Coordination compound

Chemical terminology

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synonym complex compound (Complex) generally refers to coordination compound

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Coordination compounds are a class with characteristics Chemical structure Compounds of, by Central atom (or ions, collectively referred to as central atoms) and molecules or ions surrounding it (referred to as Ligands /Ligands) are formed completely or partially by the combination of coordination bonds.

It consists of atom Or ions and several ligand Molecules or ions Coordination bond Complex molecules or ions formed by combination are usually called coordination units. All compounds containing coordination units are called coordination compounds. The branch of chemistry that studies complexes is called Coordination chemistry 。

Complexes are a large subcategory of compounds, which are widely used in daily life, industrial production and life sciences In recent years, the development is particularly rapid. It is not only related to inorganic compound 、 Organometallic compound Related to and related to the current chemical frontier Cluster chemistry , coordination catalysis and molecular biology have great overlap.

Chinese name: Coordination compound
Foreign name: Coordination complex/metal complex

Alias: Malocclusion 、 complex compound
Abbreviation: Complex

catalog

one General definition of high school textbooks
two term
three history
four Basic composition

General definition of high school textbooks

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Coordination compounds are generally composed of atoms or ions of transition metals (partial d orbitals, s orbitals and p orbitals of valence electron layer are empty orbitals) and molecules containing lone pair electrons (such as CO, NH three 、H two O) Or ions (such as Cl - 、CN - 、NO 2- Etc.) Compounds formed by coordination bonds.

Obviously, compounds containing coordination bonds are not necessarily coordination compounds. For example, although there are coordination bonds in sulfuric acid, ammonium salt and other compounds, they are not coordination compounds because there are no atoms or ions of transition metals. Of course, compounds containing transition metal ions are not necessarily coordination compounds, such as ferric chloride, zinc sulfate and other compounds are not coordination compounds.

term

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

Coordination bond Coordination covalent bond: the chemical bond existing in the coordination compound, which is provided by one atom with two bonding electrons to become the electron donor, and the other bonding atom becomes the electron acceptor. See Acid-base reaction and Lewis acid-base theory 。 Coordination unit: The part of a compound containing coordination bonds, which can be a molecule or an ion. Coordination ion: ion containing coordination bond, which can be cation or anion. Internal boundary and external boundary: the internal boundary refers to the coordination unit, and the external boundary is opposite to the internal boundary. ligand 、 Ligands Coordination group: molecule or ion providing electron pair. Coordination atom: The atom in a ligand that provides an electron pair. Central atom, metal atom: generally refers to the atom receiving the electron pair. coordination number : Number of coordination atoms around the central atom. Chelate: containing chelate Complexes of ligands.

In addition, complexes containing multiple central atoms are called polynuclear complexes, and ligands connecting two central atoms are called bridging ligand , with hydroxyl Bridged olation , oxygen bridged Oxygenation 。

Schematic Diagram of Coordination Compounds

In the complex, the central atom and the ligand share two electrons, and the chemical bond formed is called the coordination bond. These two electrons are not provided by two atoms, but from the ligand atom itself, such as [Cu (NH three ) four ]SO four Medium, Cu 2+ And NH three They share two electrons to form a coordination bond, both of which are provided by the N atom. The condition for the formation of coordination bond is that the central atom must have an empty orbit, and the transition metal atom best meets this condition.

history

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People have been exposed to coordination compounds for a long time. At that time, they were mostly used for daily life. The raw materials were basically obtained from nature, such as fungicides Bile alum And used as dyes prussian blue 。 The earliest research on complexes began in 1798. French chemist Tassel First use of divalent cobalt salt ammonium chloride And ammonia Preparation of CoCl 3· 6NH three , and found that chromium 、 nickel , copper platinum And Cl, H two O、CN、 CO and C2H4 They can also form similar compounds. At that time, it was impossible to explain the bonding and properties of these compounds, and most of the experiments carried out were limited to the observation of color differences of complexes, the number of moles of aqueous solution that could be precipitated by silver ions, and conductance Determination of. As for the bonding of these complexes, the popular view at that time borrowed the idea of organic chemistry, thinking that such molecules were chain like, and only the terminal halogen ions could be dissociated and precipitated by silver ions. However, this statement is far fetched, and there are many facts that cannot be explained.

