Coordination Compounds and Nomenclature

A ligand is a molecule or ion with one or more electron pairs that acts as a Lewis base, donating electrons to a central metal ion. Ligands form coordinate covalent bonds with the metal, which acts as a Lewis acid. Common ligands include ammonia, water, and chloride ions, each contributing electron pairs to stabilize the complex.

Start with the central metal ion symbol, then list all ligands alphabetically. Enclose the complex ion in square brackets, and place counter ions outside the brackets last. Use numerical subscripts to indicate the number of each ligand and counter ion. For example, [Co(NH3)5Cl]Br2 shows cobalt with five ammonia ligands and one chloride, balanced by two bromide counter ions.

Primary valence is the oxidation number of the central metal ion, while secondary valence is the coordination number—the total number of ligands bonded to the metal. Alfred Werner's coordination theory explains that if fewer ligands bind to the metal, counter ions can attach directly to maintain the secondary valence constant.

Anionic ligands are named by adding the suffix -o to the stem name of the group. For example, chloride becomes chloro, and hydroxide becomes hydroxo. These are listed alphabetically before neutral ligands when naming the complex, and their count is indicated using Greek prefixes like di-, tri-, or tetra-.

Four common neutral ligands have special names: aqua for water (H2O), ammine for ammonia (NH3), carbonyl for carbon monoxide (CO), and nitrosyl for nitrogen monoxide (NO). These exceptions are used instead of the molecular names. When multiple identical ligands are present and the ligand name contains di-, tri-, or tetra-, use bis-, tris-, or tetrakis- prefixes instead.

Set the overall charge of the coordination compound equal to the sum of the ligand charges plus the metal's oxidation state. Water and neutral molecules contribute zero charge, while anionic ligands contribute their negative charge. For example, in [Cr(H2O)4Cl2]Br, the complex has a +1 charge: +1 = 0 − 2 + x, so the metal's oxidation state is +3.

The coordination sphere consists of the central metal ion plus all attached ligands. It is enclosed in square brackets in the chemical formula, while counter ions remain outside the brackets. The coordination number is the total number of donor atoms bonded to the metal. For instance, in [Co(H2O)6]2+, the coordination sphere contains cobalt and six water molecules with a coordination number of six.