Coordinate Compounds: A complex compound that has a central metal atom or ion that has a fixed number of atoms or molecules bound to it is known as a coordination compound. The group of atoms that surrounds the central metal atom and are joined by chemical bonds are known as ligands. These compounds are also known as complex compounds.

Alfred Werner \((1866-1919)\) was a Swiss chemist who was the first to propose his ideas about the different structures of coordination compounds. He grouped many coordination compounds prepared by him and studied their physical and chemical behaviour by simple experimental techniques and proposed the concept of a primary valence and a secondary valence for a metal ion. Some examples of coordination compounds are: \({\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_6}} \right]^{3 + }},{\left[ {{\rm{CoCl}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_5}} \right]^{2 + }},\left[ {{\rm{Ni}}{{({\rm{CO}})}_4}} \right],{\left[ {{\rm{Pt}}{{({\rm{Cl}})}_4}} \right]^{2 – }}\) and many more. In this article, we will learn about various types and features of coordination compounds.

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What are Coordinate Compounds?

Coordinate compound or coordination compound is a class of substances with chemical structures in which a central metal atom is surrounded by the atoms or group of atoms of nonmetals called ligands and are joined to it by chemical bonds. Such compounds are the product of a Lewis acid-base reaction in which neutral molecules or anions (known as ligands) bond to a central metal atom (or ion) by coordinate covalent bonds. Ligands contain at least one pair of electrons to donate to a metal atom/ion and hence, act as Lewis bases.

Some Terminologies of Coordination Compounds

The definitions of some important terms used in coordination chemistry can be found below:

1. Coordination Entity: A chemical compound in which the coordination centre or the central ion or atom is bound to a group of number of atoms, molecules or ions is called a coordination entity. For example, \(\left[ {{\rm{Ni}}{{({\rm{CO}})}_4}} \right],{\left[ {{\rm{Pt}}{{({\rm{Cl}})}_4}} \right]^{2 – }}\) etc.

2. Ligands: The group of atoms, molecules, or ions that are bound to the central atom or ion are referred to as ligands. These ligands can either be in the form of a simple ion or molecule (such as \({\rm{N}}{{\rm{H}}_3},{\rm{C}}{{\rm{l}}^ – }\) or \({\rm{CO}})\) in the form of relatively large molecules, such as ethane – \(1,2\) – diamine \(\left( {{\rm{N}}{{\rm{H}}_2} – {\rm{C}}{{\rm{H}}_2} – {\rm{C}}{{\rm{H}}_2} – {\rm{N}}{{\rm{H}}_2}} \right)\)

3. Coordination Number: The total number of sigma bonds through which the ligands are bound to the coordination centre in coordination compounds are known as coordination number. For example, in the given coordination complex \(\left[ {{\rm{Ni}}{{({\rm{CO}})}_4}} \right]\) the coordination number of Nickel is \(4\).

4. Coordination Sphere: The part of a complex compound which is enclosed in the square bracket and consists of central transition metal ions surrounded by neighbouring atoms or groups is known as coordination sphere.

5. Oxidation Number: The charge associated with the coordination compound when all the electron pairs that are donated by the ligands are removed from it is known as oxidation number. For example, in \({\left[ {{\rm{Pt}}{{({\rm{Cl}})}_4}} \right]^{2 – }}\), the oxidation number of Pt is \(+2\).

Coordination Compounds Nomenclature

Different rules for the nomenclature of coordination compounds are as follows:

