20.3:
Metal-Ligand Bonds
In a complex ion, the metal ion is bound to anionic or neutral molecules, known as ligands. Ligands have one or more lone pairs of electrons and act as electron-pair donors or Lewis bases. They donate the electron pair to the metal ion, which acts as an electron-pair acceptor or a Lewis acid. Thus, a coordinate covalent bond between the metal ion and ligand forms a Lewis acid-base adduct.
The ligand atom donating an electron pair is known as the donor atom. The number of donor atoms surrounding the metal ion correlates to the coordination number of the metal ion.
Depending upon the number of donor atoms present, ligands are further classified as monodentate, bidentate, or polydentate ligands, which can be charged or neutral.
Monodentate ligands have only one donor atom. Oxygen is a donor atom in a neutral water molecule; nitrogen is a donor atom in a charged cyanide ion.
Bidentate ligands have two donor atoms that are sufficiently far apart to bind simultaneously to a metal ion. The nitrogen atoms of three bipyridine molecules bind to a ruthenium ion with the coordination number six.
A polydentate ligand has two or more donor atoms present in its structure. Here, six nitrogens from two molecules of diethylenetriamine bind to a cobalt ion.
As multiple donor atoms appear to hold the central metal ion like a claw, the bidentate and polydentate ligands are also called chelating agents.
A chelating agent has more affinity for the central metal ion than a monodentate ligand, forming a more stable coordination complex. This is known as the chelating effect.
Chelating agents are versatile. They are used to complex interfering metal ions in a reaction—to increase the shelf life of food products by complexing trace metal ions involved in catalyzing decomposition reactions or treating lead poisoning using ethylenediaminetetraacetate or EDTA.
The complex ion formation influences the physicochemical properties of the metal ion and ligands, such as change in oxidation and reduction potentials, or colors. Furthermore, ligands can undergo reactions such as deprotonation of hydrated metal ions or displacement by another strongly-attracted ligand. Here, the water molecule is replaced by ammonia, which binds stronger to the metal ion.
20.3:
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.