Metal atoms or ions interact with a ligand containing two or more coordination atoms to form a complex with a ring structure, which is called a chelate.The ligand that can generate chelates is called chelating agent, also called complexing agent.
Metal atoms or ions interact with a ligand containing two or more coordination atoms to form a complex with a ring structure, which is called a chelate.The ligand that can generate chelates is called chelating agent, also called complexing agent.It is also called chelating ligand, chelating group or multidentate ligand.
The ligand has two or more coordination atoms, and forms a chelate ring with a central atom (or ion) at the same time.Because of the cyclization of chelating agents, the stability of chelates is higher than that of non chelating coordination compounds with similar composition and structure.
Chelating agents are mostly organic ligands.Up to now, 14 chelating agents have been found at most.Oxygen and nitrogen are the most common coordination atoms in chelating agents, followed by sulfur, phosphorus and arsenic.
This kind of compound is described as "chelate" because the molecular structure is very similar to the two "claws" of "crab", which clamp metal atoms or ions.
chemical property
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Complex ions (also called complex ions) are composed of a positive ion (called central ion) and several neutral molecules or ions (called ligands). Compounds containing coordination ions are called coordination compounds.This coordination compound has a ring structure.Its stability is higher than that of non chelates with similar composition and structure.
In the complex, the central ion and the ligand are bound by coordination bonds.Coordination bond is a special kind of covalent bond. Generally, the covalent bond is formed by two bonding atoms giving out one electron each to form a common electron pair. In coordination bond, one atom provides the electron pair and the other atom provides the empty orbit.In order to distinguish, the covalent bond is represented by "-", such as H ·+· H=H: H (H-H), the coordination bond is represented by "←", and the arrow points to the atom providing the empty orbit, such as Cu+NHthree=CuNHthree(Cu←NHthree)。
If there is only one coordination atom in the ligand, only one coordination bond can be formed between the central ion and the ligand.Some ligand molecules contain more than two coordination atoms, and the two atoms are separated by two to three other non coordination atomsatomThe ligand can form more than two coordination bonds with the central ion (or atom) at the same time.The number of atoms directly coordinating with the central ion is called the coordination number. Usually, the coordination number is 4 or 6, and there are also 2 and 8.The coordination number depends on the electronic structure and spatial factors of the central particle.
The most common coordination atoms are oxygen, sulfur and nitrogen.Phosphorus, arsenic and selenium can also form chelates.In chelating agent molecules, the coordination atoms are connected to form an appropriate structure through other atoms (mainly carbon atoms).According to the number of coordination atoms that can participate in coordination contained in the chelating agent molecule, it can be divided into two rodent, three rodent, four rodent and six rodent chelating agents.
Chelating agents includeinorganicandOrganicTwo types.Most of them are organic compounds.Common chelating agents include polyphosphates, amino carboxylic acids, 1,3-diketones, hydroxycarboxylic acids, polyamines, etc.
Chelating agents have various structural forms, including straight chain and branched chain structures.There are also ring chelating agents such as porphyrin derivatives and crown ether derivatives.
Chelating effect
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The chelate formed by the same metal ion and a chelating agent is more stable than the complex formed by a single rodent ligand with the same coordination atom. This special stability is due to cyclization. Therefore, the increased stability due to the formation of a chelating ring is called chelating effect.
The stability of the chelate is measured by the stability constant of the chelate. The greater the stability constant, the higher the stability of the chelate.It is assumed that the chelate formed by chelating agent and metal ion is ML (the simplest case).Strictly speaking, the stability constant of chelate should be expressed as:
In the formula, activity of α - type body;
KTML：At a certain temperature, it is a constant and becomes the activity constant or thermodynamic constant.[1]
purpose
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Chelation is widely used in chemical research and industrial production.
In the chemical industry and industrial production process, adding chelating agents to make metal ions form chelates with completely different properties is the main method to reduce and control the concentration of metal ions.
Chelates are widely used in mineral flotation process, hydrometallurgy, extraction and separation of metal elements, catalytic synthesis of substances, water softening, electroplating process, pharmaceutical industry, dyeing process, etc.
Chelates are widely used in the chemical, mechanical andDeinkingBleaching of pulp.Some chelating agents can be used in medicine for the treatment of lead and mercury poisoning.The specific uses are as follows:
1. Can effectively restrain metal ion pairhydrogen peroxideAnd hypochlorite bleaching agent, improving bleaching efficiency and savingBleaching liquorAnd reduce bleaching cost.
2. For Fe3+Ions have strong capture ability and dispersion effect to avoid Fe3+Ions andPhenolic groupThe reaction forms a dark color compound to protect the fiber, improve the whiteness of the pulp, and reduce the yellowing of the pulp.
3. It can effectively prevent the formation of calcium, magnesium and other metal ions during pulp bleachingprecipitateSo as to prevent scaling of system equipment and pipelines, and gradually remove the original scaling of rinsing system.
