Coordination compounds are an important chapter of class 12th chemistry as prescribed by the CBSE board. It has regularly featured in the board examinations as well in every competitive entrance examination for medical or engineering. Therefore, it is necessary for the student to study this chapter in close details. Chemical compounds which comprise of an array of anions or neutral molecules, bound to a central atom through coordinate covalent bonds are termed as coordination compounds. Coordination compounds are also termed as coordination complexes.
Another important term to know is called as ligands, that are also termed as complexing agents. These ligands are the molecules or ions connected to the central atom. Several coordination compounds encompass a metallic element as the main central atom and are thus addressed as metal complexes. These kinds of coordination complex usually comprise of a transition element as its central atom. The central atoms are called as coordination centres. In order to understand the coordination compounds, one must first understand the important terms related to this topic. Following are the important terms to be noted,
- Coordination Entity- It is defined as a chemical compound where the central ion or atom (also called as the coordination center) is linked to a specified number of atoms, molecules, or ions. Few examples of these coordination entities are [CoCl3(NH3)3], and [Fe(CN)6]4.
- Central Atoms and Central Ions – These are the central atoms or ions to which specified atoms or ions are linked are called as central atoms or central ions. A point to note is that usually Lewis acids are central atoms or ions and thus suited for electron pair acceptors.
- Ligands – The atoms or ions that are connected to the central atom or ion are called as Ligands. They can be simple ions and molecules or can also form long molecules.
- Coordination Number – The total number of sigma bonds by which ligands are connected to coordination central atom is called as coordination number. An example of the same is that for the coordination complex given by [Ni(NH3)4]2+, the coordination number of nickel is 4.
- Coordination Sphere – The non-ionizable part of a complex compound consisting of central transition metal ion that is bounded by neighbouring atoms or groups and is enclosed in a square bracket. When the coordination centre, its ligands and the net charge of the chemical compound is written as a whole, then it forms a coordination sphere.
- Coordination Polyhedron – It is defined as the geometric shape created by the connection of the ligands to the coordination central atom is called the coordination polyhedron. Common spatial arrangements include square planar and tetrahedral.
- Oxidation Number – This refers to the oxidation number computed by determining the associated charge with it, when contribution of electron pair from ligands are removed from it.
Important properties of coordination complexes
We will now have a look at the important properties of coordination complexes below:
- The coordination compounds moulded by the presence of transition elements are coloured because of the existence of unpaired electrons. These electrons tend to absorb light in their electronic transitions. An example of this is where the complexes comprising Iron(II) can display green and pale green colours, but the coordination compounds having iron(III) tend to have a brown or a yellowish-brown colour.
- In cases where the coordination centre is a metal, then the corresponding coordination complexes tend to have a magnetic property. This is due to the occurrence of the unpaired electrons in the central complex.
- A wide range of chemical activates is exhibited by these coordination compounds. They are usually a part of the inner-sphere electron transfer reactions along with electron transfers on the outer sphere.
- Complex compounds with specified ligands have the capacity to help in the conversion of molecules in a catalytic or a stoichiometric means.
Do you know the answer of the below question?
D.Tetrammine copper (II) sulphate
Important Rules to be followed for Nomenclature of Coordination Complexes
The IUPAC has laid down strict standards to be followed in order to name coordination compounds. Following are the important rules,
- While naming complex coordination complexes, then the ligand name must always be written before the central atom or ion.
- Alphabetical order must be followed when a central ion or atom is connected by more than one ligand.
- Prefixes such as di-. Tri- or tetra etc can be used when there is a presence of multiple monodentate ligands connected to central atom.
- Only after the correcting naming of ligands is done that the naming oc the central atom or ion can be proceeded with. It must be borne in mind that when a coordination compound has an anion charge linked to it, then the suffix of ‘-ate’ must be used.
- Another important point is that the oxidation state of the central metal atom/ion has to be stated using roman numerals. It is important to note that these roman numerals must be enclosed in brackets.
- If the coordination compound is linked through a counter ion, then the cationic unit has to be mentioned prior to mentioning the anionic part.