Swiss chemist in 1893 Werner This paper summarized the previous theories, first proposed a series of basic concepts such as modern coordination bond, coordination number and coordination compound structure, and successfully explained the conductivity, isomerism and magnetism of many complexes. Since then, coordination chemistry has essentially developed. Werner is also known as the "father of coordination chemistry", and thus obtained Nobel Prize in Chemistry 。

In 1923, the British chemist Sidgwick proposed the "effective atomic number" rule (EAN), suggesting the relationship between the number of electrons in the central atom and its coordination number. Many complexes, especially carbonyl complexes, conform to this rule, but there are also many examples of non-compliance. Although this rule only partially reflects the essence of the formation of complexes, its idea has also promoted the development of coordination chemistry.

Coordination compound

Modern coordination chemistry is no longer confined to the donor acceptor relationship of electron pairs, but largely relies on Molecular orbital theory And began to study new types of complexes, such as sandwich complexes and clusters. One typical example is Chua's salt K [Pt (C two H four )Cl three ]。 Although the compound had been prepared as early as 1827, it was not until 1950 that the properties of the feedback π bond were clearly studied.

Basic composition

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Coordination compound

Formerly known as "complex". The coordination compound is composed of Central atom 、 Ligands And the outside world, such as Copper tetraammonium sulfate (II) The molecular formula is [Cu (NH three ) four ]SO four 。 The central atom can be a charged ion, such as [Cu (NH three ) four ]SO four Cu in 2+ 。 ligand give Lone pair electron Or multiple delocalized electrons, Central atom Accepts a lone pair of electrons or multiple nonlocal electrons to form a coordination bond that combines the two. For example, K four [Fe(CN) six ]、[Cu(NH three ) four ]SO four 2- 、[Pt(NH three ) two Cl two ]And [Ni (CO) four ]They are all complexes. Including: CN: -,: NH three , and: CO: are ligands, both have lone pair electrons (:), Fe 2+ 、Cu 2+ 、Pt 2+ And Ni are Central atom , can accept lone pair electrons. ligand And a central atom to form a coordination body, which is listed in square brackets. The complex is partially dissociated in solution, but still tends to maintain its bulk. All metals in the periodic table can be used as central atoms, where transition metal (See Transitional element ）It is easy to form complexes. Nonmetal It can also be used as a central atom. There are two kinds of ligands: monodentate ligand and multidentate ligand. Only one single tooth Coordination atom , e.g. CN - 、CO、NH three And Cl - They are monodentate ligands. The coordination atoms are C, N and Cl, which are directly bonded to the central atom. Multiple teeth have two or more Coordination atom : ethylenediamine H two NCH two CH two NH two It is a bidentate ligand, and the coordination atom is two N atoms; EDTA Root (EDTA4 for short) (- OOCCH two )2N-CH two -CH two -N(CH two COO -) 2 is a hexadentate ligand, and the coordination atom is two N and O on four carboxyl groups. Ligands are negative ions or neutral molecules, occasionally Positive ion (e.g. NH two NH + )。 The charged coordination body is called Coordination ion , with positive charge The anion with negative charge is called anion. The charge of the coordination ion is the sum of the charge of the metal ion and the ligand, such as Fe 2+ And 6CN - Coordination generation [Fe (CN) six ] 4- Anion, Cu 2+ And 4NH three Generate [Cu (NH three ) four ] 2+ Coordination cations, each of which forms complexes with opposite charged cations or anions. The neutral coordination body is a complex, such as Pt 2+ And 2NH three And 2Cl - Generate [Pt (NH three ) two Cl two ]; Ni and 4CO produce [Ni (CO) four ]。 Complexes can be mononuclear or polynuclear, and a single nucleus has only one central atom; A polynucleus has two or more central atoms. The above complexes are mononuclear complexes; Polynuclear complexes such as [(CO) three Fe(CO) three Fe(CO) three ]。

