Knowledge database: 4.3 Complex reactions

As complex cases of reactions, we could mention the reactions of synthesis and analysis of coordination complex type of compounds, protolytic reactions and reactions of sedimentation and dissolution.

Complex compounds are compounds that are composed of two main parts - the central ion and the ligands. Ligands are basically molecules or ions that are connected to the central ion. This central ion is mostly one of the transition elements, and thereby its last electron shell isn't full. Because of that, the central ion attracts molecules or ions that have lone pairs of electrons in their shells. This kind of bond between ligands and the central ion is known as the coordinate covalent bond. Although understanding the creation of complex compounds isn't that simple, it isn't necessary for the understanding of reactions in which ions of this type take part.

On the picture above, we can see an example of a complex compound in which there is a cobalt central ion, to which chloride and ammonium ions are connected to, as ligands. We can also see that this ion has its own charge number, and in reactions, it basically behaves like a cation.

The other mentioned type of reactions are the so called protolytic reactions. They are characterized by the transfer of protons (i.e. hydrogen ions) from one ion or molecule to another. As a typical example we can mention the following reaction: NH3 + H2O <-> NH4(+) + OH(-). In this reaction, the water molecule loses a proton, and thereby becomes a hydroxide ion. On the other side, the ammonia molecule recieves a proton, and thereby becomes a ammonium ion.

The last type of reactions that we mentioned are the reactions of sedimentation and dissolution. In cases of such reactions, we usually deal with two solutions in which there are different ions dissolved. By combining these solutions, a insoluble compound is created in form of a sediment. This kind of reactions are often used in analytical chemistry for the detection of certain particles in different samples. As a typical example, we can mention the following reaction: Ag(+) + Cl(-) <-> AgCl. When the solutions containing silver and chloride ions are mixed together, silver chloride is created which forms immidiately in form of a white sediment.