General Chemistry

Metal Complexes in the Body

The ability of metal ions to coordinate with (bind) and then release ligands in some processes, and to oxidize and reduce in other processes makes them ideal for use in biological systems. The most common metal used in the body is iron, and it plays a central role in almost all living cells. For example, iron complexes are used in the transport of oxygen in the blood and tissues.

Metal-ion complexes consist of a metal ion that is bonded via "coordinate-covalent bonds" (Figure 1) to a small number of anions or neutral molecules called ligands. For example the ammonia (NH3) ligand used in this experiment is a monodentate ligand; i.e., each monodentate ligand in a metal-ion complex possesses a single electron-pair-donor atom and occupies only one site in the coordination sphere of a metal ion. Some ligands have two or more electron-pair-donor atoms that can simultaneously coordinate to a metal ion and occupy two or more coordination sites; these ligands are called polydentate ligands. They are also known as chelating agents (from the Greek word meaning "claw"), because they appear to grasp the metal ion between two or more electron-pair-donor atoms. The coordination number for a metal refers to the total number of occupied coordination sites around the central metal ion (i.e., the total number of metal-ligand bonds in the complex).

Ligands

Figure 1

You have already learned that a covalent bond forms when electrons are shared between atoms. A coordinate-covalent bond (represented by a green arrow in this diagram) forms when both of the shared electrons come from the same atom, called the donor atom (blue).

An anion or molecule containing the donor atom is known as a ligand. The top illustration shows a coordinate-covalent bond between a metal ion (e.g., Fe, shown in red) and a monodentate ligand (a ligand that contains only one electron-pair-donor atom, shown in light blue). The bottom illustration shows a metal ion with coordinate-covalent bonds to a bidentate ligand (a ligand that contains two donor atoms simultaneously coordinated to the metal ion, shown in yellow).

 


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This page created by Matt Traverso, Washington University in St Louis.
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Revised: 2004-08-08