Coordination Compounds Metal Atom and Neutral Molecules ions

Coordination compounds are a class of compounds with a characteristic chemical structure. The central atom (or ion, collectively referred to as the central atom) and the molecules or ions surrounding it (called ligands/ligands) are completely or partially bound by coordination bonds form. Coordination Compounds

It contains complex molecules or ions formed by combining a central atom or ion with several ligand molecules or ions with coordination bonds and is commonly referred to as a coordination unit. Any compound containing a coordination unit is called a coordination compound. The branch of chemistry that studies complexes is called coordination chemistry.

Complexes are a larger subclass of compounds and are widely used in daily life, industrial production, and life sciences. They have developed particularly rapidly in recent years. It is not only related to inorganic compounds and organometallic compounds, but also has a lot of overlaps with atomic cluster chemistry, coordination catalysis, and molecular biology.

The general definition of high school textbooks

Coordination compounds generally refer to the transition metal atoms or ions (part of the valence electron layer d orbitals and s, p orbitals are empty orbitals) and molecules containing lone pair electrons (such as CO, NH₃, H₂O) or ions ( such as CI^–, the CN^–, NO^2-, etc.) formed by a coordination bond compound binding.

Obviously, compounds containing coordination bonds are not necessarily coordination compounds. Although compounds such as sulphuric acid and ammonium salts have coordination bonds, they are not coordination compounds because there are no transition metal atoms or ions. Of course, compounds containing transition metal ions are not necessarily complex compounds, and compounds such as ferric chloride and zinc sulfate are not complex compounds.

The term of Coordination Bond

When discussing classical coordination compounds, the following terms are often mentioned:

Coordination bond, covalent bond: A chemical bond existing in a coordination compound. Two electrons provided by one atom to form a bond become an electron donor, and the other bond-forming atom becomes an electron acceptor. See acid-base reaction and Lewis acid-base theory. 

Coordination unit: A compound contains a part of a coordination bond, which can be a molecule or an ion. Coordination ion: An ion containing a coordination bond, which can be a cation or an anion. Inner boundary, outer boundary: The inner boundary refers to the coordination unit. The outer boundary is opposite to the inner boundary. Ligand, ligand, ligand: A molecule or ion that provides an electron pair. 

Coordinating atom: An atom that provides an electron pair in a ligand. Central atom, metal atom: Generally refers to an atom that accepts an electron pair. Coordination number: the number of coordination atoms around the central atom. Chelate: A complex containing a chelating ligand.

In addition, a complex containing multiple central atoms is called a multinuclear complex, a ligand connecting two central atoms is called a bridged ligand, a hydroxyl bridged one is called a hydroxyl group, and 5oxygen bridged one is called an oxygen Link.

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In a complex, two electrons are shared between the central atom and the ligand, and the chemical bond formed is called a coordination bond. The two electrons are not provided by each of the two atoms, but from the ligand atom itself, such as In [Cu(NH₃)₄]SO₄, Cu²⁺ and NH₃ share two electrons to form a coordination bond, both of which are provided by N atoms. The condition for forming a coordination bond is that the central atom must have an empty orbital, and the transition metal atom best meets this condition.

Basic Component

Formerly known as “complex”. The coordination compound is composed of a central atom, a ligand and the outside world. For example, the molecular formula of tetraamminecopper (II) sulfate is

[Cu (NH₃)₄]SO₄. The central atom may be a charged ion, such as Cu²⁺ in [Cu (NH₃)₄] SO₄. The ligand gives a lone pair of electrons or multiple unlocalized electrons, and the central atom accepts the lone pair of electrons or multiple unlocalized electrons to form a coordination bond that combines them. 

For example, K₄[Fe(CN)₆], [Cu(NH₃)₄]SO₄ ^2- , [Pt (NH₃)₂Cl₂], and [Ni (CO)₄] are all complexes. Among them: CN:-,: NH₃, and: CO: are ligands, all have lone pair electrons (:), Fe ²⁺, Cu ²⁺, Pt ²⁺ and Ni are central atoms, Can accept lone pair electrons. 