Naming method

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① When naming the coordination ion, the name of the ligand is placed first, and the name of the central atom is placed after. ② Ligands And the name of the central atom. ③ If the central atom is an ion, add Roman numerals with parentheses after the name of the metal ion to indicate the Valence state 。④ coordination number Use Chinese numerals before the ligand name. ⑤ If there are multiple 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. The names of different ligands should also be separated by middle dots. According to the above rules, [Cu (NH three ） four 〕SO four It is called tetraammonium copper sulfate (Ⅱ), [Pt (NH three ） two Cl two It is called dichloro diammonium platinum (II), K [Pt (C two H four ）Cl three ] said trichloride·（ ethylene ）Potassium platinum (Ⅱ) complexes. In fact, complexes are also commonly known as K four 〔Fe（CN） six Call Xanthate ，K three 〔Fe（CN） six Red blood salt, Fe four 〔Fe（CN） six 〕 three call prussian blue 。

When naming coordination compounds, generally follow Chinese IUPAC nomenclature , naming rules are:^[1-3]

Ionic complexes are treated as salts. When naming the coordination unit, the ligand comes first, and different ligands are separated by dots, and the word "combination" should be added between the last ligand and the name of the central atom. The names of ligands are listed in the right table, and their order mainly follows two items: "inorganic before organic" and "anion before neutral molecule". The number of ligands shall be added before ligands, and parentheses shall be added if necessary to enclose the name of ligands to avoid ambiguity. The central atom needs to be marked afterwards Oxidation number , represented by Roman numerals with parentheses. Positive ion complex scale chloride 、 Nitrate 、 sulfate The anionic complex is called potassium/sodium or acid.
bridging ligand Fill before μ ； η It means that the ligand has n atoms bonded to the central atom (n is the Haptoe number ）。 For possible Bonded isomerism The coordination atom should be indicated after the ligand.

Complex	name
[NiCl four ]-	Tetrachloro nickel acid (II) ion
[Cu(NH three )Cl five ]3-	Pentachloro monoammonium cupric acid (II) ion
[Cd(en) two (CN) two ]	Dicyano bis (ethylenediamine) cadmium (II)
[Co(NH three ) five Cl]SO four	Monochloro pentaammonium cobalt sulfate (III)
Fe two Cl six ( Ferric chloride Dimer)	Tetrachloride- μ -Ferrous chloride (III)
(NH four ) three [Cr(NCS) six ]	Ammonium hexa (thiocyanate) - N-dichromate (III)

Naming rules

Schematic diagram of energy level splitting in crystal field theory of complexes

1、 Naming of complexes (for senior high school students)

(1) The key to the naming of complexes lies in the naming of the inner boundary of complexes (i.e. coordination ions)

The naming method of the coordination ions 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 [the middle dot is used between the names of different ligands（ · ）Separation] -- combination -- name of the central ion -- valence of the central ion.

The valence of the central ion is calculated from the external ion charge/ligand charge according to the fact that the complex charge is zero. The central ion is followed by parentheses and Roman numerals.

The examples are omitted.

(2) The complexes can be regarded as salts if the inner boundary is cation , the outside world must be anion ； If the inner boundary is an anion, the outer boundary must be a cation. It can be named according to the naming method of salt. From right to left, it can be named as an acid or a chemical.

If there are multiple 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.

The examples are omitted.

Complexes dissolved in water are easily ionized into internal bound ligand and external ion, while internal bound ligand and central atom are usually unable to ionize.

Eg：[Co（NH three ） five Cl]Cl two ——〉[Co（NH three ）5Cl] 2+ + 2 Cl - One third of Cl cannot be ionized.

2、 Detailed description (for academic use)

(1) In complexes

Anion is first followed by cation. The word "hua" or "acid" is added between anion and cation, and the anion is regarded as acid radical.

(2) In coordination unit

① The ligand is first 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 ligands is indicated by one, two, three, etc. in front of the ligand. "One" may be omitted. If it is easy to cause misunderstanding, it is necessary to add parentheses to the ligand.

For example, two (methylamine), two（ Triphenylphosphine ）Etc.

③ Used between several different ligands“ ˙” separate.

