The ability of the transition metals to form the complexes is referred to as coordination compounds. This property is not only applicable to transition metal only but also shown by some other metals. Werner, in 1893 was the first scientist to propose the theory of coordination compounds and also purposed the concept of the primary and secondary valence of a metal ion.

The branch of inorganic chemistry dealing with the study of coordination compounds is known as coordination chemistry.

What are Coordination Compounds?

Coordination compounds are the compounds in which the central metal atom is attached to a number of ions or neutral molecules by coordinate bonds either by donation of lone pairs of electrons by these ions or neutral molecules to the central metal atom.

Coordination compounds are used for the extraction of metals, in Analytical (qualitative or quantitative analysis), chemistry (like colour tests, titrations etc.), inorganic chemistry (can be used to remove toxics from the substances or any bodies) and even use in the treatment of chemotherapy and can be used for preparing various medicines. It can also be used as a catalyst.

e.g. Nickel tetracarbonyl, [Ni(CO)₄], in which CO molecules are linked to the central nickel atom by coordinate bonds by donating lone pairs of electrons, is a coordination compound.

Types of Complexes

There are following three types of complexes-

Cationic complex – The complex ion carries a net positive charge. e.g.: [Co(NH₃)₆]³⁺, [Ni(NH₃)₆]²⁺, etc.
Anionic complex – The complex ion carries a net negative charge. e.g.: [Ag(CN)₂]^–, [Fe(CN)₆]^4-, etc.
Neutral complex – A complex carrying no net charge. e.g.: [Ni(CO)₄], [Co(NH₃)₃Cl₃], etc.

Terminology used in Coordination Compounds

Before proceeding with the study of coordination compounds first we need to understand some basic terms related to it:

Ligands and central metal atom/ion: The ion or neutral molecules which are bonded to a central metal ion or atoms is called a ligand. The atom or ion to which these ligands are attached is called a central metal atom or ion. While in the formation of the coordinate bonds, the anions or the neutral molecules act as the electron pair donors and the metal atom or ion acts as the electron pair acceptor.

For example: In [Ni(NH₃)₆]²⁺, central metal atom or ion is lewis acid.

The ligand is attached to a single donor atom, then it is called unidentate. The number of coordinating groups attached to a ligand is called the denticity of the ligand.

When ligands are attached to different donor atoms then it is called ambi dentate ligands. e.g. NO^–₂ can coordinate through either nitrogen or oxygen. Similarly, CN^– can coordinate through either carbon or nitrogen.

Coordination number: The number of ligands attached to the central metal ion through coordinate bonds is called the coordination number of the metal ion.

For example, in the complex ions [Ag(CN)₂]^–, [Cu(NH₃)₄]²⁺ and [Cr(H₂O)₆]³⁺, the coordination numbers of Ag, Cu and Cr are 2, 4 and 6 respectively.

Coordination sphere or coordination entity: A entity attached to a central atom in a fixed number of ions or molecules and is enclosed in square brackets and are called the coordination sphere or coordination entity. The ionizable groups which are written outside the square brackets are called counter ions.

For example, in the coordination compound.[Cu(NH₃)₄] SO₄, the complex ion, [Cu(NH₃)₄]²⁺, in which Cu²⁺ is the central metal ion and four NH₃ molecules are ligands, forms the coordination sphere and SO₄^2-ions are the counter ions.

Coordination polyhedron: This spatial arrangement of the ligand atoms which are directly attached to the central atom/ ion is called coordination polyhedron around the central atom/ion.
Oxidation number or oxidation state: The oxidation number of the central atom is defined as the charge that it carries as if all the ligands are removed with electron pairs that are shared with the central atom. It is represented by the roman numbers, I, II, III, etc. in the parentheses after the name of the central atom.

For example, the oxidation number of copper in [Cu(NH₃)₄]²⁺is +2 but the coordination number is 4. Similarly, the oxidation number of Fe in [Fe(CN)₆]^3- is 3 but the coordination number is 6.

Charge on the complex ion: The charge carried by a complex ion is the algebraic sum of the charges carried by the central metal ion and the ligands coordinated to the central metal ion.

For example, in the complex ion, [Ag(CN)₂], Ag⁺ion carries a change of +1 and each CN^–ion carries a charge of – 1. Therefore, the net charge on the complex ion [Ag(CN)₂]^– is + 1 – 2 = -1.

Homoleptic and Heteroleptic complexes: Complexes in which the metal atom is attached to only one type of ligands are called homoleptic complexes. For example, [Co(NH₃)₆]³⁺. The complexes in which the metal atom is attached to more than one kind of ligand are called heteroleptic complexes. For example, [Co(NH₃)₄Cl₂]⁺.
Homonuclear and polynuclear complexes: Complexes in which only one metal atom is present are known as homonuclear complexes whereas in which more than one metal atom is present are known as polynuclear complexes.