The ligand and the central atom constitute the coordination ontology and are listed in square brackets. The complex undergoes partial dissociation in solution but still tends to retain its body. All metals in the periodic table can be used as central atoms, and transition metals (see transition elements ) are more likely to form complexes. 

Non-metals can also serve as central atoms. There are two types of ligands: monodentate and multidentate. Only a monodentate ligand atoms, e.g. the CN ^–, CO.’S, NH2_{. 3} and CI ^– are monodentate ligands, coordinating atoms are C, N, and Cl, which is directly bonded to the central atom. 

Polydentate has two or more coordination atoms ethylenediamine H ₂ NCH ₂ CH ₂ NH ₂ is a bidentate ligand, the coordination atoms are two N atoms; ethylenediamine tetraacetate (referred to as EDTA4-) (-OOCCH ₂ ) 2N-CH ₂ -CH ₂ -N (CH ₂ COO-) 2 is a hexadentate ligand, and the coordination atoms are two N and O on four carboxyl groups. 

The ligand is a negative ion or a neutral molecule, and occasionally a positive ion (such as NH ₂ NH ⁺). The charged coordination body is called coordination ion, the positively charged coordination ion is called coordination cation, and the negatively charged coordination ion is called coordination anion. 

The charge of the ligand ion is the sum of the charge of the metal ion and the ligand, for example, Fe ²⁺ and 6CN ^– coordinate to produce [Fe (CN) ₆ ] ^4- coordinate anion, Cu ²⁺ and 4NH ₃ produce [Cu (NH ₃ ) ₄ ] ²⁺ complex cations, each of which forms a complex with an oppositely charged cation or anion. 

Neutral coordination bodies are complexes, such as Pt ²⁺ and 2NH ₃ and 2Cl ^-to produce [Pt (NH ₃ ) ₂ Cl ₂ ]; Ni and 4CO produce [Ni (CO) ₄ ]. The complex can be single-core or multi-core, with a single core having only one central atom; a multi-core has two or more central atoms. The above complexes are all mononuclear complexes; multinuclear complexes such as [(CO) ₃Fe(CO)₃Fe (CO)₃].

Coordination Compounds Theory

The chemical bond theory of coordination compounds mainly studies the nature of the binding force between the central atom and the ligand to explain the physical and chemical properties of the complex, such as magnetism, stability, reactivity, coordination number and geometric configuration. The theory of complexes starts with electrostatic theory. 

Then Sidwick and Pauling proposed the covalent covalent model, that is, the application of the valence bond theory in complexes, has ruled this field for more than two decades, which can better explain the coordination number, geometric configuration, and magnetic properties. Wait for some properties, but cannot help the color and spectrum of the complex.

Valence theory suggests that the lone pair of electrons provided by the ligand has entered the empty atomic orbital of the central ion, so that the ligand and the central ion share the two electrons. The formation of coordination bonds has gone through three processes: (excitation), hybridization, and bonding.

Hybridization, also known as orbital hybridization, is the process of linearly combining atomic orbitals of similar energy into equal numbers and degenerate hybridized orbitals. 

From this, the concept of outer or inner orbital complexes can also be derived. By judging the electronic configuration and hybrid type of the complex, the magnetic properties, redox reaction properties, and geometric configuration of the complex can be derived. For many classical complexes, the results of the valence bond theory are relatively close to the facts.

In addition to valence bond theory, the crystal field theory and coordination field theory developed later are also more important complex theory.

Crystal field theory treats ligands as point charges and treats coordination bonds as ionic bonds, which can be seen as an extension of electrostatic theory. 

Moreover, it takes the effect of ligands on d orbits of different spatial orientations as entry points in different geometric configurations, and concludes that d orbitals of different orientations will split energy levels and establish the concepts of splitting energy and crystal field stabilization energy.