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 article

① Inorganic ligands before organic ligands

E.g. PtCl two （Ph three P） two Dichloride ˙ II（ Triphenylphosphine ）Platinum (II)

② Anionic ligands first, then cationic ligands, and finally molecular ligands

E.g. K [PtCl three （NH three ）]Trichloride ˙ Potassium monoammonium platinum (Ⅱ) acetate

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

For example, [Co (NH three ） five H two O]Cl three Pentaammonia trichloride ˙ Cobalt monohydrate (III)

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

E.g. [Co (Py) (NH three ）（NO two ）（NH two OH)] Cl chloromononitro ˙ Monoammonia ˙ Monohydroxyammonia ˙ Cobalt (Ⅱ) monopyridine

⑤ If the number of atoms in the ligand is the same, the element symbols of other atoms in the ligand directly connected with the coordination atom shall be in the order of the English alphabet. As NH 2- And NO 2- , then NH 2- before.

Basic classification

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According to the classification of ligands, there are:

① Hydrated complexes. It is a complex formed by metal ions and water molecules. Almost all metal ions aqueous solution Can form hydrated complexes, such as [Cu (H two O） four 〕 2+ 、〔Cr（H two O） six 〕 3+ 。

② Halogen complexes. Most metals can form halide complexes, such as K two 〔PtCl four 〕、Na three 〔AlF six 〕。

③ Ammonia complex. Complexes formed by metal ions and ammonia molecules, such as [Cu (NH three ） four 〕SO four 。

④ Cyanide complex. Complexes formed by metal ions and cyanide ions, such as K four 〔Fe（CN） six 〕。

Schematic Diagram of Coordination Compounds

⑤ Metal carbonyl chemical compound. Metal complexes with carbonyl group (CO). Such as [Ni (CO) four 〕。

Press Central atom Classification, including:

① Mononuclear complexes. Only one central atom, such as K two 〔CoCl four 〕。

② Polynuclear complexes. The number of central atoms is greater than 1, such as [(H three N） four Co（OH）（NH two ）Co（H two NCH two CH two NH two ） two 〕Cl four 。

According to the type of bonding, there are:

① Classical complexes. Metal and organic Group Interformation σ Coordination bond, such as [Al two （CH three ） six 〕。

② Cluster complexes. It contains at least two metals as the central atoms, and also contains metals- Metal bond , such as [W] six （Cl twelve ）Cl six 〕。

③ Complexes containing unsaturated ligands. Metal and Ligands Form π between- σ key Or π - π* Feedback key , such as K ［ PtCl two （C two -H four ）〕。

④ Sandwich complexes. Central atom Metal, organic ligand Group Metal atoms are sandwiched between two parallel carbon ring systems, such as Ferrocene 〔Fe（C five H five ） two 〕。

⑤ Cavity complexes. The ligand belongs to macrocyclic polydentate Organic compound , such as N (CH two CH two OCH two CH two OCH two CH two ） three N. They are related to alkali metal and alkaline-earth metal To form cave like complexes.

According to the type of discipline, there are:

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

② Organic Metal complex 。 Metal and organic matter Ligands A complex formed between.

③ Bioinorganic complexes. Complexes formed by biological ligands and metals, such as Metalloenzyme , chlorophyll Vitamin B12 。

Coordination compounds can be divided into traditional coordination compounds and Organometallic compound 。

Traditional coordination compounds consist of more than one Coordination ion (also called ionic complex), the electrons in the coordination bond are almost all provided by the ligand. Typical ligands include H2O 、 NH3 , Cl, CN and en 。

Example: [Co（ EDTA )]、 [Co(NH3)6]Cl3 、 [Fe(C2O4)3]K3 And [Cr (H two O) six ]Cl three 。

Organometallic compounds refer to compounds containing metal carbon chemical bonds, and ligands are organic Group (e.g olefin 、 Alkyne 、 alkyl 、 Aromatic ring ）Or chemicals of similar nature, such as phosphine 、 Hydrogen anion , carbon monoxide.

example: (C5H5)Fe(CO)2CH3 、 Fe(CO)5 、Cp two TiMe two 。

The chemical branches that overlap coordination chemistry are as follows:

Bioinorganic Chemistry ——The coordination ligand exists in nature, often amino acid Side chains and coenzyme , such as porphyrin 。 Examples include hemoglobin 。
Cluster chemistry ——Use metal atoms as ligands, such as Ru3(CO)12 。

Main properties

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stability

Generally, the stability of coordination compounds mainly refers to the thermal stability and whether the complexes are easy to ionize out their components (central atoms and Ligands ）。 The coordination body can be weakly dissociation There are very few central atoms (ions) and ligands, such as [Cu (NH three ） four 〕 2+ A small amount of Cu can be dissociated 2+ And NH three ：

