When it comes to complexes, the first thing to say is coordination bonds. When two substances form a valence bond, the two electrons that form the bond are provided by the same element, and the other atom only provides an empty orbit. This bonding method is called coordination bond (coordination bond is a kind of valence bond, but the bonding method is different from ordinary valence bond, so it is named separately).

Compounds with coordination bonds can be called complexes, and complexes are also called complexes. (In fact, many particles we see contain coordination bonds, but not all substances with coordination bonds are called complexes. Ammonium (NH4)+ can be understood as an ammonia molecule providing a pair of electrons to a hydrogen ion to form coordination bonds, but we generally don't say that ammonium is a complex). Let me talk about the characteristics of the complex.

Generally, complexes have a central atom (mostly metal atoms or ions, with many empty orbitals). There are also some particles that provide them with lone electron pairs, called ligands. For example, (Fe(SCN)3, where iron ion is the central atom and bisulfate is the ligand (S-coordinated lone electron pair). The coordination bond existing in the molecule is S-Fe bond. This is a common complex. If a ligand can provide a pair of electrons, it is called a monodentate ligand, and a ligand can provide multiple pairs of electrons to the central ion at the same time, this ligand is called a multidentate ligand, and the formed complex is also called a chelate, which is generally organic.

For example, the chelated calcium of Rolex brand we eat is difficult to absorb, but the absorption rate of chelated calcium is much higher. This is the application of complexes in life.