To infer the electronic configuration and stability of the complex. The crystal field theory can well explain the complex color, thermodynamic properties, and complex distortion, but it cannot reasonably explain the spectrochemical sequence of the ligand, nor can it be applied well to special high / low-cost complexes and sandwich complexes.

Compounds, carbonyl complexes and olefin complexes.

The coordination field theory combines molecular orbital theory with crystal field theory. It is more rigorous in theory, but quantitative calculation is very difficult. In the calculation process, approximate processing has to be introduced, so only approximate results can be obtained.

Reaction

Ligand exchange reaction

Ligand coordination compound may be other ligands substituted called ligand exchange reactions, the general reaction mechanism is a nucleophilic substitution reaction . Taking octahedral complexes as an example, the general formula for such reactions is:

In the formula, X is a substituted ligand, which is usually called a leaving group; Y is a substituted group, which is usually called an entering group. The rates of such ligand exchange reactions vary widely, some reactions can be completed within 10 seconds, while others can take months. For the difference in activity, there is an artificial rule that the complex with a half-life greater than one minute at a concentration of about 0.1 M and a temperature of 25 ° C belongs to the so-called “inert” complex, otherwise it is called an active complex.

Both the valence bond theory and the coordination field theory explain the difference in the rate of such reactions, and generally the following rules exist:

1. The increase of the central metal ion charge will reduce the reaction rate;
2. The central ions are d ⁰ , d ¹ , d ² , d ⁹ , d ¹⁰ configuration, and the complexes with high spin d ⁵ , d ⁶ , d ⁷ configuration and high spin d ⁴ configuration are all active for ligand exchange reactions. ;
3. When the central ion is in the d ³ , d ⁸ configuration, or the low-spin d ⁴ , d ⁵ , d ⁶ configuration, it is inert to the ligand exchange reaction.

In addition, the reaction rate also depends on the type and arrangement of the solvent and ligand.

The coordination reaction can be seen as the acid-base reaction in Lewis acid-base theory: metal ions provide empty orbitals for the acid, and ligands provide electron pairs for the base. The reaction between the transition metal and the ligand is often accompanied by a color change. For example, adding HCl to [Cu (NH ₃ ) ₄ ] sequentially produces [Cu (H ₂ O) ₄ ] (light blue), [CuCl (H ₂ O) ₃ ], [CuCl ₂ (H ₂ O) ₂ ], [CuCl ₃ (H ₂ O)], [CuCl ₄ ], [Cu (NH ₃ ) ₄ ] (dark blue); for another example, adding excess ammonia to [Cu (H ₂ O) ₄ ] Light blue to dark blue:

Redox reactions

Redox reactions of coordination compounds include two types, one is a redox reaction between a central atom and a ligand, and the other is a redox reaction between two complexes. The latter can be divided into two categories:

• Electron transfer mechanism, outer layer reaction mechanism: The first coordination layer of both reactants remains unchanged. The reaction rate is mainly related to the structure of the reactants and the electron spin state. The complexes containing π-conjugated system ligands such as bipyridine and CN tend to have faster reaction rates. In addition, bridged ligands can also transfer electrons, but are generally not as effective as direct electron transfer reactions.
• Bridge mechanism and inner layer reaction mechanism: two metal atoms are connected to a bridged ligand at the same time to form a transition state.

Whether the reaction proceeds through the outer layer mechanism or the inner layer mechanism depends on the structure of the complex. 

Complexes that are inert to ligand exchange reactions, have no bridging ligands, or have very low electron transfer activation energies, their mechanism is dominated by the outer layer mechanism. 

The ligands that are reactive to the ligand exchange mainly have a bridge mechanism. The energy barrier to be overcome by the bridge mechanism is much lower than that of the outer reaction mechanism, because the bridged ligand transfers electrons and reduces the electron penetration of the outer layer and water Energy.