Coordination noumenon Dissociation equilibrium in solution and Weak electrolyte The ionization equilibrium of is very similar, but also has its dissociation equilibrium constant, which is called the Stability constant K。

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

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 ionic potential of the complex: φ= Z/r φ Is the ionic potential Z is the charge number r is the radius. transition metal With high nuclear charge, small radius, empty d orbitals and free d electrons, they are easy to accept the Electron pair And it is easy to feedback d electrons to the ligand. Therefore, they can form stable complexes. Alkali metals and alkaline-earth metal Contrary to transition metals, they have low polarization, inert gas structure, poor ability to form complexes and poor stability of their complexes.

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

Basic structure

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There are many kinds, the most common are octahedron and tetrahedron. The former is like [Fe (CN) six ] 4- , the latter such as [Ni (CO) four ]: There are also plane squares such as [Cu (NH three ) four ] 2+ ，[Cu(H two O) four ] 2+ .

configuration

The configuration of coordination compounds is determined by coordination number That is, the number of coordination atoms around the central atom of the compound. The sum of coordination number and radius, charge number of metal ions and ligands Electronic configuration Relevant, generally between 2-9, Lanthanide and Actinide element The coordination number of 10 or more often appears in the complexes of.

The coordination atom around the central atom is regarded as a point, and the points are connected by lines Coordination polyhedron 。 The relationship between coordination number and complex configuration is shown in the following table:

coordination number	configuration	example
two	Linear	HgCl2 、 Ag(NH3)2 、[Au(CN) two ]
three	Plane triangle	HgI three 、Pt( PPh3 ) three 、Fe[N(Si(CH three ) three ) two ] three
four	tetrahedron	Ni(CO)4 、 MnO4 、 SnCl4 、SiO₂
	Plane square	Pt(NH3)2Cl2 、PtCl four 、Ni(CN) four
five	Trigonal bipyramid	Fe(CO)5 、CdCl five
	Tetragonal cone	[InCl five ]、SbF five
six	Octahedron	[Ti(H two O) six ]、[Co( en ) three ]、[Cu( NH3 ) six ]
seven	Pentagonal bipyramid	[ZrF seven ]、[UO two F five ]

In five coordination Trigonal bipyramid and Tetragonal cone As a result, the structure of a large part of five coordination compounds is an intermediate structure between the two structures. Six coordinated compounds except the extremely common Octahedron Besides, it may be Triangular prism Structure, such as mononuclear complex [Re (S two C two Ph two ) three ]It belongs to this category. In the seven coordination, the complex may also be a single capped octahedron or a single capped triangular prism structure.

Among compounds with higher coordination number, the octagonal can be tetragonal Antiprism 、 Dodecahedron 、 Cube , double hat triangular prism or hexagonal double cone structure; The nine coordinated structure can be a three capped triangular prism or a single capped square antiprism; The decagonal coordination can be a double capped tetragonal antiprism or a double capped dodecahedron structure; Eleven coordinated compounds are rare, which may be single capped pentagonal prism or single capped pentagonal antiprism; Twelve coordinated, such as [Ce (NO three ) six ], ideal Icosahedron ； The tetragonal coordination is a double capped hexagonal antiprism. The highest coordination number is very rare, such as PbHe fifteen The coordination number of lead in the ion is at least 15.

The above is only the ideal configuration of the complex. In practice, the structure of complexes often suffers from distortion, which may be caused by Steric effect 、 Electronic effect (See jahn-teller effect ）Or the type of ligand.

Isomerism

isomerism Phenomenon is one of the important properties of complexes. It not only affects the physical and chemical property And its stability, reactivity and biological activity There are also close relationships. Important complex isomerism phenomena include Stereoisomerism and Structural isomerism 。

Stereoisomerism

Stereoisomerism yes chemical formula It is a heterogeneous phenomenon that the atoms are arranged in the same order as the atoms, but the atoms are arranged in different spaces. Stereoisomerism is mainly divided into Geometric isomerism and Optical isomerism 。

Geometric isomerism

Geometric isomerism It is the isomerization phenomenon caused by different ligands with the same composition in different spatial geometric arrangements, mainly in the coordination number of 4 Plane square And coordination number 6 Octahedron In the structure Cis trans isomerism Body and Areal meridional isomerism The form of body exists.

