Werner’s Theory of Coordination Compounds was proposed by a Swiss Chemist Alfered Werener in 1898. Werner studied the physical, chemical, and isomeric properties of several coordination compounds and postulated some theories.

In this article, we will learn about, Werner’s Coordination theory, its postulates, and others in detail.

Werner Coordination Theory

Werner postulated the following theories regarding Coordination Compounds after his study:

• In a coordination compound, the metal atom exhibits two types of valencies namely primary valency and secondary valency.
• Primary Valencies are ionizable and satisfied by negative ions
• Secondary Valencies are non-ionizable and satisfied by neutral atoms or negative ions
• The Secondary Valency is equal to the Coordination Number and is fixed for a metal.
• The ions or atoms attached via Secondary Valency to the metal have a characteristic spatial arrangement and hence give definite shape to the coordination compound. Such arrangements are called Polyhedra.
• The Coordination Compounds of Transition Metals generally exhibit Tetrahedra, Octaderal, and Square Planar geometry.

Since Werner’s Coordination Theory talks about Primary or Secondary valencies also known as Primary and Secondary Linkages, let’s learn about them in detail.

Postulate of Werner’s Theory

Werner’s Theory explains that the central metal atom in the coordination compound has two types of vacancies that are,

• Primary Valency
• Secondary valency

Let’s learn about them in detail.

Primary Valency

The valencies that Metal exhibits in the production of simple salts generally Binary compounds  CoCl₃, NaCl, and CuSO₄, are known as primary valencies. In modern terms, it refers to a Metal’s oxidation number. For example, in CoCl₃, the Primary Valency of Co is 3 and the Oxidation State is +3.

Features of Primary Valency

• Ionizable and satisfied by negative ions
• Non-Direction hence not give shape to coordination compounds
• They are written outside the square bracket in the coordination compound. For Example, in [Co(NH₃)₆]Cl₃, 3 is the Primary Valency of Co.

Secondary Valency

Secondary Valency refers to the number of ions or groups of atoms directly to the Metal in a coordination compound. They are inside the square bracket in Coordination Compound molecular formula. For Example, in [Co(NH₃)₆]Cl₃ the secondary valency of Co is 6 as 6 molecules of NH₃ are attached to Co. They are called Metal’s Coordination Number.

Features of Secondary Valency

• Non-ionizable and satisfied by either neutral atoms or negative ions
• Directional hence give shape to coordination compounds
• They are written inside the square bracket and called as Metal’s Coordination Number.

Structures of Coordination Compound based on Werner’s Theory

Let’s study the following Coordination Compounds of Cobalt on the basis of Werner’s Theory. It should be noted that as per convention, dotted lines (…..) show primary valency and thick lines ( __ ) show secondary valency

[Co(NH₃)₆]Cl₃

In this, there are three primary bonds that are ionizable and are satisfied by Cl^– ion also called Counter ion, which is represented outside the square bracket. The number of secondary valency is 6 which is satisfied by neutral NH₃ molecules and written inside the square bracket. These six secondary valences determine the shape of the molecule.

 

[CoCl(NH₃)₅]Cl₂

In this, there are two primary bonds that are ionizable and satisfied by Cl^– ion also called Counter ion, which is represented outside the square bracket. The number of secondary valency is 6 which is satisfied by 5 neutral NH₃ molecules and 1 Cl atom and written inside the square bracket. These six secondary valences determine the shape of the molecule

 

[CoCl₂(NH₃)₄]Cl

In this, there are two primary bonds that are ionizable and satisfied by Cl^– ion also called Counter ion, which is represented outside the square bracket. The number of secondary valency is 6 which is satisfied by 4 neutral NH₃ molecules and 2 Cl atoms and written inside the square bracket. These six secondary valences determine the shape of the molecule.

 

[CoCl₃(NH₃)]

In this, there are no primary bonds that are ionizable. The number of secondary valency is 6 which is satisfied by 3 neutral NH3 molecules and 3 Cl atoms and written inside a square bracket.  Here all the 3 Cl atoms are non-ionizable means that they can’t be precipitated out. These six secondary valences determine the shape of the molecule.

 

Limitations of Werner’s Theory

The limitations of Werner’s Coordination Theory are as follows:

• It could not account for all elements’ failure to create coordination compounds.
• Werner’s coordination theory fails to explain the nature of the bonding between the core metal atom and the ligands.
• Werner’s coordination theory failed to describe complicated geometry when secondary valency was equal to 4.
• Werner’s hypothesis explains some of the features of coordination compounds, but it does not explain their color or magnetic properties.