There are two more special types of redox reactions:

• Two-electron transfer reaction: The change in oxidation state during the reaction is ± 2, and the mechanism is more likely to be a bridge mechanism.
• Non-complementary reactions: The valence states of oxidant and reducing agent are not equal. The general reaction mechanism is carried out in several steps.

Nomenclature

① When naming ligand ions, the name of the ligand is placed first, and the name of the central atom is placed after. 

② The name of the ligand and the central atom are connected by the word “he”. 

③ If the central atom is an ion, add a Roman numeral with parentheses after the name of the metal ion to indicate the valence of the ion. 

④ The coordination number is in Chinese before the ligand name. 

⑤ If there are multiple kinds of ligands in the complex, they are arranged in the following order: anionic ligands come first, neutral molecular ligands come last; inorganic ligands come first, and organic ligands come last. 

The names of different ligands are separated by a dot. According to the above rules, [Cu(NH₃)₄]SO₄ is called tetra ammonium copper (II) sulfate, [Pt(NH3)2Cl2] is called dichloro · diamine platinum (Ⅱ), and K[Pt(C₂H₄)Cl₃] is called trichloro· ( ethylene ) platinum (II) acid potassium. In fact, the complex is also commonly used by common names, such as K₄[Fe(CN)₆] is called yellow blood salt, K₃[Fe(CN)₆] is called red blood salt, and Fe₄[Fe(CN)₆₃ called Prussian blue.

When naming coordination compounds, the Chinese IUPAC nomenclature is generally followed. The naming rules are:

• Ionic complexes are processed in the form of salts. When naming the coordination unit, the ligand is in front, and the different ligands are separated by dots, and the word “H” is added between the last ligand and the central atom name. The names of the ligands are listed in the table on the right. The order of the ligands is mainly followed by “inorganic first, then organic” and “anion first, then neutral molecules”. The number of ligands should be added before the ligand, and the name of the ligand should be enclosed in parentheses if necessary to avoid ambiguity. The central atom is followed by the oxidation number, which is indicated by Roman numerals in parentheses. Positive ion complexes are called chlorides, nitrates, sulfates, etc., while anionic complexes are called potassium/sodium or an acid.
• Μ should be added before bridging the ligand, η means that the ligand has n atoms bonded to the central atom (n is the Hapto number of the ligand ). For complexes that may produce bonded isomers, the coordinating atom must be indicated after the ligand.

Complex	name
[NiCl 4 ]-	Tetrachloronickelate (II) ion
[Cu (NH 3 ) Cl 5 ] 3-	Pentachloro · monoammonium copper (II) ion
[Cd (en) 2 (CN) 2 ]	Dicyanobis (ethylenediamine) cadmium (II)
[Co (NH 3 ) 5 Cl] SO 4	Monochlorosulfate · pentaammonium cobalt (III)
Fe 2 Cl 6 ( ferric chloride dimer)	Tetrachlorodi- μ -ferrous (III) chloride
(NH 4 ) 3 [Cr (NCS) 6 ]	Hexa (thiocyanate) -N-ammonium (III) chromate

Naming rules

I. Naming of complexes (for high school students)

(1) The naming of the complex lies in the naming of the inner boundary of the complex (ie, the complex ion).

The naming method of the complexions in the inner boundary of the complex is generally in the following order: from right to left is the number of ligands-the name of the ligand [separated by a dot ( · ) between different ligand names ] ——He——the name of the central ion—the valence of the central ion.

The valence of the central ion is calculated from the charge of the external ion/ligand based on the charge of the complex being zero. It is indicated by parentheses and Roman numerals after the central ion.

Examples are omitted.

(2) The complex can be regarded as a salt. If the inner boundary is a cation, the outside must be an anion, if the inner boundary is an anion, the outside must be a cation. It can be named according to the salt naming method, and it can be named as an acid or a certain from right to left.

If there are multiple kinds of ligands in the complex, their arrangement order is: anionic ligands come first, neutral molecular ligands come last; inorganic ligands come first, and organic ligands come last.

