Coordination Compounds

Coordination compounds are a class of chemical compounds in which there is a central metal atom that is bonded to several ligands simultaneously. The metal acts as a Lewis acid and the ligands as Lewis bases. 

Ligands are the molecules or atoms that contain a lone pair of electrons that can be donated to the empty orbitals of the metal. They have several applications, the primary one being in catalysis. Double salts form another class of compounds and are often confused with coordination compounds. Double salts and coordination compounds have stark differences, ranging from the nature of bonding to physical and chemical properties. Here is everything you need to know about coordination compounds.

Meaning of Coordination Compounds

A typical coordination compound has a metal, mostly a transition metal, that bonds covalently to the ligands (neutral and anionic, and rarely cationic). There are certain terminologies associated with them whose understanding is crucial to grasp the concept of coordination compounds:

1. Ligand: The atoms, molecules, or ions that bind with the metal via coordinate covalent bonding are known as ligands. Mostly, they act as Lewis bases and provide electron density to the metal. Common ligands include CN-, NH3, H2O, and Cl-.

1. Central atom or ion: The metal atom or ion to which the ligands bind is called the central atom. Example: in K3[Fe(CN)6], the central ion is Fe3+. 

1. Coordination Number: It is defined as the total number of ligands attached to the central metal. In K3[Fe(CN)6], the central ion is directly attached to six cyanide ions, making the coordination number of this compound 6. Common coordination numbers are 4 and 6.

1. Denticity: One ligand may have more than one donor site or multiple electron-donating groups. Denticity refers to the number of donor groups that the ligand has. Ligands can be divided into the following categories based on their denticity:

• Unidentate: If a ligand has one donor site/group, it is called unidentate. Examples are Cl-, NH3, etc.
• Multidentate: If a ligand has more than one donor site/group, it is called multidentate. It can further be divided into bidentate, hexadentate, etc., depending upon the number of available donor sites. Ethylenediamine is an example of a bidentate in which two nitrogen atoms can donate electrons simultaneously to the metal centre. 
• Polydentate: If a ligand can exhibit multiple denticities, it is called polydentate. EDTA (ethylenediaminetetraacetate) is an example of a polydentate ligand that can donate different numbers of electron pairs, ranging from one to six.
• Ambidentate: A ligand that has more than one donor site but only one of them can be involved in bonding at one time is called an ambidentate. 

For example, SCN- is ambidentate because it can donate from both sulphur and nitrogen centres. NO2- is ambidentate as it can donate from both oxygen and nitrogen centres.

1. Coordination Sphere: Ligands coordinating with the metal centre form a coordination sphere, which is enclosed in square brackets. In K3[Fe(CN)6], the coordination sphere is formed by Fe(CN)63-. Three potassium ions outside the coordination sphere balance the three negative charges on the coordination sphere.

1. Coordination Polyhedron: The coordination sphere has a particular spatial geometry known as the coordination polyhedron. 

Double Salts and Coordination Complex

A double salt is another class of chemical compound that is the combination of two salts in a stoichiometric ratio. It is crystalline and ionic in nature. Two different ionic salts exist in the same crystal lattice. 

The most common example of a double salt is potash alum (K2SO4.Al2(SO4)3.24H2O). It is composed of two salts namely potassium sulphate and aluminium sulphate. It has 24 molecules of hydration and is a good conductor of electricity in its aqueous form as the compound breaks down into constituent cations and anions. There is no strong bond between the two salts.

A coordination complex, on the other hand, has strong covalent bonds between the ligands and the metal. It does not dissociate into its constituents in water.

Werner’s Postulates 

Werner gave the following postulates concerning coordination complexes:

1. The central atom exhibits primary and secondary valencies. Each metal wants to satisfy both valencies.
2. The primary valency satisfies the charge and the secondary valency refers to the number of ligands attached to the metal. 
3. The geometry of the complex is fixed by the secondary valency of the metal.

Conclusion

Ligands are of several types depending on the denticity. Werner gave his postulates regarding coordination compounds that help explain their structure and bonding nature. In this article, we talked about coordination compounds and defined different terminologies associated with them. Now that the meaning of coordination compounds is clear, let’s move on to the coordination compounds questions.