From the perspective of spatial relationship, cis（ cis -) means that the same ligand is in the neighborhood, trans（ trans -) means that the same ligand is in alignment. Octahedron [MA three B three ]Among the two isomers of（ fac -) or cis cis means that 3 A and 3 B respectively occupy the vertex of the triangular surface of the octahedron（ mer -) or cis trans means that 3 A and 3 B are in octahedron Outside catch Of meridian Top side by side.

cis -[CoCl two (NH three ) four ]

trans -[CoCl two (NH three ) four ]

fac -[CoCl three (NH three ) three ]

mer -[CoCl three (NH three ) three ]

Asymmetry Double tooth Planar square complex of ligand [M (AB) two ]There may also be geometric isomerism, and the structure is similar to the above cisplatin.

Polynuclear complexes also exhibit geometric isomerism. For example, the dinuclear complex of Pt (II) [Pt two (PPr three ) two (SEt) two Cl two ]Of Cis trans isomer Have been prepared, and at room temperature benzene The solutions are stable. But trans adds trace amount to hot or cold benzene solution Tripropylphosphine As a catalyst, it can be completely converted into cis.

Optical isomerism

Optical isomerism yes Stereoisomerism In another form, two optical isomers can make Plane polarized light It deflects the same amount but in different directions, so it is also called optical isomerism or Enantiomerism 。 Most complexes will gradually lose in solution optical rotation This process is called Racemization 。 Depending on the specific situation, the racemization mechanism may be intermolecular or intramolecular.

The simplest optical isomer of the complex is tetrahedral, the central atom is connected to four different groups, and the molecule cannot coincide with the mirror image. For example, [Be (C six H five COCHCOCH three ) two ]。 For octahedral complexes, optical isomerization mainly occurs in the following cases:

one
[M(AA) three ]Type, such as three-（ Oxalate ）Close chromium (III)、[Co{(OH) two Co(NH three ) four } three ]Cl six (The first optically active carbon free compound produced - Hexol )。
two
[M(AA) two X two ]Type, such as [Rh (en) two Cl two ]。
three
[M(AB) three ]Type, such as [Co (gly) three ]。
four
[M(AA)B two X two ]Type, such as [Co (en) (NH three ) two Cl two ]。
five
Involving multidentate ligands, such as [Co（ edta )]。

Λ-[Fe(ox) three ]

Δ- [Fe(ox) three ]

Λ- cis -[CoCl two (en) two ]

Δ- cis -[CoCl two (en) two ]

Structural isomerism

Structural isomerism yes chemical formula Isomers with the same but different atomic order can be mainly divided into the following categories:

[Co(NH three ) five (NO two ）]Two bonded isomers of.

Bonded isomerism The ligand coordinates with the central atom through different coordination atoms. Ligands are called two ligands, which contain more than two Lone pair electron And can coordinate with the central atom respectively. Two common ligands are: NO2 、 SCN And CN.
configurational isomerism : The complex can take more than one configuration. For example, [NiCl two (Ph two PCH two Ph) two ]It can be tetrahedron and plane quadrilateral respectively. Common isomers have five coordination compounds Trigonal bipyramid and Tetragonal cone Isomerism between configurations, and eight coordination compounds Dodecahedron and Tetragonal antiprism Heterogeneity between configurations.
Ligand isomerism : Mutual Isomer Similar complexes formed by ligands of, such as 1,3-diaminopropane and 1,2-diaminopropane amino propane Separately formed cobalt complex [Co (H two N-CH two -CH two -CH two -NH two )Cl two ]、[Co(H two N-CH two -CH(-NH two )-CH three )Cl two ]。
Ionic isomerism : The complexes have the same molecular formula but different coordination anions, so the ions produced in the aqueous solution are different, such as [Co (NH three ) five SO four ]Br and [Co (NH three ) five Br]SO four 。
Solvent isomerization : The position of water in the complex is different, and there are differences between the internal boundary and the external boundary, such as [Co (H two O) six ]Cl three And [Cr (H two O) five Cl]Cl·H two O。
Coordination isomerism ： cation and anion They are coordination ions, and the ligands can exchange components with each other. Examples are: [Co (NH three ) six ][Cr(CN) six ]And [Cr (NH three ) six ][Co(CN) six ]、[Cr(NH three ) six ][Cr(SCN) six ]And [Cr (SCN) two (NH three ) four ][Cr(SCN) four (NH three ) two ], and [Pt (NH three ) four ][PtCl six ]And Pt (NH three ) four Cl two ][PtCl four ]。
Polymerization isomerism : It is a kind of coordination isomerism, used to express complexes relative molecular mass Multiple relationship on, and polymerization Aggregates in are not the same. For example, [Co (NH three ) six ][Co(NO two ) six ]Can be regarded as [Co (NH three ) three (NO two ) three ]Of Dimer 。

