To give a more or less accurate definition of what complex compounds are, modern chemistry has to rely on the basic principles of the coordination theory, which was proposed by A. Werner back in 1893. The complexity of this issue lies in the diversity and multiplicity of various chemical compounds falling within under the definition of complex.
In general terms, complex compounds are chemicals that contain a number of complex particles. Until now, science does not have a rigorous definition of the term "complex particle". Often they use the following definition: a complex particle is understood to mean a complex particle that can independently exist both in a crystal and in a solution. It consists of other simple particles, which in turn have the ability to exist independently. Also, complex chemical particles, in which all or part of the bonds are formed according to the donor – acceptor principle, often fall under the definition of complex particles.
A common feature that all complex compounds possess is the presence of a central atom in their structure, called the “complexing agent”. Given the diversity that these compounds possess, it is not necessary to talk about some common features of this element. Often, the complexing agent is an atom forming a metal. But this is not a strict sign: complex compounds are known in which the central atom is an atom of oxygen, sulfur, nitrogen, iodine and other elements, which are bright non-metals. Speaking about the charge of the complexing agent, we can say that it is mainly positive, and in the scientific literature has received the name of a metal center, but there are examples where the central atom had a negative charge, and even zero.
Accordingly, isolated groups of atoms or individual atoms that are located around a complexing agent are called ligands. These can be particles, which, before entering the complex compound, were molecules, for example, water (H2O), carbon monoxide (CO), nitrogen (NH3), and many others; they can also be OH–, PO43–, Cl−, or a hydrogen cation H +.
An attempt to classify complex compounds by the type of charge of the complex divides these chemical compounds into cationic complexes, which are formed around a positively charged ion of neutral molecules. There are also anionic complexes, the complexing agent of which is an atom with a positive oxidation state. Simple and complex anions are ligands. A separate group can distinguish neutral complexes. Their formation occurs by coordination around the neutral atom of the molecule. Also included in this category of complex substances are compounds formed by the simultaneous coordination around a positively charged ion and molecules, and negatively charged ions.
If we take into account the number of places occupied by ligands in the so-called coordination sphere, then monodentate, bidentant, and multidentate ligands are determined.
The preparation of complex compounds in various ways allows classification by the nature of the ligand. Among them are ammonia, in which the ligands are represented by ammonia molecules, aquacomplexes, where the ligands are water, carbonyls - carbon monoxide plays the role of a ligand. In addition, acid complexes exist in which acid residues surround the central atom. If it is surrounded by hydroxide ions, then the compounds are referred to as hydroxocomplexes.
Complex compounds play an important role in nature. Without them, the vital activity of living organisms is impossible. Also, the use of complex compounds in human activities allows for complex technological operations.
Analytical chemistry, metal mining from ores, electroplating, production of varnishes and paints - this is just a short list of industries in which the use of complex chemicals was found.