1. In the naming of complex coordination complexes, the ligands are always written before the central metal ion.
2. When the coordination centre is attached to more than one ligand, the ligand’s name is written in alphabetical order irrespective of the numerical prefixes that are applied to the ligands.
3. When many monodentate ligands are present in the coordination compound, the prefix used to denote the number of ligands is of the type: di-, tri-, tetra-, and so on.
4. When many polydentate ligands are attached to the central metal ion, the prefix used is of the form bis-, tris-, and so on.
5. In the case of the anions present in a coordination compound, their names must end with the letter ‘\({\rm{o}}\)’, which is generally replaced by the letter ‘\({\rm{e}}\)’. For example, the sulphate anion must be written as ‘sulphato’ and the chloride anion must be written as ‘chlorido’.
6. Some neutral ligands are assigned specific names in coordination compounds such as: \({{\rm{H}}_2}{\rm{O}}\) (aqua or aquo), \({\rm{N}}{{\rm{H}}_3}\) (ammine), \({\rm{CO}}\) (carbonyl), \({\rm{NO}}\) (nitrosyl), etc.
7. The name of the central metal atom is written after the ligands are named. The negatively charged coordination complex has the suffix ‘-ate’ with it.
8. Priority is given to the existing Latin name of the metal while writing the name of the central metallic atom in an anionic complex (except mercury).
9. The oxidation state of the central metal atom/ion must be represented in roman numerals enclosed in a set of parentheses.
10. In case, if the coordination compound is accompanied by a counter ion, the cationic species must be written before the anionic species.

Here are some examples of the nomenclature of coordination compounds that follows the above rules:

Coordination Compound Examples

S.No.	Formula	Name	Coordination Number	Oxidation Number
1.	\(\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_6}} \right]{\rm{C}}{{\rm{l}}_3}\)	Hexaamminecobalt (III) chloride	\(6\)	\(+3\)
2.	\({\rm{C}}{{\rm{u}}_2}\left[ {{\rm{Fe}}{{({\rm{CN}})}_6}} \right]\)	Copper(II) hexacyanoferrate(II)	\(6\)	\(+2\)
3.	\(\left[ {{\rm{Cu}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}{{\left( {{{\rm{H}}_2}{\rm{O}}} \right)}_2}} \right]{\rm{C}}{{\rm{l}}_2}\)	Tetraamminediaquacopper(II) chloride	\(6\)	\(+2\)
4.	\({{\rm{K}}_3}\left[ {{\rm{Co}}{{\rm{F}}_6}} \right]\)	Potassium hexafluorocobaltate(III)	\(6\)	\(+3\)
5.	\({\rm{Ba}}{\left[ {{\rm{FeB}}{{\rm{r}}_4}} \right]_2}\)	Barium tetrabromoferrate(III)	\(4\)	\(+3\)
6.	\(\left[ {{\rm{Ag}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_2}} \right]{\rm{Cl}}\)	Diamminesilver(I) chloride	\(2\)	\(+1\)
7.	\({\left[ {{\rm{Cu}}{{({\rm{CN}})}_4}} \right]^{3 – }}\)	Tetracyanocuprate(I) ion	\(4\)	\(+1\)
8.	\({\rm{Cr}}{({\rm{CO}})_6}\)	Hexacarbonylchromium (0)	\(6\)	\(0\)
9.	\({\rm{K}}\left[ {{\rm{AuC}}{{\rm{l}}_4}} \right]\)	Potassium tetrachloroaurate(III)	\(4\)	\(+3\)
10.	\(\left[ {{\rm{Ni}}{{\left( {{{\rm{H}}_2}{\rm{O}}} \right)}_6}} \right]{\rm{C}}{{\rm{l}}_2}\)	Hexaaquanickel(II) chloride	\(6\)	\(+2\)

Properties of Coordination Compounds

1. The coordination compounds that contain the transition elements are mostly coloured due to the presence of unpaired electrons that absorb light of specific wavelength during their electronic transitions. For example, Iron (II) complexes can exhibit green and pale green colours, but Iron (III) complexes exhibits a brown or yellowish-brown colour.
2. The coordination complexes have a magnetic nature due to the presence of a metallic central metal and an unpaired electron.
3. Coordination compounds are very reactive and can perform various chemical reactions. Electron transfer reactions of both types: inner-sphere electron transfer reactions, as well as outer-sphere electron transfers, take place.
4. Complex compounds with certain ligands can help in the transformation of molecules in a catalytic or a stoichiometric manner. They also show isomerism of different types.

Summary

The chemistry of coordination compounds is a very important and challenging area of modern inorganic chemistry as in the past few years, advances in this area, have developed new concepts and models of bonding and molecular structure. Werner’s theory postulated the use of two types of linkages (primary and secondary) by a metal atom or ion in a coordination compound. In the modern language of chemistry, these linkages are described as ionisable (ionic) and non-ionisable (covalent) bonds, respectively. Coordination compounds are very important as these compounds provide critical understandings into the functioning and structures of vital components of biological systems. Coordination compounds also find elaborated applications in metallurgical processes, analytical and medicinal chemistry.

