{{\left( {{\text{N}}{{\text{H}}_3}} \right)}_5}{\text{Cl}}} \right]^{2 + }} + 2{\text{C}}{{\text{l}}^ – }\)

 Thus, ionisation will give three ions, only two of which are \({\text{C}}{{\text{l}}^ – }\) ions, although the complex has three chlorine.

Similarly compounds \({\text{CoC}}{{\text{l}}_3}.4{\text{N}}{{\text{H}}_3}\) and \({\text{CoC}}{{\text{l}}_3}.3{\text{N}}{{\text{H}}_3}\) maybe formulated as below.

\(\left[{{\text{Co}}{{\left({{\text{N}}{{\text{H}}_3}} \right)}_4}{\text{C}}{{\text{l}}_2}} \right]{\text{Cl}}\) (It has only one ionisable \({\text{C}}{{\text{l}}^ – }\) ion)

\(\left[{{\text{Co}}{{\left( {{\text{N}}{{\text{H}}_3}} \right)}_3}{\text{C}}{{\text{l}}_3}} \right]\) (It has no ionisable \(\mathrm{Cl}\) and hence, behaves as a non \(-\) electrolyte)

Hence, their structures can be drawn as III and IV, respectively.

The important aspect of the structures of five different complexes of \({\text{PtC}}{{\text{l}}_4}\) with ammonia prepared by Werner can now be tabulated below. All these compounds, platinum, exhibit a primary valency (oxidation number) of four and secondary valency (coordination number) of six.

Evidence for Werner’s Theory

1. Chloride ion activity: Werner’s theory of coordination compounds of \({\text{Co}}\)(III) and \({\text{Pt}}\)(IV) with ammonia explains a different number of ionisable chloride ions in different complexes. He was also able to correctly assign whether a particular chloride ion only satisfied the primary valency or had a dual role.
2. A total number of ions formed: The total number of ions formed by a complex proposed by Werner is found to be following the molar conductivity of its solution.
3. Number and type of isomers: The number and structure of isomers proposed by Werner were found to be following the observed fact.

Applications of Werner’s Theory

Some of the applications of Werners theory of coordination compounds are listed below:

1. It predicts the exact structure of each complex.
2. It explains why a particular metal atom and particularly ligand form different complexes. It also explains the different properties of each complex.
3. It predicts the structure of different complexes with C.N. \(4\) and \(6\).
4. The last postulate of Werner’s theory explained isomerism and predicted the existence of isomerism of types that had not previously been observed. Werner showed that the complex of divalent platinum \(\left[{{\text{Pt}}{{\left({{\text{N}}{{\text{H}}_3}} \right)}_2}{\text{C}}{{\text{l}}_2}} \right]\); existence in cis – and trans – isomeric forms.

The existence of the isomerism established the proof of geometrical structures of these complexes, viz, the existence of the cis- and trans -isomers of the above complex indicates the planner arrangement of the coordination groups around platinum because if the arrangement were tetrahedral, then groups could be interchanged and only structure should exist. Hence isomerism would not have been possible. Similarly, Werner suggested that the two compounds (violet and green) of these compositions \({\text{CoC}}{{\text{l}}_2}.4{\text{N}}{{\text{H}}_3}\) are due to cis- and trans – isomerism. The six coordination groups are at the corners of an octahedron.

Werner’s contribution is a unique one. The fundamental postulates proposed by Werner are as valid today as when they were presented over \(70\) years ago, despite the tremendous advances in theory, the remarkable increase in the number of coordination compounds and enormous data of such compounds.

Limitations of Werner’s Theory of Coordination Compounds

1. It could not explain the inability of all elements to form coordination compounds.
2. Werner’s coordination theory failed to explain the bonding nature between the central metal atom and the ligands.
3. Werner’s coordination theory failed to explain the geometry of complexes when secondary valency was equal to \(4\).
4. It does not explain the colour, the magnetic and optical properties shown by coordination compounds.

Nomenclature of Coordination Compounds

The present system of nomenclature derived from the suggestion of Alfred Werner and recommended by the inorganic nomenclature committee of the International Union of pure and applied chemistry in \(1957\) is discussed here. The essential features of the system are summarised below:

1. Like simple salts, the positive part of the coordination compound is named first, followed by the negative part.
   In case the complex is a non-ionic or molecular species, a one-word name is given to it. Example., \(\left[{{\text{Co}}{{\left({{\text{N}}{{\text{H}}_3}} \right)}_3}{\text{C}}{{\text{l}}_3}} \right]\)
2. The naming of the coordination sphere
   While naming a coordination sphere, ligands are named first and then the metal. Following points are followed while naming ligands and the central metal.
   (i) (a) The neutral ligands are named as the molecules. For example \({{\text{C}}_5}{{\text{H}}_5}{\text{N}}\) -pyridine, \({\left({{{\text{C}}_6}{{\text{H}}_5}} \right)_3}{\text{P}}\) –  triphenylphosphine
   
   However, the following neutral ligands are named differently and not as a molecule.
   \({{\text{H}}_2}{\text{O}}\) – aquo
   \({\text{CO}}\) – carbonyl
   \({\text{NO}}\) – nitrosyl
   
   (b) The names of negative ligands end with – o, example.,
   \({\text{C}}{{\text{l}}^ – }\) – chlorido
   \(\mathrm{NH}^{2-}\) -imido
   \({\text{ON}}{{\text{O}}^ – }\) -nitrito
   
   (c) The names of positive ligands end with -ium, example.,
   \({\text{NO}}_2^ + \) – nitronium
   \({\text{N}}{{\text{O}}^ + }\) – nitrosonium
   
   (ii) If there are several ligands in the coordination sphere, these are listed in alphabetical order. In case simple ligands like chloro, nitro, amido, ammine, aquo, etc., are present more than once, their number is indicated by the prefixes di-, tri-, tetra, etc. On the other hand, the number of other ligands which already have prefixes di-, tri-, etc., in their names, as ethylenediamine, is indicated by the prefixes bis-, tetrakis-, etc.
   