Examples are omitted:

The complexes dissolve in water and are easily ionized as internal ligand ions and external ions, while internal ligands and central atoms are usually not ionizable.

Detailed instructions (for academic use)

(1) In the complex

The word “chemical” or “acid” is added between the anion and the cation first, followed by the anion as an acid radical.

(2) In the coordination unit

① The ligand is followed by the central ion (or atom), and the word “combination” is added between the ligand and the central ion (or atom).

② The number of the ligands is represented by one, two, or three in front of the ligand. “One” can be omitted. If it is easy to misunderstand, the ligand needs to be bracketed.

Such as bis (methylamine), bis (triphenylphosphine), etc.

③ Several different ligands are separated by “˙”.

④ Add “()” after the central ion, and use Roman numerals to indicate the valence state of the central ion (or atom).

(3) Sequence of ligands

Each of the following provisions is based on the previous one

① first inorganic ligand, then the organic ligand

Such as PtCl₂(Ph₃P)₂ dichlorophosphonium bis (triphenylphosphine) platinum (Ⅱ)

② First anionic ligand, then cationic ligand, and finally molecular ligand

Such as K [PtCl₃(NH₃)] potassium trichlorophosphonium monoammonium platinum (Ⅱ) acid

③ In the same kind of ligands, according to the order of the element symbols of the coordination atoms in the English alphabet,

Such as [Co(NH₃)₅H₂O]Cl₃Co (Ⅲ)

④ The coordination atoms are the same, and the number of atoms in the ligand is the first.

Such as [Co (Py) (NH ₃ ) (NO ₂ ) (NH ₂ OH)] Cl mononitro-ammonium-ammonium-hydroxyl ammonium-pyridine cobalt (Ⅱ) chloride

⑤ If the number of atoms in the ligand is the same, the order of the element symbols of other atoms in the ligand directly connected to the coordination atom in the English alphabet. If NH ^2- and NO ^2- , then NH ^2- comes first.

Basic classification

Classified by ligands, there are:

① Hydrated complex. It is a complex of metal ions and water molecules, almost all of the metal ions in an aqueous solution of the hydrated complex can form, [such as a Cu (H ₂ O) _{. 4} ] ²⁺, [Cr (H ₂ O)_.6 ]³⁺ .

② halogenated complex. The complex formed by metal ions and halogen (fluorine, chlorine, bromine, iodine) ions, most metals can form halogenated complexes, such as K ₂ [PtCl ₄ ], Na ₃ [AlF ₆ ].

③ Ammonia complex. Complexes of metal ions and ammonia molecules, such as [Cu (NH ₃)₄]SO₄.

④ cyanide complex. Complexes of metal ions and cyanide ions, such as K ₄ [Fe (CN) ₆ ]

⑤ Metal carbonyl compound. A complex formed by a metal and a carbonyl group (CO). Such as [Ni (CO)₄].

Classified by central atom, there are:

①Single core complex. There is only one central atom, such as K ₂ [CoCl₄].

② Multinuclear complex. The number of central atoms is greater than 1, such as [(H₃N)₄Co(OH) (NH₂)Co(H₂NCH₂CH₂NH₂)₂]Cl₄.

Classification by key type can be:

① Classical complexes. A sigma coordination bond is formed between the metal and the organic group, such as [Al₂(CH₃)₆].

② Cluster-like complexes. It contains at least two metals as the central atom, which also contains metal-metal bonds, such as [W₆(Cl₁₂)Cl₆].

③ Complexes containing unsaturated ligands. A π- σ bond or a π-π * feedback bond is formed between the metal and the ligand, such as K [PtCl ₂ (C ₂ -H ₄ )].

④ Sandwich complex. The central atom is a metal and a ligand is an organic group. The metal atom is sandwiched between two parallel carbocyclic systems, such as ferrocene [Fe(C₅H₅)₂].

⑤ Acupoint complex. The ligands belong to macrocyclic polydentate organic compounds, such as N (CH₂OCH₂CH₂)₃ N having a bicyclic structure, and they form a cavity complex with alkali metals and alkaline earth metals.

