Written By Sahana Soma Kodarkar
Last Modified 25-01-2023

Important Terms Related to Coordination Compounds: Check Here

Coordination compounds are ligand-bound molecules that include one or more metal centres (atoms, ions, or molecules that donate electrons to the metal). Certain terms and definitions are necessary to better understand the concept of coordination compounds. To learn more about the important terms related to coordination compounds, read the below article.

Coordination Compounds

Definition of some Important Terms Pertaining to Coordination Compounds

The definitions of some important terms pertaining to coordination compounds are given below:

1. Coordination Entity

A coordination entity contains a central metal atom or ion which is bonded by a fixed number of atoms or groups of atoms or anions or neutral molecules, each of which is called a ligand.

For example, \(\left[ {{\rm{CoC}}{{\rm{l}}_3}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_3}} \right]\) is a coordination unit in which the cobalt ions are surrounded by three ammonia molecules and three ions of chloride.

A coordination entity is enclosed in square brackets whether it is charged or neutral.

A coordination entity carrying a positive charge is called a cationic complex. For example, \({\left[ {{\rm{Ni}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_6}} \right]^{2 + }},{\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}} \right]^{2 + }}\).
A coordination entity carrying a negative charge is called anionic complex. For example, \({\left[ {{\rm{Fe}}{{({\rm{CN}})}_6}} \right]^{4 – }},{\left[ {{\rm{PtC}}{{\rm{l}}_6}} \right]^{2 – }}\).
A coordination entity with no charge is a neutral complex. For example, \(\left[ {{\rm{Ni}}{{({\rm{CO}})}_4}} \right],\,\left[ {{\rm{Ni}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}{\rm{C}}{{\rm{l}}_2}} \right]\).

2. Central Atom / Ion

In a coordination entity, the atom/ion that is attached to a fixed number of ions/groups which are bound in a definite geometrical arrangement around it is called the central atom or ion.

For example, in the complex \({\rm{K}}\left[ {{\rm{Ag}}{{({\rm{CN}})}_2}} \right],{\rm{A}}{{\rm{g}}^ + }\) is the central metal ion.

Any metal atom or ion that can accept electrons given by ligands can act as a central metal atom or ion and involve complex formation. Transition metal atoms or ions usually form complexes because they possess sufficient vacant d-orbitals to accommodate electrons given by ligands.

3. Ligands

The molecular or ionic species which gets directly attached to the central metal atom or ion during the formation of a complex is called a ligand. For example, in the complex \({{\rm{K}}{\rm{4}}}\left[ {{\rm{Fe(CN}}{{\rm{)}}{\rm{6}}}} \right]{\rm{,C}}{{\rm{N}}^{\rm{ – }}}\) ions are ligands because they get directly attached to the central \({\rm{F}}{{\rm{e}}^{{\rm{2 + }}}}\) ion during the formation of this complex.

Ligands form coordinate bonds with the central metal atom or ion. As a result, ligands are atoms, ions, or molecules that have a proclivity to donate a pair of electrons to the central metal atom. Ligands may be neutral, positive as well as negatively charged.

Classification of Ligands

Ligands can be classified into two classes. They are as follows;

a) Classification on the Basis of Charge

Depending Upon the nature of charge present, ligands can be classified as follows:

1) Neutral ligands: Neutral ligands possess no electrical charge present on them. They are usually molecular species having one or more lone pairs of electrons.

For example, \({\rm{N}}{{\rm{H}}_3}({\rm{ ammine }}),{\rm{CO}}({\rm{ carbonyl }}),{{\rm{H}}_2}{\rm{O (aqua), }}{{\rm{C}}_5}{{\rm{H}}_5}\;{\rm{N (pyridine), }}\) etc.