Since all the above discussions were based on Coordination Compounds hence let’s have a glance at Coordination Compounds’ definitions and properties

What are Coordination Compounds

Coordination compounds are chemical compounds composed of an array of anions or neutral molecules linked by coordinate covalent bonds to a central atom. Coordination compounds are also known as coordination complexes. The molecules or ions that are connected to the center atom are referred to as ligands (also known as complexing agents).

Metal complexes are coordination compounds in which the central atom is a metallic element. In this type of coordination complex, the central tom is frequently a transition element. It should be noted that the coordination center is the central atom in these complexes.

Properties of Coordination Compounds

• Because unpaired electrons absorb light during electronic transitions, transition element coordination compounds are colored. For Example, Iron (II) complexes, are green or pale green in color, whereas iron(III) coordination compounds are brown or yellowish-brown in color.
• When the coordination center is a metal, the resulting coordination complexes have a magnetic property due to the presence of unpaired electrons.
• The chemical reactivity of coordination molecules varies. They have the ability to participate in both inner-sphere and outer-sphere electron transfer reactions.
• Complex compounds with specific ligands have the ability to catalyze or stoichiometrically aid in the transition of molecules.

Applications of Coordination Compounds

Coordination compounds’ unique features, make them particularly helpful in various processes and industries. Some of these coordination compound applications are listed below.

• Because of the color of coordination compounds containing transition metals, they are widely employed in industries for material coloration. In the dye and pigment industries, they are used.
• In the electroplating process, some complex molecules using cyanide as a ligand are employed. These chemicals are also beneficial in the field of photography.
• Many metals can be extracted from their ores with the use of coordination complexes. Nickel and cobalt, for example, can be recovered from their ores via hydrometallurgical procedures using coordination complex ions.

Similar to Coordination Complex, Double Salts are also formed by the combination of two or more stable compounds in a stoichiometric ratio. However, differences exist between them.

Difference between Double Salt and Coordination Complex

The basic difference between Double Salt and Coordination is that in aqueous solutions, Double Salts are totally ionizable, and each ion in the solution delivers the corresponding confirmatory test. Potash alum, for example, is a Double Sulfate. K₂SO₄ is the chemical formula. When Al₂(SO₄)₃.24H₂O is ionized, it produces K⁺, SO₂^-4, and Al⁺³ ions, which respond to the tests while In aqueous solutions, coordinate complexes are only partially ionizable. These produce a complex ion that isn’t completely ionized. Potassium Ferrocyanide is one example. [K₄Fe(CN)₆]. K⁺ and [Fe(CN)₆]^4- [ferro cyanide ions] are formed when it ionizes.

Heteroleptic Complex

The complex compound in which the metal ion is surrounded by more than one type of ligand is called the Heteroleptic complex. Various examples of heteroleptic compounds are,

• [CoCl₃(NH₃)]
• [CoCl₂(NH₃)₄]Cl, etc

These types of compounds have more than one type of donor atom.

Read More,

• Nomenclature of Coordination Compounds
• Applications of Coordination Compounds
• Isomerism in Coordination Compounds

FAQs on Werner’s Theory

Q1: What is Werner’s Theory?

Answer: 

Werner’s Coordination Theory states that in a coordination compound, Metal atom has two linkages Primary and Secondary. The Primary Linkages are ionizable and satisfied by negative ions while secondary linkages are non ionizable and satisfied by negative ion or neutral atoms.

Q2: What is the Primary Valency according to Werner’s?

Answer: 

The valencies that a metal exhibits in the production of simple salts are known as primary valencies. It represents the metal’s oxidation condition. They’re ionisable and can be written outside of the coordination sphere.

Q3: What is Secondary Valency?

Answer: 

Secondary Valencies are non-ionizable valencies of metal in a coordination compund that are ionizable and satisfied by negative ion or neutral atoms. They give shape to the coordination compound.

Q4: What is a Double Salt?

Answer:

In aqueous solutions, double salts are totally ionizable, and each ion in the solution delivers the corresponding confirmatory test.

Q5: What is a Coordination Complex?

Answer:

In aqueous solutions, coordinate complexes are only partially ionizable. These produce a complexion that isn’t completely ionised.

Q6: Why do Coordination Compounds have color?

Answer: 

When ligands form a coordination complex with a transition metal, electrons in the d orbitals split into high energy and low energy orbitals. In this process, some wavelengths are absorbed, subtractive colour mixing happens, and the coordination complex solution becomes coloured.