Classified by subject type:

① Inorganic complex. Both the central atom and the ligand are inorganic.

② Organometallic complexes. A complex formed between a metal and an organic ligand.

③ Biological inorganic complex. Complexes formed by biological ligands and metals, such as metalloenzymes, chlorophyll, and vitamin B12.

Coordination compounds can be divided into traditional coordination compounds and organometallic compounds.

• Traditional coordination compounds are formed by more than one coordination ion (also called an ion complex), and the electrons in the coordination bond are provided “almost” by the ligand. Typical ligands include H₂O, NH2 ₃, CI, the CN and EN.
• Examples: [Co( EDTA)], [Co(NH₃)₆]Cl₃, [Fe(C₂O₄)₃] K₃ and [Cr(H₂O)₆]Cl₃ .
• An organometallic compound refers to a compound containing a metal-carbon chemical bond, and a ligand is an organic group (such as an olefin, an alkyne, an alkyl group, or an aromatic ring ) or a chemical having similar properties, such as phosphine, a hydroxide ion, or carbon monoxide.
• Examples: (C₅H₅) Fe (CO)₂CH₃ , Fe(CO)₅ , Cp₂TiMe₂.

Chemical branches that cover the coordination chemistry, such as:

• Bioinorganic chemistry —the complex ligands exist in nature, often amino acid side chains and coenzymes, such as porphyrins. Examples include heme.
• Cluster chemistry -using metal atoms as ligands, such as Ru₃(CO)₁₂.

Main properties

Stability

Generally, the stability of a coordination compound mainly refers to the thermal stability and whether the complex can easily ionize its components (central atom and ligand) in solution. The coordination body can weakly dissociate a small number of central atoms (ions) and ligands in the solution. For example, [Cu (NH₃)₄]²⁺ can dissociate into a small amount of Cu²⁺ and NH₃.

The dissociation equilibrium of the coordination body in solution is very similar to that of the weak electrolyte. It also has its dissociation equilibrium constant, which is called the stability constant K of the complex.

The larger K is, the more stable the complex is, that is, the degree of dissociation in the aqueous solution is small.

The stability of the complex in solution is related to the radius, charge of the central atom and its position in the periodic table, that is, the ion potential of the complex: φ = Z ​​/ r φ is the ion potential Z is the number of charges r is the radius . 

The transition metal has a high nuclear charge, a small radius, and empty d orbitals and free d electrons. They easily accept the electron pairs of the ligands, and it is easy to feedback the d electrons to the ligands. Therefore, they can all form stable complexes. Alkali metals and alkaline earth metals are just the opposite of transition metals. They have low polarizability, have an inert gas structure, have poor ability to form complexes, and their complexes have poor stability.

The stability of the complex conforms to the theory of soft-hard affinity, that is, soft-soft, hard-hard.

Basic structure

There are many, the most common is octahedron and tetrahedron. 

The former is like [Fe(CN)₆]^4- and the latter is like [Ni(CO)₄]: There are also flat squares like [Cu(NH₃)₄]²⁺, [Cu(H₂O)₄]²⁺.

Structure

The configuration of the coordination compound is determined by the coordination number, that is, the number of coordination atoms around the central atom of the compound. The coordination number is related to the radius, charge number and electronic configuration of the metal ion and the ligand. Generally, the coordination number is between 2-9, and the coordination number of the lanthanide and actinide complexes often exceeds 10.