   Always remember that when ligands are listed alphabetically, spellings (alphabet) of their proper names are taken into consideration and not the spellings of the prefixes di-, tri-, and further. Thus, di-amine should be listed under ‘a’ and not under ‘d’.
   
   (iii) When the coordination sphere is either a cation or a neutral molecule, the name of the central atom remains as such, and the oxidation state of the metal is shown by Roman numerals in bracket immediately following the name of the metal. For example,
   \(\left[{{\text{Co}}{{\left( {{\text{N}}{{\text{H}}_3}} \right)}_6}} \right]{\text{C}}{{\text{l}}_3}\) – Hexamminecobalt(III) chloride
   \(\left[{{\text{Pt}}{{\left( {{\text{N}}{{\text{H}}_3}} \right)}_2}{\text{C}}{{\text{l}}_4}} \right]\) – Diamminetetrachloroplatinum(IV)

When the coordination sphere is an anion, the name of the metal atom ends with -ate. The oxidation state is then indicated in the last by the Roman numeral in the bracket. For example,

\({{\text{K}}_3}\left[ {{\text{Co}}{{\left({{\text{N}}{{\text{O}}_2}} \right)}_6}} \right]\) – Potassiumhexanitrocobaltate (III)

Note that the whole coordination sphere is written as one word, i.e., without any space throughout.

(iv) If the complex ion has two or more metal atoms, it is termed polynuclear. The ligands which connect the two metal atoms are called bridge groups; these are preceded by the Greek letter \(\mu \), which is repeated before the name of each different kind of bridging group, example.,

(V) In case a unidentate ligand contains more than one donor atoms (ambidentate ligands), its point of attachment with the central atom is designated by the symbol (in italics) of the donor atom after the name of the ligand. For example
\({{\text{K}}_3}\left[ {{\text{Co}}{{({\text{NCS}})}_6}} \right]\) – Potassium hexathiocyanato – N – cobaltate (III)

(Vi) Geometrical isomerism is named cis and trans depending on whether similar groups are on the same or opposite sides. Similarly, optically active isomers are designated by the symbols \((+)\) or d and \((-)\) or l
for dextro \((+)\) and laevo \((-)\) rotatory compounds. For example,

(vii) Complicated ligands are customarily denoted by abbreviations in complexes. For example, ethylenediamine is commonly written as “en”.

Summary

Werner’s theory is responsible for the formation of structures of various cobalt amines. Coordination compounds are used as catalysts for many industrial processes and have many qualitative/quantitative chemical analysis applications within analytical chemistry. The coordination compounds have importance in a biological system. They play a vital role in metallurgy and medicine. In this article, we learned about the nomenclature of Werners theory, postulates, applications, limitations and evidence of Werner’s theory of coordination compounds.

FAQs on Werner’s Theory of Coordination Compounds

Frequently asked questions related to Werner’s theory of coordination compounds is listed as follows:

Q.1: What is primary Valency, according to Werner?

Ans: Primary valencies are those which a metal normally exhibits in the formation of its simple salts. Thus, in the formation of \({\text{PtC}}{{\text{l}}_4},{\text{CuS}}{{\text{O}}_4}\) and \({\text{AgCl}}\), The primary valencies of \({\text{Pt, Cu}}\) and \({\text{Ag}}\) are \(4, 2\) and \(1\), respectively. Primary valencies are satisfied by negative ions.

Q.2: What is the importance of coordination compounds?

Ans: Coordination compounds are a significant feature of the chemistry of over half of the elements. They have essential roles as industrial catalysts in controlling reactivity, and they are essential in biochemical processes.

Q.3: What is Werner’s coordination number?

Ans: Coordination numbers generally range between \(2\) and \(12\), with \(4\) (tetracoordinate) and \(6\) (hexacoordinate) being the most common. Werner referred to the central atom and the ligands surrounding it as the coordination sphere.

Q.4: How did Werner first explain bonding in complexes?

Ans: Werner was the first to propose correct structures for coordination compounds containing complex ions, in which neutral or anionic ligands surround a central transition metal atom. Werner proposed the structure, \(\left[ {{\text{Co}}{{\left( {{\text{N}}{{\text{H}}_3}} \right)}_6}} \right]{\text{C}}{{\text{l}}_3}\) with the \({\text{C}}{{\text{o}}^{3 + }}\) ion surrounded by six \({\text{N}}{{\text{H}}_3}\) at the vertices of an octahedron.

Q.5: Why do coordination compounds have colour?

Ans: When ligands attach to a transition metal to form a coordination complex, electrons in the d orbitals split into high energy and low energy orbitals. Certain wavelengths are absorbed in this process, subtractive colour mixing occurs, and the coordination complex solution becomes coloured.

Q.6: Explain Werner’s theory of coordination compounds.

Ans: The postulates of the Werner’s theory of coordination compounds are:

(1) Every complex compound has a central metal atom (or) ion.

(2) The central metal shows two types of valencies namely primary valency and secondary valency.

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