theory

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Coordination compound Chemical bond Theory, mainly studying the nature of the binding force between the central atom and ligand, to explain the physics and chemical property , such as magnetism, stability, reactivity, coordination number and geometric configuration. The theory of complexes starts from static electricity Theory. Then Sidgwick and Pauline Propose coordination Covalent Model, that is, the application of Valence bond theory , has dominated this field for more than 20 years, and can better explain some properties such as coordination number, geometric configuration, magnetism, but can do nothing about the color and spectrum of complexes.

According to valence bond theory, ligands provide Lone pair electron Into the void of the central ion Atomic orbital So that the ligand shares these two electrons with the central ion. The formation of coordination bond has gone through three processes: (excitation), hybridization and bonding, of which hybridization is also called Track hybridization Is an atomic orbital with similar energy linear combination Become equal quantity and energy Degeneracy The process of hybridizing orbits. The outer rail can also be derived/ Inner rail type complex By judging the electronic configuration and hybrid type of the complex, we can get the magnetic properties Redox reaction Properties and geometry. For many classical complexes, the results of valence bond theory are relatively close to the facts.

In addition to valence bond theory Crystal field theory And Coordination field theory It is also an important complex theory.

The crystal field theory treats the ligand as a point charge and the coordination bond as an ionic bond, which can be seen as an extension of the electrostatic theory. In addition, in different geometric configurations d The role of the orbit as a starting point to draw different orientations d The orbital will undergo energy level splitting, and the concepts of splitting energy and crystal field stabilization energy are established to speculate the electronic configuration and stability of the complex. The crystal field theory can well explain the color thermodynamics Properties and complex distortion, but can not reasonably explain the Spectrochemical sequence And can not be well applied to special high/low price complexes Sandwich complex 、 carbonyl complex and Olefin complex 。

Coordination field theory Combined Molecular orbital theory And Crystal field theory 。 It is more rigorous in theory, but the quantitative calculation is very difficult. In the calculation process, approximate treatment has to be introduced, so only approximate results can be obtained.

reaction

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Ligand exchange reaction

The ligand in the coordination compound can be possessed by other ligands replace , called Ligand exchange reaction , the general reaction mechanism is Nucleophilic substitution reaction 。 with Octahedral complex For example, the general formula of such reaction is:

Where X is the substituted ligand, usually called the leaving group; Y is the substituent group, usually called the entering group. The rate of such ligand exchange reactions varies greatly, some reactions can be completed within 10 seconds, while others take several months. There is an artificial regulation for the difference of activity, and it is considered that the concentration is about 0.1 M At 25 ° C, half life Complexes larger than one minute are so-called "inert" complexes, otherwise they are called active complexes.

Valence bond theory and Coordination field theory The rate difference of this kind of reaction has been explained, and the following rules generally exist:

one
The increase of the central metal ion charge will reduce the reaction rate;
two
The central ion is d zero 、d one 、d two 、d nine 、d ten Configuration, high spin d five 、d six 、d seven Configuration and high spin d four The coordination compounds of configuration are active for ligand exchange reaction;
three
The central ion is d three 、d eight Configuration, or low spin d four 、d five 、d six It is inert for ligand exchange reaction.

In addition, the reaction rate is also related to the type and arrangement of solvents and ligands.