Regarding the coordination atoms around the central atom as points and connecting the points with lines, a coordination polyhedron is obtained. The relationship between coordination number and complex configuration is listed in the following table:

Coordination number	structure	Examples
2	Straight	HgCl 2 , Ag (NH 3 ) 2 , [Au (CN) 2 ]
3	Flat triangle	HgI 3 , Pt (PPh 3 ) 3 , Fe [N (Si (CH 3 ) 3 ) 2 ] 3
4	tetrahedron	Ni (CO) 4 , MnO 4 , SnCl 4 , SiO₂
	Flat square	Pt (NH 3 ) 2 Cl 2 , PtCl 4 , Ni (CN) 4
5	Triangular double cone	Fe (CO) 5 , CdCl 5
	Square cone	[InCl 5 ], SbF 5
6	Octahedron	[Ti (H 2 O) 6 ], [Co ( en ) 3 ], [Cu ( NH 3 ) 6 ]
7	Pentagonal double cone	[ZrF 7 ], [UO 2 F 5 ]

In the pentacoordination, the interconversion of two configurations of triangular double cone and tetragonal cone is often involved. Therefore, a large part of the structure of the pentacoordinate compound is an intermediate structure between the two structures. 

Compound hexacoordinated other very common octahedral outside, there may be a triangular prism structure, such as mononuclear complexes [Re(S₂C₂Ph₂)₃] i.e. belong to this category. In the seven coordination, the complex may also be a single-cap octahedron or a single-cap triangular prism structure.

Among the higher coordination compounds, octacoordinates can be tetragonal prisms, dodecahedrons, cubes, double-hat triangular prisms or hexagonal double-pyramid structures; nine-coordination can be triple-hat triangular prisms or single Cap tetragonal anti-prism structure; ten-coordinates can be double-cap tetragonal anti-prisms or double-cap dodecahedral structures; eleven coordination compounds are rare, and maybe single-cap pentagonal prisms or single-cap pentagonal prisms. Twelve coordinations such as [Ce(NO₃)₆] are ideal icosahedrons, Fourteen coordinations are double cap hexagonal prisms. No matter how high the coordination number is very rare, such as PbHe, the coordination number of lead in this ion is at least 15.

The above is just the ideal situation of the complex configuration. In practice, the structure of the complex is often distorted, which may be due to steric effects, electronic effects (see Ginger-Taylor effect), or the type of ligand.

Heterogeneous phenomenon

Isomerism is one of the important properties of complexes. It not only affects the physical and chemical properties of the complex, but also has a close relationship with its stability, reactivity and biological activity. Important complex isomerization phenomena include stereoisomerism and structural isomerism.

Stereoisomerism

Stereoisomerism is a phenomenon of isomerism in which the chemical formula and the arrangement of the atoms are the same, and only the atoms are arranged differently in space. Stereoisomerism is mainly divided into geometric isomerism and optical isomerism.

Geometric heterogeneity

Geometric isomerism is the heterogeneity caused by the different geometrical arrangement of different ligands that make up the same complex. It mainly occurs in planar squares with a coordination number of 4 and octahedral structures with a coordination number of 6. The formula-trans isomer and the face-warp isomer exist.

From the viewpoint of spatial relationships, cis- ( CIS -) refer to the same ligand in the ortho, trans- ( Trans -) refer to the same ligand in the para position. Octahedral [MA _{. 3} B_{. 3} ] of the two isomers, a surface of formula ( FAC -) or cis – cis pointing tip cam face A 3 and B each accounted for 3 octahedron by formula ( Mer – ) or cis-trans refers 3 a and 3 B octahedral circumscribed spheres of the meridian in parallel on. See below:

cis- [CoCl ₂ (NH ₃ ) ₄ ]

trans- [CoCl ₂ (NH ₃ ) ₄ ]

fac- [CoCl ₃ (NH ₃ ) ₃ ]

mer- [CoCl ₃ (NH ₃ ) ₃ ]

The planar square complex [M(AB)₂] of the asymmetric bidentate ligand may also have geometric isomerism, and its structure is similar to the above cisplatin, as shown in the figure below:

Multinuclear complexes also have geometric isomerism. For example, the cis and trans isomers of the binuclear complex [Pt₂(PPr₃)₂(SEt)₂Cl₂] of Pt (II) have been prepared, and their benzene solutions are stable at room temperature. However, the transform can be completely converted to the cis form by adding a trace amount of tripropylphosphine as a catalyst to a hot or cold benzene solution.