FAQs

Q1. Which is a coordination compound?
Ans. Coordinate compound or coordination compound is any of a class of substances with chemical structures in which a central metal atom is surrounded by the atoms or group of atoms of nonmetals called ligands and are joined to it by chemical bonds. Such compounds are the product of a Lewis acid-base reaction in which neutral molecules or anions (known as ligands) bond to a central metal atom (or ion) by coordinate covalent bonds. Ligands contain at least one pair of electrons to donate to a metal atom/ion and hence, act as Lewis bases.

Q2. How do you identify coordinate compounds?
Ans. A coordination compound can be identified based on the presence of the following entities:
(i) Coordination Entity – A chemical compound in which the coordination centre or the central ion or atom is bound to a group of number of atoms, molecules or ions is called a coordination entity. For example, \(\left[ {{\rm{Ni}}{{({\rm{CO}})}_4}} \right],{\left[ {{\rm{Pt}}{{({\rm{Cl}})}_4}} \right]^{2 – }}\)−, etc.
(ii) Ligands – The group of atoms, molecules, or ions that are bound to the central atom or ion are referred to as ligands. These ligands can either be in the form of a simple ion or molecule (such as \({\rm{N}}{{\rm{H}}_{{3^\prime }}}{\rm{C}}{{\rm{l}}^ – }\)− or \({\rm{CO}})\) in the form of relatively large molecules, such as ethane \(- 1,2\) – diamine \(\left( {{\rm{N}}{{\rm{H}}_2} – {\rm{C}}{{\rm{H}}_2} – {\rm{C}}{{\rm{H}}_2} – {\rm{N}}{{\rm{H}}_2}} \right)\).
(iii) Coordination Sphere – The part of a complex compound which is enclosed in the square bracket and consists of central transition metal ion surrounded by neighbouring atoms or groups is known as coordination sphere.

Q3. What are the different types of coordination compound?
Ans. Different types of coordination compounds based on the charge present on them are as follows: (i) Cationic complexes are the coordination complexes in which the coordination sphere is a cation. For example: \({\left[ {{\rm{Zn}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}} \right]^{2 + }}\) (ii) Anionic complexes are the coordination complexes in which the coordination sphere is an anion. For example: \(\left[ {{{\rm{K}}_3}{\rm{Fe}}{{({\rm{CN}})}_6}} \right]\). (iii) Neutral complexes are the coordination complexes in which the coordination sphere is neutral, i.e., neither cationic nor anion. For example: \(\left[ {{\rm{Cr}}{{({\rm{CO}})}_6}} \right]\)

Q4. How is a coordination compound formed?
Ans. A coordination compound is formed by the chemical reaction between a metal ion and a ligand. If strong force of attractions exists in coordination complexes, they become quite stable. Some of these complexes get dissociated to a little extent in an aqueous solution.

Q5. Which is the example of coordinate compound?
Ans. A complex compound that has a central metal atom or ion that has a fixed number of atoms or molecules bound to it is known as coordination compound. The group of atoms that surrounds the central metal atom and are joined by chemical bonds are known as ligands. Some examples of coordination compounds are as follows: \(\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_6}} \right]{\rm{C}}{{\rm{l}}_3}\) – Hexaammine cobalt (III) chloride, \({\rm{C}}{{\rm{u}}_2}\left[ {{\rm{Fe}}{{({\rm{CN}})}_6}} \right]\)- Copper(II) hexacyanoferrate(II), \({{\rm{K}}_3}\left[ {{\rm{Co}}{{\rm{F}}_6}} \right]\)- Potassium hexafluorocobaltate(III), \(\left[ {{\rm{Cu}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}{{\left( {{{\rm{H}}_2}{\rm{O}}} \right)}_2}} \right]{\rm{C}}{{\rm{l}}_2} – \) Tetraamminediaqua copper(II) chloride, etc.

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