Coordination atom refers toCoordination compoundMiddle Direct and Centeratom(orIons）CoordinationOfLigandsOfatom。The lone pair electrons provided by the coordination atom are accepted by the central ion to form a coordination bond.Example The atoms of nonmetallic elements with high electronegativity, such as F, I, Br, are often coordination atoms.The coordination atom must haveLone pair electron。OneMonodentate ligandOnly one coordination atom can be provided, andMultidentate ligandThere may be two or more coordination atoms.
In coordination ions, the central ion and the ligand are bound by coordination bonds.For example, [Zn (NHthree)four]2+In the coordination ion, it is the central ion Zn2+Accepts the electron pair given by the nitrogen atom in the ligand to form a coordination bond.The nitrogen atoms in the ammonia molecule are called coordination atoms or donor atoms.
Therefore, to form a complexTwo conditions: One is that the central ion, as the electron "acceptor", must have an empty orbit. The ions of various sub group elements and VIII group elements in the periodic table all have empty orbits, which are easy to accept lone pair electrons, so it is easiest to generate complexes, while the elements at both ends of the periodic table have the weakest ability to generate complexes;The second is that the ligand as the electron "donor" must have lone pairs of electrons (refers to the pairs of electrons that are not shared, also known as the pairs of electrons that are not shared).[1]
Number of coordination atoms
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The number of coordination atoms in the coordination ion (or coordination molecule) that are directly bound with the central atom by coordination bonds is called coordination number.In essence, coordination number is the number of coordination bonds formed between the central atom and the ligand.If all ligands are unidentate ligands, the coordination number of central atoms is equal to the number of ligands.If there are multidentate ligands in the ligand, the central original coordination number is not equal to the number of ligands.The coordination number mainly depends on three factors: the structure of the electron layer of the central atom, the space effect and the electrostatic effect.[2]
For example, [Cu (NHthree)four]SOfourThe ligand in is NHthreeMolecule, where the N atom is directly and centrally locatedmetal ionBondingSo the coordination atom is N instead of H.Similarly, at Kthree[Co(CN-)five(SCN)] is CN-C atom and SCN in ligand-The S atom in the ligand.In a multidentate ligand, there may be two or more coordination atoms.For example, in [Co (NHtwoCHtwoCHtwoNHtwo)three]SOfourMedium,ethylenediamine The two N atoms in the ligand are coordination atoms, so thecoordination numberIt is still 6.[3]
Polymer chelating agents, also known as chelating resins, are a class of important functional polymers.It is characterized by that the polymer skeleton is connected with a chelating functional group that can coordinate with metal ions, and has selective chelating effect on a variety of metal ionsAdsorption resinIt can concentrate and enrich various metal ions.The resin can be widely used in analysis and detection, pollution control, environmental protection and industrial production.In addition, whenChelating resinAfter chelating with specific metal ions, the polymer complexes formed will have many useful new physical and chemical properties, which are widely used as catalysts, photosensitive materials and antistatic agents.[4]
Oxygen is the most common coordination atom, which has six outer electrons. Normally, two outer electrons form bonds with other atoms, and the other four form two lone pairs of electrons. These two lone pairs of electrons can form coordination bonds independently.Oxygen atoms exist in various types of coordination groups, which are the most common polymer coordination groups.According to the different coordination groups contained, the polymer chelating agents with oxygen as the coordination atom are classified as follows:
The most common alcohol polymer chelating resin isPolyvinyl alcoholIts structure is to connect a hydroxyl group as a coordination group every one carbon atom on the saturated carbon chain. Generally, two adjacent hydroxyl groups coordinate with the same central ion, so that after forming a coordination bond, they will form a six membered ring stable structure with the central ion.[4]
The β - diketone structure refers to the chemical structure in which a saturated carbon atom is separated between two carbon groups, and carbonyl oxygen is used as the coordination atom.The structure of β - diketone is an important multi coordination group, in which there are three carbon atom intervals between the coordination atoms, so it can also form a six membered ring structure when forming the complex, and the tension in the ring is small.The existence of double bonds in the ring makes the formed chelate more stable.[4]
3. Phenolic chelating resin
Compared with the alcohol hydroxyl group, the lone pair electrons of the phenol hydroxyl group on the benzene ring are conjugated with the benzene ring, which is more acidic and easy to ionize under alkaline conditions.There are many polymers containing phenolic hydroxyl, includingpolystyreneClass A and epoxy resin, etc.The complex formed by phenolic hydroxyl group as coordination group is also relatively stable, but due to the rigidity of benzene ring, there are special requirements for the structure of polymer when forming multi coordination chelate, and the chelate structure formed is also relatively complex.[4]
