Metal-Ligand Bonds

In a complex ion, the metal ion is bound to anionic or neutral molecules, known as ligands. Ligands have one or more lone pairs of electrons and act as electron-pair donors or Lewis bases. They donate the electron pair to the metal ion, which acts as an electron-pair acceptor or a Lewis acid. Thus, a coordinate covalent bond between the metal ion and ligand forms a Lewis acid-base adduct.

The ligand atom donating an electron pair is known as the donor atom. The number of donor atoms surrounding the metal ion correlates to the coordination number of the metal ion.

Depending upon the number of donor atoms present, ligands are further classified as monodentate, bidentate, or polydentate ligands, which can be charged or neutral.

Monodentate ligands have only one donor atom. Oxygen is a donor atom in a neutral water molecule; nitrogen is a donor atom in a charged cyanide ion.

Bidentate ligands have two donor atoms that are sufficiently far apart to bind simultaneously to a metal ion. The nitrogen atoms of three bipyridine molecules bind to a ruthenium ion with the coordination number six.

A polydentate ligand has two or more donor atoms present in its structure. Here, six nitrogens from two molecules of diethylenetriamine bind to a cobalt ion.

As multiple donor atoms appear to hold the central metal ion like a claw, the bidentate and polydentate ligands are also called chelating agents.

A chelating agent has more affinity for the central metal ion than a monodentate ligand, forming a more stable coordination complex. This is known as the chelating effect.

Chelating agents are versatile. They are used to complex interfering metal ions in a reaction—to increase the shelf life of food products by complexing trace metal ions involved in catalyzing decomposition reactions or treating lead poisoning using ethylenediaminetetraacetate or EDTA.

The complex ion formation influences the physicochemical properties of the metal ion and ligands, such as change in oxidation and reduction potentials, or colors. Furthermore, ligands can undergo reactions such as deprotonation of hydrated metal ions or displacement by another strongly-attracted ligand. Here, the water molecule is replaced by ammonia, which binds stronger to the metal ion.