Coordination Compounds and Nomenclature

Coordination compounds are neutral species consisting of either a neutral complex or charged complex ions and oppositely-charged counter ions. The complex ion is a central metal ion bound to molecules or ions, known as ligands.

Ligands are compounds with one or more electron pairs. They act as Lewis bases by donating a pair of electrons to the metal ion, which acts as a Lewis acid to form a coordinate covalent bond. Together, they constitute a coordination sphere.

In 1893, Alfred Werner proposed a coordination theory to explain why specific stable molecules react and violate the valence theory, to form coordination compounds. By studying various cobalt-ammonia complexes, Werner postulated that the central metal atom exhibits two types of valence interactions.

The primary valence is the oxidation number of the central metal, while the secondary valence is the number of ligands bound to it, also called the coordination number. Thus, if the number of bounded ligands decreases, counter ions can bind directly to the metal ion, maintaining the secondary valence.

Werner’s coordination theory is nowadays used to write chemical formulas of coordination compounds. Start with the symbol of the metal ion, then list all ligands in alphabetical order. Put square brackets around the complex ion, and list counter ions last. Use numerical subscripts to indicate the number of ligands and the counterions involved.

For a systematic nomenclature of coordination compounds, first, name the complex ions. Start by enlisting the ligands alphabetically. An anionic ligand ends with the letter -o, while a neutral ligand is denoted by its molecular name with a few exceptions such as aqua and ammine.

Use Greek-prefixes such as di-, tri- or tetra- to denote the number of ligands. If a prefix is present in the ligand name already, alternate prefixes to bis- tri- or tetrakis with the ligand name in parentheses.

Then name the metal name and its oxidation state using Roman numerals in parentheses. In the case of an anionic complex, the metal name is suffixed with -ate.

Lastly, the coordination compound is named by adding the cation name first, followed by the anion. Thus, the two coordination compounds are called amminechlorobis(ethylenediammine)cobalt(III) bromide and ammonium diaquatetrachlorocobaltate(III).