Optical Isomerism

Optical isomerism is another form of stereoisomerism. Two optical isomers can deflect plane-polarized light by the same amount but in different directions, so it is also called optical isomerism or enantiomer. Most complexes gradually lose optical activity in solution, a process called racemization. Depending on the circumstances, the racemization mechanism may be intermolecular or intermolecular.

The simplest optical isomer of the complex is a tetrahedral type. The central atom is connected to four different groups, and the molecule cannot coincide with the mirror image. For example [Be (C₆H₅COCHCOCH₃)₂]. For octahedral complexes, optical isomerization mainly occurs in the following cases:

1. [M (AA)₃] type, such as tri- (oxalato) chromium (III), [Co{(OH)₂Co(NH₃)₄}₃]Cl₆ (the first one with optical rotation And carbon-free compound—Hexol).
2. [M (AA)₂X₂] type, such as [Rh(en)₂Cl₂].
3. [M (AB)₃] type, such as [Co(gly)₃].
4. [M (AA) B₂X₂] type, such as [Co(en)(NH₃)₂Cl₂].
5. Multidentate ligands are involved, such as [Co( edta )].

Λ- [Fe (ox)₃]

Δ- [Fe (ox)₃]

Λ- cis- [CoCl₂(en)₂]

Δ- cis- [CoCl₂(en)₂]

Structural heterogeneity

Structural isomers are isomers with the same chemical formula but different atomic order, which can be divided into the following categories:

Two bonded isomers of [Co (NH₃)₅(NO₂)].

• Bonding isomerism: The ligand is coordinated to the central atom through different coordination atoms. Ligands are called amphiphilic ligands. Such ligands contain more than two atoms containing lone pairs of electrons and can each coordinate with the central atom. Common amphiphilic ligands are: NO₂, SCN and CN.
• Configurational isomerism: The complex can take more than one configuration. For example, [NiCl₂(Ph₂PCH₂Ph)₂] can have tetrahedral and planar quadrilateral configurations, respectively. Common configurational isomers are the isomers between the triangular double- pyramid and tetragonal cone configurations of the pentacoordinate compound and the isomers between the dodecahedral and tetragonal anti-prism configurations of the octacoordinate compound.
• Ligand isomer: mutually isomers Similar complexes formed by ligands, such as 1,3-diaminopropane and 1,2-amino-propane cobalt complexes are formed [of Co(H₂N-CH₂-CH₂-CH₂-NH₂)Cl₂], [Co(H₂N-CH₂-CH(-NH₂)-CH₃)Cl₂].
• Ionic isomerism: The complexes have the same molecular formula but different coordination anions, so the ions generated in the aqueous solution are different, such as [Co(NH₃)₅SO₄]Br and [Co(NH₃)₅Br]SO₄.
• Solvent isomerism : the position of water in the complex is different, there are differences between the inner boundary and the outside, such as [Co(H₂O)₆]Cl₃ and [Cr(H₂O)₅Cl] Cl·H₂O.
• Coordination isomerism: Both cations and anions are coordination ions, and the ligands can exchange components with each other. 

Examples are: [Co(NH₃)₆] [Cr(CN)₆] and [Cr(NH₃)₆] [Co(CN)₆], [Cr(NH₃)₆] [Cr(SCN)₆] And [Cr(SCN)₂(NH₃)₄] [Cr(SCN)₄(NH₃)₂], and [Pt(NH₃)₄][PtCl₆] and Pt(NH₃)₄Cl₂][PtCl₄].

• Polymerization isomerism: It is a kind of coordination isomerism, which is used to indicate the multiple relationships of the relative molecular mass of the complex. It is not the same as “polymerization” in the polymerization reaction.

For example [Co(NH₃)₆][Co(NO₂)₆] can be regarded as a dimer of [Co(NH₃)₃(NO₂)₃].