2) Anionic ligands: Anionic ligands have a negative charge. These are anionic species that contain one or more lone pairs of electrons. For example, \({{\rm{F}}^ – }{\rm{(fluorido), C}}{{\rm{l}}^ – }({\rm{chlorido }}),{\rm{B}}{{\rm{r}}^ – }\left( {{\rm{bromido), }}{{\rm{I}}^ – }{\rm{(iodide), C}}{{\rm{N}}^ – }{\rm{(cyanido), O}}{{\rm{H}}^ – }} \right.{\rm{(hydroxo), C}}{{\rm{H}}_3}{\rm{CO}}{{\rm{O}}^ – }{\rm{(acetato)}}\) etc.

3) Cationic ligands: Such ligands carry a positive charge and occur very rarely in complexes. For example, \({\rm{N}}{{\rm{O}}^ + }\left( {{\rm{nitrosonium), N}}{{\rm{H}}_2}\;{{\rm{N}}^ + }{{\rm{H}}_3}} \right.{\rm{ (hydrazinium), }}\) etc.

b) Classification on the Basis of the Number of Donor Atoms

The mode of attachment of a ligand to the central metal ion or atom is determined by the number of donor atoms present in it. The number of these connecting groups (donor atoms) is called the denticity of the ligand. Depending upon the number of donor atoms, ligands can be classified as follows:

1) Unidentate/monodentate ligands: Ligands that have only one donor atom and can form only one coordinated bond with the central metal atom or ion are called unidentate ligands or monodentate ligands. Such ligands can be attached to the central atom through one point only. They may be neutral as well as anionic. Some important monodentate ligands, their names, and donor atoms are as follows;

Ligand	Name	Donor atom
\({{\rm{H}}_2}{\rm{O}}\)	Aqua	\({\rm{O}}\)
\({\rm{N}}{{\rm{H}}_3}\)	Ammine	\({\rm{N}}\)
\({\rm{CO}}\)	Carbonyl	\({\rm{O}}\)
\({\rm{NO}}\)	Nitrosyl	\({\rm{N}}\)
\({{\rm{F}}^ – }\)	Fluorido/fluoro	\({\rm{F}}\)
\({\rm{C}}{{\rm{l}}^ – }\)	Chlorido/chloro	\({\rm{CI}}\)
\({\rm{B}}{{\rm{r}}^ – }\)	Bromido/Bromo	\({\rm{Br}}\)

2) Bidentate ligands: Ligands that have two donor atoms and can form two coordinated bonds with the central metal atom or ion are called bidentate ligands. Such ligands can be attached to the central metal atom or ion via two points. Some important ligands named are as follows.

3) Tridentate ligands: The ligands which possess three donor atoms and can form three coordinate bonds with the central metal atom or ion are called tridentate ligands. For example,

4) Tetradentate ligands: The ligands which possess four donor atoms are called tetradentate ligands. For example,

5) Pentadentate ligands: The ligands which contain five donor atoms are called pentadentate ligands. For example,

6) Hexadentate ligands: The ligands which contain six donor atoms are termed hexadentate ligands. For example,

7) Polydentate ligands: Ligands with more than one donor atoms are collectively called polydentate ligands.

8) Ambidentate ligands: Some unidentate ligands have more than one donor atoms through which they can ligate to the central atom. These are known as ambidentate ligands.

9) Bridging ligands: The monodentate ligands which can simultaneously attach themselves to more than one metal atom or ion are called bridging ligands. The ligands \({\rm{O}}{{\rm{H}}^ – },{\rm{NO}}_2^ – ,{\rm{C}}{{\rm{l}}^ – },{\rm{CO}},\) etc., act as bridging ligands.

Chelating Ligands and Chelates

When a polydentate ligand attaches itself to a central ion through two or more donor atoms, it forms a (five or six-membered generally) ring with the central ion; the ligand is called a chelating ligand, and the ring thus formed is called a chelate.

Some common chelating ligands are ethylenediamine (en), oxalate (ox), diethylenetriamine (dien), EDTA, etc.

4. Coordination Sphere

In the coordination compounds, the central metal atom/ion and the non-ionisable ligands attached to it are always enclosed in a square bracket ([]) and collectively termed as coordination sphere. The portion present outside the square brackets is called the ionisation sphere.

