To give a more or less precise definition of what,which are complex compounds, modern chemistry has to rely on the basic provisions of the coordination theory, which was proposed by A. Werner as far back as 1893. The complexity of this question lies in the diversity and multiplicity of the most diverse chemical compounds that fall under the definition of complex compounds.
In general terms, complex connections arechemical substances, which contain a number of complex particles. Until now, science does not have a strict definition of the concept of "complex particle". Often use the following definition: a complex particle is understood as a complex particle that is capable of independently existing both in a crystal and in solution. It consists of other simple particles, which in turn have the ability to exist on their own. Also often complex chemical particles fall into the definition of complex particles, in which all bonds or a part of them are formed according to the donor-acceptor principle.
A common feature that all complexcompound, is the presence in their structure of a central atom, called "complexing agent." Given the diversity that these compounds have, 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 that are bright nonmetals. Speaking about the charge of the complexing agent, we can say that it is mostly positive, and in the scientific literature it was called the metal center, but there are examples when the central atom had both a negative charge and even zero.
Accordingly, isolated groups of atoms orindividual atoms that are located around the complexing agent, are called ligands. This may be particles that before the entry into the complex compound were molecules, for example, water (H2O), carbon monoxide (CO), nitrogen (NH3), and many others, as well as OH, PO43, Cl-, or the hydrogen cation H +.
An attempt to classify complex compoundsby the type of charge of the complex, separates these chemical compounds into cationic complexes that are formed around a positively charged ion of neutral molecules. Also there are anionic complexes, the complexing agent in which is an atom with a positive oxidation state. Simple and complex anions are ligands. A separate group can be distinguished neutral complexes. Their formation occurs by coordinating around a neutral atom of molecules. Also to this category of complex substances include compounds formed by 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, bidentate and polydentate ligands are determined.
Preparation of complex compounds by variousmethods makes it possible to classify by the nature of the ligand. Among them are ammonia, in which ligands are represented by ammonia molecules, aqua complexes, where ligands are water, carbonyls-carbon monoxide plays the role of a ligand. In addition, there are acido complexes in which the central atom is surrounded by acid residues. If it is surrounded by hydroxide ions, then the compounds are referred to as hydroxo complexes.
Complex compounds play an important role in thenature. 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, extraction of metals from ores, electrotype, varnish and paint production is only a short list of industries in which complex chemicals have been applied.
</ p>>
