INTRODUCTION

1.1  COORDINATION COMPOUNDS

The transition metals and ions have a 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 dissolving in water or any other suitable solvent. The properties of the compounds are totally different from those of their constituents.

The most distinctive aspect of transition metal chemistry is the formation of coordination compounds. A coordination compound is a complex substance in which atoms or groups of atoms have been added beyond the number possible on 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 bonds 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 ethylenediamine (H2 N  – CH2 – CH2  – NH2), are bidentate because they contain two pairs or sites that enter into bonds with the central ion. Examples are oxalate (C2  O­42- ), acetylacetonate (aCac–), ethane – 1, 2 – diamine (en) etc.

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