Chelates – what they are, how they work and why they are so important

How are chelate complexes formed?

The chelation process

A chelate complex is a type of coordination compound in which a metal ion is connected to a ligand through multiple coordination bonds (donor-acceptor), forming a ring structure. This unique bond increases the stability of the metal complex compared to simple coordination complexes, which is important in various chemical reactions and biological processes.

Chelation is a special case of coordination chemistry theory. A bidentate or polydentate ligand binds to a metal or metal ion, forming a stable ring structure called a chelate ring. The way in which the coordination bond is formed plays an important role here. In this case, the shared electron pair comes from only one of the atoms (which already has a stable electron configuration). In the process of chelate formation, at least two free electron pairs from different donor atoms coordinate with the same central ion.

Properties of chelates

The formation of multiple coordination bonds by chelates, and in particular the presence of a ring in the molecule, determines their unique properties. The most important of these are listed below:

• Chelate complexes are generally more stable than non-chelated complexes due to the entropy advantage gained by forming multiple bonds with the metal centre.
• The chelate effect refers to the increased stability of chelate complexes compared to complexes formed with monodentate ligands. The greater the number of ring closures around the metal atom, the more stable the compound.
• The stability constants of chelate complexes can vary significantly depending on the nature of both the metal ion and the ligands involved.
• Chelates exhibit good buffering properties.
• The presence of metal and the complex structure give them fluorescent properties.

Natural chelates – examples from nature

Haemoglobin

Haemoglobin – a polypeptide found in red blood cells, enables the transport of oxygen in the blood from the lungs to other tissues in the body. A single haem ligand contains four nitrogen atoms that bind to iron in haemoglobin to form a chelate. Oxygen molecules are transported by haemoglobin in the blood, binding to the iron centre. When haemoglobin loses oxygen, its colour changes to blue-red. Importantly, haemoglobin only transports oxygen when iron is in the Fe2+ form; oxidation of iron to Fe3+ prevents oxygen transport.

Chlorophyll

Chlorophyll is a green pigment found in plants. It is an extremely important component of photosynthesis, absorbing light energy and converting it into chemical energy. The central ion in chlorophyll is magnesium, which is bound to four nitrogen atoms, thus forming a stable ring structure.

Vitamin B12

Vitamin B12 is a naturally occurring compound containing cobalt. This metal is the central ion of the vitamin B12 chelate. Four nitrogen atoms are coordinatively bound to cobalt, forming a ring structure. The chelate structure of vitamin B12 is crucial for its biological functions, particularly its enzymatic role.

The use of chelates in medicine, chemistry and agriculture

The unique structure of chelates, which gives them remarkable properties, determines their wide application in many fields.

These compounds are used in medicine and pharma,  especially in the treatment of heavy metal poisoning. Chelating agents bind and remove toxic metals from the body. This group mainly includes lead and mercury. Cadmium, cobalt, gallium, lithium and zinc are also listed in this category, although they occur less frequently. All of the above elements, when ingested, act as metabolic poisons, but also as teratogens, i.e. substances that cause birth defects. The activity of these elements in the body and their subsequent removal from the body takes place, among other things, through chelation.

In analytical chemistry, chelates are used to detect and quantitatively determine metal ions in various samples. They are particularly important in classical analysis, e.g. complexometric titrations. They have the ability to selectively and stably bind metals.

Chelating agents are also used as extractants in industrial and laboratory metal separation and as metal ion buffers and indicators in analytical chemistry. Many commercial dyes and a number of biological substances, including chlorophyll and haemoglobin, are chelate compounds.

Chelates also perform important functions in agriculture, e.g. in the form of fertilisers that supplement mineral deficiencies or agents for controlling plant diseases.

Summary

Chelates are chemical compounds with a unique structure and wide range of applications. Their ability to form stable complexes with metal ions makes them indispensable in biology, medicine, analytical chemistry and agriculture. Thanks to their structure and properties, they play a key role in many life and technological processes.