Metal-Ligand Bonds

The hemoglobin in the blood, the chlorophyll in green plants, vitamin B-12, and the catalyst used in the manufacture of polyethylene all contain coordination compounds. Ions of the metals, especially the transition metals, are likely to form complexes.

In these complexes, transition metals form coordinate covalent bonds, a kind of Lewis acid-base interaction in which both of the electrons in the bond are contributed by a donor (Lewis base) to an electron acceptor (Lewis acid). The Lewis acid in coordination complexes, often called a central metal ion (or atom), is the 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 with a lone pair of electrons that can form a coordinate bond to the metal. 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.

Ligands are monodentate, from the Greek for “one-toothed,” when they connect with the central metal through only one atom. Here, the number of ligands and the coordination number are equal. Ligands with one donor atom, such as NH₃, Cl⁻, and H₂O, are monodentate ligands. Many other ligands coordinate to the metal in more complex fashions.

Bidentate ligands are those in which two atoms coordinate to the metal center. For example, ethylenediamine (en, H₂NCH₂CH₂NH₂) contains two nitrogen atoms, each of which has a lone pair and can serve as a Lewis base (Figure 1a). Both of the atoms can coordinate to a single metal center. In the complex [Co(en)₃]³⁺, there are three bidentate en ligands, and the coordination number of the cobalt(III) ion is six (Figure 1b). The most common coordination numbers are two, four, and six, but examples of all coordination numbers from 1 to 15 are known.

Figure 1. (a) ethylenediamine (en) contains two nitrogen atoms, with a lone pair each, that can coordinate with a metal ion. (b) Three bidentate en ligands coordinate with a single cobalt ion.

Any ligand that binds to a central metal ion by more than one donor atom is a polydentate ligand (or “many teeth”) because it can bite into the metal center with more than one bond. The term chelate from the Greek for “claw” is also used to describe this type of interaction. Many polydentate ligands are chelating ligands, and a complex consisting of one or more of these ligands and a central metal is a chelate. A chelating ligand is also known as a chelating agent. A chelating ligand holds the metal ion rather like a crab’s claw would hold a marble. Polydentate ligands are sometimes identified with prefixes that indicate the number of donor atoms in the ligand. The heme complex in hemoglobin is another important example (Figure 2). It contains a polydentate ligand with four donor atoms that coordinate to iron.

Figure 2: The single ligand heme contains four nitrogen atoms that coordinate to iron in hemoglobin to form a chelate.

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