Coordination bond, also called coordination covalent bond, or simply coordination bond, is a specialcovalent bond。Common when covalent bondElectron pairA coordination bond is formed when one atom provides an empty orbital and the other atom provides an empty orbital.Once the coordination bond is formed, it is the same as the common covalent bond.The two electrons shared between two bonded atoms are not provided by each atom, but come from one atom.E.g. ammonia andBoron trifluorideCan formCoordination compound: In the picture formula, → indicates coordination key.The pair of electrons between N and B comes from theLone pair electron。
Whether the nature of coordination bond and covalent bond is the same
When a covalent bond is formed between atoms, if the shared electron pair is only provided by one atom rather than two atoms, such a covalent bond is called a coordination bond, so the coordination bond must be a covalent bond, which also has the characteristics of covalent bond: directivity and saturation, so there is no essential difference between coordination bond and covalent bond.
The covalent bond is not necessarily a coordination bond. The key is to see whether the source of the shared electron pair is provided by one bonding atom or two bonding atoms. If it is provided by one bonding atom unilaterally, it is a coordination bond. If it is provided by both bonding atoms jointly, it is an ordinary covalent bond. Therefore, coordination bonds and covalent bonds are only different in the formation process.
What are the conditions for forming coordination bonds
Coordination bond is a special kind of covalent bond, which can not be formed when any two atoms meet.It requires that one atom A of the two bonded atoms has a lone pair of electrons, and the other atom B has an "empty orbit" to accept the lone pair of electrons, so the expression method of coordination bond is A → B, A is called ligand, and B is called central atom or ion.Sometimes, in order to enhance the bonding ability, the central atom or ion B uses the empty orbit with similar energy to hybridize, and then receives the lone electron pair of ligand atom A.Coordination bonds can exist in both molecules (such as H2SO4) and ions (such as ammonium ionHydrous hydrogen ionEtc.).
Characteristics of coordination bond
Compared with conventional covalent bonds, coordination bonds mainly have the following characteristics:
one
Coordination covalent bonds mainly occur between atoms of different elements, and pure covalent bonds can occur between two atoms of the same element.
two
In this bonding, atoms or Lewis bases that donate electron pairs from themselves are called donors, and atoms or Lewis acids that receive electron pairs are called receptors or receptors.
three
The other atom that receives these shared electron pairs is called a receptor or acceptor.
four
An arrow → → symbol represents this bond, pointing to the receptor from the donor atom.
five
Bond length, the strength of coordination covalent bond is almost similar to that of covalent bond.
six
After sharing an electron pair, each atom becomes stable[2]
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Coordination bond
Coordination bond is polar bond, and the electron is always biased to one side. According to the strength of polarity, it is close to ionic bond or close toPolar covalent bond。In someComplexIn addition to the ligand providing electrons to the receptor to form common coordination bonds, the receptor electrons also transfer to the ligand to form feedback coordination bonds.For example, the lone pair electrons on carbon in CO in Ni (CO) 4 are transferred to nickelatomCoordination formationσIn coordination bond, the d electron of nickel atom flows back to the empty π * anti bond orbit of CO to form four electron three center d-p π bond, which is feedback coordination bond.NonmetalCoordination compoundThis key may also exist in.Coordination bond can be explained by the following three theories:
Valence bond theory
①Valence bond theory。It is believed that the entry of electrons on the ligandCentral atomThe hybrid orbit of.For example, cobalt (Ⅲ) complexes. [Lone pair electrons of F in CoF6] 3 - enter Co3+Sp3d2 hybridOrbit, which is called outer orbital complex or high spin complex, has four unpaired electrons, and is paramagnetic. [The lone pair electrons of NH3 in Co (NH3) - 6] 3+enter the d2sp3 hybrid orbital of Co3+. This complex is called an internal orbital complex or a low spin complex. Since all electrons have been paired, it has no paramagnetism and is diamagnetic.