Coordination reaction can be seen as the same Lewis acid-base theory In Acid-base reaction : Metal ions provide empty orbits for acids, and ligands provide electron pairs for alkali ， transition metal The reaction with ligand is often accompanied by color change. For example, HCl is added to [Cu (NH three ) four ]Generate [Cu (H two O) four ](light blue), [CuCl (H two O) three ]、[CuCl two (H two O) two ]、[CuCl three (H two O)]、[CuCl four ]、[Cu(NH three ) four ](dark blue); As another example, the excessive ammonia Join [Cu (H two O) four ]The color immediately changes from light blue to dark blue:

Redox reaction

Coordination compound Redox reaction There are two types, one is the redox reaction between the central atom and the ligand, and the other is the redox reaction between two complexes. The latter can be divided into two categories:

Electron transfer mechanism, outer reaction mechanism: the first coordination layer of the two reactants remains unchanged. The reaction rate is mainly related to the structure and electron spin State, containing π conjugated system ligands, such as Bipyridine The complex reaction rate of CN and CN is often fast. In addition, bridging ligands can also transfer electrons, but generally not as direct electron transfer The response is effective.
Bridge mechanism, inner layer reaction mechanism: two metal atoms are connected to a bridge ligand at the same time to form transition state 。

Whether the reaction is carried out by outer layer mechanism or inner layer mechanism depends on the structure of the complex. yes Ligand exchange reaction Inert, no bridging ligand or electron transfer activation energy For very low complexes, their mechanism is dominated by the outer mechanism. The bridge mechanism mainly occurs for the complexes with ligand exchange reaction activity. The energy barrier that the bridge mechanism needs to overcome is much lower than that of the outer reaction mechanism, because the transfer of electrons by the bridge ligand reduces the energy of electrons penetrating the outer layer of the ligand and the hydration layer.

Redox reaction There are two more special types of reactions in:

Double electron transfer reaction: in reaction Oxidized state The change of is ± 2, and the mechanism is bridge mechanism.
Non complementary reaction: oxidant and reducing agent The change of valence state is not equal, and the general reaction mechanism is divided into several steps.

Henry Taube He has made a lot of contributions to the study of redox reaction of complexes, and thus obtained Nobel Prize in Chemistry 。

application

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Applications of coordination compounds include:

In analytical chemistry, complexes can be used to:

Ion separation: change the solubility of substances by forming complexes, so as to separate from other ions. For example, ammonia water and AgCl 、 Hg2Cl2 and PbCl2 React to separate the first group of cations:
And utilization Ammonia complex The generation of makes Zn Into solution:
Metal ionic titration : For example, when quantitatively determining the content of Fe in the solution, indicator Dark red [Fe（ phen ) three ]。
Masking interfering ions: use the formation of complexes to eliminate the factors that will interfere with the results in analytical experiments. colorimetry When measuring Co, it will be interfered by Fe, and can be added F Generate colorless stable coordination ion [FeF] with Fe six ], to mask Fe:

In industrial production:

Coordination catalysis: the mechanism of catalytic reaction often involves coordination compound intermediates, such as synthesis ammonia Industrial use Cupric diacetate Carbon monoxide removal, catalyzed by organic metal catalyst olefin And asymmetric catalysis in the preparation of drugs.
Mirror making: using silver ammonia solution as raw material Silver mirror reaction , plated a bright silver coating on the back of the glass.
Extract metal: for example Cyanidation In the gold extraction step, due to the formation of stable coordination ion [Au (CN) two ], making inactive gold enter the solution:
The thermal decomposition of many carbonyl complexes can also be used to purify metals. For example, in the Monde process, the purification of nickel uses Nickel tetracarbonyl Generated and decomposed Reversible reaction ：
Material precursor: alumina particles and Gallium arsenide (GaAs) thin films.
Hard water softening

In biology, many biological molecules are complexes and contain iron Of hemoglobin And oxygen and carbon monoxide The normal operation of many enzymes and magnesium containing chlorophyll can not be separated from the complex mechanism. Commonly used cancer treatment drugs Cisplatin , i.e cis -[PtCl two (NH three ) two ]It can inhibit the DNA replication process of cancer cells, and contains a plane square complex configuration. EDTA 、 Sodium citrate 、 2,3-dimercaptosuccinic acid etc. antidote The mechanism that can be used for detoxification of heavy metals is often that they can cooperate with heavy metal ions to transform them into coordination compounds with low toxicity, so as to achieve the purpose of detoxification.

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