For example, in a complex \(\left[ {{\rm{Cu}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}} \right]{\rm{S}}{{\rm{O}}_4}\), the part \({\left[ {{\rm{Cu}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}} \right]^{2 + }}\) is called a coordination sphere, and \({\rm{SO}}_4^{2 – }\) constitutes an ionisation sphere.

Note:

I. Species present in the coordination sphere are non-ionisable.
II. Species present in the ionisation sphere are ionisable.

5. Coordination Number (CN) or Ligancy

It is the number of ligands that are directly bonded to the central metal atom/ion in the coordination sphere.

For example,
I. In the complex \({{\rm{K}}_2}\left[ {{\rm{PtC}}{{\rm{l}}_6}} \right]\), six chloride ions are present in the coordination sphere; therefore, the coordination number of \({\rm{Pt}}\) is \(6\).
II. In the complex \(\left[ {{\rm{Ni}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}} \right]{\rm{C}}{{\rm{l}}_2}\), the coordination number of \({\rm{Ni}}\) is \(4\).

6. Coordination Polyhedron

The spatial arrangement of the ligands bound directly to the central atom or ion forms a polyhedron around the central atom. This polyhedron is called a coordination polyhedron. The coordination polyhedron may be tetrahedral, square planar, octahedral, trigonal bipyramidal, etc., in shape.

7. Oxidation number or Oxidation State or Charge on Central Metal Ion

The oxidation number is the residual charge that appears on the central atom when all the ligands are removed along with the electron pairs shared with the central atom. A Roman numeral represents the oxidation number in parenthesis following the name of the coordination entity.

Similarly, the charge of a complex ion is the sum of the charges of the constituent parts i.e., the sum of the charges on the central metal ion and its surrounding ligands. It should be noted that:

a) the sum of the charges of the constituents is zero; the complex is neutral and
b) For an anionic or cationic complex, the sum of the charges of the constituents is equal to the load in the coordination sphere.

8. Homoleptic and Heteroleptic Complexes

Complexes in which a metal is bound to only one kind of donor group, e.g., \({\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_6}} \right]^{3 + }}\), are known as homoleptic. Complexes in which a metal is bound to more than one kind of donor group, e.g., \({\left[ {{\rm{Co}}{{\left( {{\rm{N}}{{\rm{H}}_3}} \right)}_4}{\rm{C}}{{\rm{l}}_2}} \right]^ + }\)^, are known as heteroleptic.

Summary

Coordination compounds are those molecular or addition compounds in which a central metal atom or ion is permanently attached to certain atoms or groups of atoms called ligands. A coordination entity contains a central metal atom or ion which is bonded by a fixed number of atoms or groups of atoms or anions or neutral molecules, each of which is called a ligand. The molecular or ionic species which gets directly attached to the central metal atom or ion during the formation of a complex is called a ligand. Central atom, coordination Sphere, coordination Number, coordination polyhedron, etc., are some important terms related to coordination compounds.

Q.1. What are the terms used in coordination chemistry?
Ans: The terms used in coordination chemistry are as follows:
1) Coordination Entity
2) Central Atom / Ion
3) Ligands
4) Coordination Sphere
5) Coordination Number
6) Coordination Polyhedron
7) Oxidation number
8) Homoleptic and Heteroleptic Complexes

Q.2. What are important types of reactions of coordination compounds?
Ans: Electron transfer, ligand exchange, and associative processes are some of the reactions that coordination complexes can undergo.

Q.3. What are the characteristics of coordination compounds?
Ans: Coordination compounds often exhibit a wide range of distinctive physical and chemical properties, such as colour, magnetic susceptibility, solubility and volatility, as well as the ability to undergo redox reactions and catalytic activity.

Q.4. What is the importance of coordination compounds?
Ans: Coordination compounds play critical roles as industrial catalysts in controlling reactivity and are required in biochemical processes.

Q.5. What is a coordination compound?
Ans: Coordination compounds are those molecular or addition compounds in which a central metal atom or ion is permanently attached to certain atoms or groups of atoms called ligands.

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