Crystal field theory
②Crystal field theory。Think of the ligand as a point charge or dipole, and consider the effect of the electrostatic field generated by the ligand on the central atomAtomic orbitalEnergy level.For example, when the central atom is introduced into six ligands located at the six top angles of the regular octahedron, the original five fold degenerate d d orbitals are split into a group of double degenerate eg (- y2, dz2) orbitals and a group of triple degenerate t2g (dxy, dxz, dyz) orbitals.The energy difference between eg and t2g orbits is called separation energyΔ0，Δ0 ∨ 10Dq, Dq is called field strength parameter.In the above cobalt (Ⅲ) complexes, the field generated by 6 F - is not strong,Δ0 is small, d electrons are arranged according to the Hunt rule, and there are four unpaired electrons, so [CoF6] 3 - is a weak field complex or a high spin complex.The field generated by 6 NH3 is strong,Δ0 is larger, d electron is in accordance withPrinciple of minimum energyandPauli exclusion principleThere are no unpaired electrons, so [Co (NH3) 6] 3+is a strong field complex or a low spin complex.
Crystal field theory
Molecular orbital theory
Coordination bond
③molecular orbitalTheory.Assuming that the electron is moving in the molecular orbital, the energy level diagram of the molecular orbital can be obtained by applying group theory or according to the basic principles of bonding.Then fill in the electrons one by one from the molecular orbital with the lowest energy according to the Pauli principle, that isElectronic configuration。Molecular orbitals are divided into bonding orbitals and anti bonding orbitals.The degree of molecular bonding depends on the difference between the number of bonding electrons and the number of anti bonding electrons in the molecule.
Formation of coordination bond
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A typical example
(1) The O atom can provide an empty 2p orbital, which can accept external coordination electron pairs to form bonds, such as in the oxide R3NO of organic amine.
(2) The O atom can not only provide an empty 2p orbital to accept external coordination electron pairs to form bonds, but also provide two pairs of isolated electron pairs to feed back to the empty orbital of the original coordination atom to form feedback bonds. For example, the feedback bond in H3PO4 is called d-p bond, and P ∨ O bond still only hasdouble bondThe nature of.
Ligands
Coordination compounds are a kind of relatively complex intermolecular compounds, which contain a complex ion and are stableStructural unit, can exist in the crystal, can also exist in the solution, can be positive ions, can also be negative ions.For example:
CuSO4+4NH3 [Cu(NH3)4]SO4 [Cu(NH3)4]2++SO42-
3Na++6F-+Al3+=Nathree[AlFsix] =3Na++AlFsix3-
Coordinationcovalent bondThe "coordination bond" for short refers to the covalent bond formed when all bonding electrons of two atoms are provided by one atom, in which all bonding electrons are provided“Ligands(referred to as ligand for short).Common ligands are:ammonia(nitrogen atom)carbon monoxide（carbon atom）, cyanogen ion (carbon atom), water (oxygen atom), hydroxide (oxygen atom);Receptors are diverse: hydrogen ions, represented by boron trifluoride (boron atom)Electron deficient compound, and a large number ofTransition metal elements。The study of coordination compounds has developed into a specialized discipline,Coordination chemistry。
Lewis acid base pair
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Lewis acid-base theoryThe definition of is that the substance providing the electron pair is alkali, and the substance receiving the electron pair is acid.When Lewis acid reacts with Lewis base, the acid accepts the electron pair of the base and forms a coordination bond[1]。
"Hydroxide" belongs to ligand, while "hydrogen ion" belongs to receptor, which indicates that the acid-base neutralization reaction between hydrogen ion and hydroxide can be seen as the process of forming coordination bond between hydrogen ion and hydroxide.From this point of view, chemist Lewis proposed the concepts of "Lewis acid" and "Lewis base", and believed that anything that can give electrons in the process of coordination bonding is called "base";What can accept electrons is called "acid".Lewis' acid-base theory expands the range of acids and bases. Lewis acid-base pairs not only include all Arrhenius acid-base pairs, but also include some neutral or even insoluble substances.
In fact, the essence of Lewis acid is the "receptor" in the coordination bond;The essence of Lewis base is the "ligand" in the coordination bond, and the two are equal.
