Complex: Compounds containing coordination units formed by central atoms (ions) and ligands (anions or molecules) with coordination bonds, such as,.

Central atom: a form body, containing a void orbital in the valence electron layer, most of which are cations.

Ligand: can provide lone pairs of electrons, such as, etc.

Monodentate ligand: Only one coordination atom in a ligand forms a bond with the central atom.

Multidentate ligand: contains multiple coordination atoms, such as the bidentate ligand ethylenediamine.

Valence bond theory (hybrid orbital theory):

Crystal field theory:

Definition: A complex that forms a ring-like structure from a multidentate ligand and the same central atom.

Chelating agent: a ligand that forms chelating with central ions.

Chelation ratio: The ratio of the number of central ions to chelating agent molecules (or ions).

Inner boundary (combination ions): The central ion (atom) forms a bond with the ligand with a coordination bond, written in square brackets, including the matching ions.

External world: The opposite ions with charge balanced to the inner world, the molecules with no external world.

Coordination atoms: The atoms in the ligand that are directly connected to the central atom are generally non-metallic atoms with high electronegativity.

Coordination number: the total number of coordination atoms bonded to the central atom, and its influencing factors include the ligand radius, charge, the radius and charge of the central atom.

If it is a cation, it is called "Small and Certain Acid" or "Small and Certain Acid", such as copper nitric acid hexahydrate, and diamino chloride and silver.

If it is a combination anion, the word "acid" is connected between the combination anion and the external cation (the combination ions are regarded as acid root), such as potassium hexacyanoferrate.

Stability constant: The larger the complex, the more stable it is.

Unstable constant: the constant of the dissociation reaction.