Ligand

The bonding atoms of ligands are usually non-metal elements such as oxygen, nitrogen, or chlorine. Whether alone or in molecules such as water or ammonia, these atoms have pairs of electrons that are not involved in chemical bonds. The electron pairs can enter the space around the metal atom and bond with it.

Thus, the metal and ligand are joined by a covalent bond, consisting of two electrons shared between them. However, both electrons are provided by the ligand itself.

Metal atoms or ions usually bond to two, four or six ligands. These are arranged with geometric symmetry around the central metal atom. The metal together with its ligands are called a coordination compound. If the structure has an overall electrical charge it is called a complex ion.

Because of their shapes, there can be different coordination compounds having exactly the same atoms and bonds, but arranged differently. Such molecules are called geometric isomers.

The different arrangement causes differences in physical properties such as color and melting temperature. Geometric isomers also differ in their chemical reactions, especially when these occur in living organisms. That is because the molecules which make up living things are themselves usually geometric isomers with very specific shapes. Reactions only occur between molecules whose shapes match each other, like a key fitted to a lock.

An example of a biologically active geometric isomer is cisplatin, a coordination compound used in medicine to suppress tumors. The molecule consists of a platinum atom surrounded by two ammonia molecules and two chlorine atoms. The four ligands lie at the corners of a square, with ligands of the same kind as neighbors.

The isomer transplatin, in which they are diagonally opposite each other, has no affect on tumors.

Compounds of metals with ligands are often brightly colored. This results from repulsion between the electrons of the ligand and those of the metal atom itself. The atom's electrons are also geometrically arranged around its nucleus. They occupy regions called orbitals. In an isolated metal atom, groups of similar orbitals have the same energy. But when ligands bond to the atom, they approach some orbitals more closely than others. Electrons in the closer orbital are repelled more strongly. They must have more energy Figure 5. Ethylenediaminetetraacetic acid (EDTA) is one of the best-known chelating agents. Illustration by Hans & Cassidy. Courtesy of Gale Group.

to occupy those orbitals. The energy difference is called "crystal field splitting."

The metal's electrons can move to a higher energy orbital by absorbing energy from visible light. Therefore, such compounds appear colored.