20.2: Coordination Compounds and Nomenclature

In most main group element compounds, the valence electrons of the isolated atoms combine to form chemical bonds that satisfy the octet rule. For instance, the four valence electrons of carbon overlap with electrons from four hydrogen atoms to form CH₄. The one valence electron leaves sodium and adds to the seven valence electrons of chlorine to form the ionic formula unit NaCl (Figure 1a). Transition metals do not normally bond in this fashion. They primarily form coordinate covalent bonds, a form of the Lewis acid-base interaction in which both of the electrons in the bond are contributed by a donor (Lewis base, e.g. ammonia molecule ) to an electron acceptor (Lewis acid, e.g. cobalt ion) (Figure 1b). The Lewis acid in coordination complexes, called a central metal ion (or atom), is often a transition metal or inner transition metal. The Lewis base donors, called ligands, can be a wide variety of chemicals - atoms, molecules, or ions. The only requirement is that they have one or more electron pairs, which can be donated to the central metal. Most often, this involves a donor atom (nitrogen atom, Figure 1b) with a lone pair of electrons that can form a coordinate bond to the metal.

Figure 1 (a) Covalent bonds involve the sharing of electrons, and ionic bonds involve the transferring of electrons associated with each bonding atom, as indicated by the colored electrons. (b) However, coordinate covalent bonds involve electrons from a Lewis base being donated to a metal center. The lone pairs from six ammonia molecules form bonds to the cobalt ion to form an octahedral complex.

The coordination sphere consists of the central metal ion or atom plus its attached ligands. Brackets in a formula enclose the coordination sphere; species outside the brackets are not part of the coordination sphere. The coordination number of the central metal ion or atom is the number of donor atoms bonded to it. The coordination number for the silver ion in [Ag(NH₃)₂]⁺ is two; for the copper(II) ion in [CuCl₄]²⁻, the coordination number is four; whereas for the cobalt(II) ion in [Co(H₂O)₆]²⁺ the coordination number is six.

The Naming of Complexes

The nomenclature of the complexes is patterned after a system suggested by Alfred Werner, a Swiss chemist and Nobel laureate, whose outstanding work more than 100 years ago laid the foundation for a clearer understanding of these compounds. The following five rules are used for naming complexes:

1. If a coordination compound is ionic, name the cation first and the anion second, in accordance with the usual nomenclature.
2. Name the ligands first, followed by the central metal. Name the ligands alphabetically. Negative ligands (anions) are named by adding -o to the stem name of the group. For most neutral ligands, the name of the molecule is used. The four common exceptions are aqua (H₂O), ammine (NH₃), carbonyl (CO), and nitrosyl (NO). For example, name [Pt(NH₃)₂Cl₄] as diamminetetrachloroplatinum(IV).
3. If more than one ligand of a given type is present, the number is indicated by the prefixes di- (for two), tri- (for three), tetra- (for four), penta- (for five), and hexa- (for six). Sometimes, the prefixes bis- (for two), tris- (for three), and tetrakis- (for four) are used when the name of the ligand already includes di-, tri-, or tetra-, or when the ligand name begins with a vowel. For example, the ion bis(bipyridyl)osmium(II) uses bis- to signify that there are two ligands attached to Os, and each bipyridyl ligand contains two pyridine groups (C₅H₄N).

When the complex is either a cation or a neutral molecule, the name of the central metal atom is spelled exactly like the name of the element and is followed by a Roman numeral in parentheses to indicate its oxidation state.

When the complex is an anion, the suffix -ate is added to the stem of the name of the metal, followed by the Roman numeral designation of its oxidation state. Sometimes, the Latin name of the metal is used when the English name is awkward. For example, ferrate is used instead of ironate, plumbate instead leadate, and stannate instead of tinate.

The oxidation state of the metal is determined based on the charges of each ligand and the overall charge of the coordination compound. For example, in [Cr(H₂O)₄Cl₂]Br, the coordination sphere (in brackets) has a charge of 1+ to balance the bromide ion. The water ligands are neutral, and the chloride ligands are anionic with a charge of 1− each. To determine the oxidation state of the metal, the overall charge is set equal to the sum of the ligands and the metal: +1 = −2 + x, so the oxidation state (x) is equal to +3.

This text is adapted from Openstax, Chemistry 2e, Chapter 19.2 Coordination Chemistry of Transition Metals.

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).