The transition metals and ion have much higher tendency to form coordination compounds as compared to the S- and P-block elements. It is because of their relatively smaller sizes, higher ionic charges and the availability of d- orbital for bond formation. Coordination compounds, unlike normal compounds retain their identity even when dissolve in water or any other suitable solvent. The properties of the compounds are totally different from those of their constituents.

The most distinctive aspects of transition metal chemistry is the formation of coordination compounds. A coordination compound is a complex substance in which atoms or group of atoms have been added beyond the number possible in the basis of electrovalent binding or covalent binding. In such a compound the linked atom of the coordinated group furnishes both electrons of the additional linkages. More simply, when a metal ion combines with an electron donor, the resulting substance is said to be a coordination compound. For example the ferrous ion combines with four ammonia molecules, in which the nitrogen atom is an electron donor to produce [Fe(NH3)4 ]2+ (2)

The molecules or ions that surround a metal ion in a complex are known as ligands. Ligands are normally either anions or polar molecules. Furthermore, they have at least one unshared pair of valence electrons, as illustrated in the following examples.

The type of bonding between the metal and ligand may be electrostatic or covalent. Ligands may be classified on the basis of their ability to form two bond with the metal ion unidentate ligands, such as    and     use a single donor atom or their ability to form two bonds (bidentate). Ligands that are divalent or contain two donor atoms, like ethlenediamine (H2 N  – CH2 – CH2 – NH2), are bidentate because they contain two pairs or sites that enter into bonds with the central ion. Examples are oxalato (C2 O­42- ), acetylacetonate (aCac), ethane – 1, 2 – diamine (en) etc.


Alfred Werner can be considered to be the pioneer in the field of coordination chemistry and he received noble prize in 1913 in recognition of his efforts. He showed that Neutral molecules were bound directly to the metal so that complex salt such as CoCl     is correctly formulated as [Co CNH3]6)3+(cl )3. G.N lewis and N.V sidgwick proposed that a chemical bond recquired the sharing of an electron pair. This led to the idea that a neutral molecule with an electron pair can donate these electrons to a metal ion or other electron acceptor. Thus, in a coordination compound, the metal species acts as electron acceptor (lewis acid) and neutral molecule with lone pair of electrons or anins as electron donor (lewis base). A metal atom or ion when bonded directly to a fixed number of anions or molecules constitutes a coordination entity. For example [pt (NH3)2cl2] is a coordination entity in which platinum is surrounded by two ammonia molecules and two chloride ions. Other examples are [Fe (CN)6]+,

2+ [Ni (co)4] etc. In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it is called the central atom or ion. For example the central atom in [Ni (NH3)6]2+ is [Ni (ii)] in [mo (CN8)]3+ mo  (v) and in [co (pr3)3], co (i). The charged ions or neutral molecules bound to the central atom in the coordination number of Mo and Co are 8 and 4 respectively. The central atom and the ligands bonded to it are enclosed in square bracket [ ] and is collectively termed as the coordination sphere. For example in the complex [Ni (NH3)6]cl2, the coordination sphere is [Ni (NH3)6]2+.The spatial arrangement of the ligands around the central atom is termed as coordination polyhedron. The most common coordination polyhedral are octahedral, tetrahedral, and square planar.