There are three theories used to explain and explain the structure and properties of complexes:Valence bond theory、Crystal field theoryandMolecular orbital theoryThe coordination field theory discussed in this entry is mainly explained through the molecular orbital theory.
Valence bond theory
Coordination field theory
Coordination compoundAccording to the properties of coordination compounds, according to the valence bond theory ofhybrid orbital theory useCovalentCoordination bond and electrovalence coordination bond explainmetal ionandLigandsInterBinding force。For example, renderingDiamagnetism, due toCentral ionWith unfilled d orbitals and s, pEmpty orbitThese empty orbits are composed by hybridizationHybrid orbit, provided by ligandLone pair electron；L → M is formed between ligand L and central ion Mσ bondCoordination key.yesParamagnetismOf.Central ionicUnpaired electronThe number is the same as that of free ions. It is believed that metal ions and ligands are bound together by electrostatic attraction.The valence bond theory explains the geometric configuration and magnetic properties of coordination compounds concisely.The valence bond theory does not mentionAntibonding orbitThe spectral data of coordination compounds cannot be satisfactorily explained.
Crystal field theory
Fig. 1 Coordination field theory
The crystal field theory isElectrostatic actionModel.Transfer the central ion (M) and ligand (L)InteractionThink alikeIonic crystalinPositive and negative ionsStatic electricity.When L approaches M, the d orbit in M is subjected to Lnegative chargeThe degenerate d d orbitals of the original energy levels are split by the electrostatic perturbation of.PressPerturbation theorycomputableFission energyDue to the complexity of calculation, the ligand is qualitatively regarded asSymmetryArrayedPoint chargeD orbit with MElectronic cloudRejection.Due to the characteristics of the distribution of d orbitals, the energy levels of the original five degenerate d d orbitals in the coordination field split, causingElectronic layoutAnd a series of other property changes, such as electron redistribution, system energy will be reduced, which explains various properties of coordination compounds.for exampleOctahedronIn the coordination ion, the d orbital is divided into two groups:Low energy levelD ofxy，dxzDyz, their energies are equal, called t2g(2g issubscript）Orbit, both of which have equal energy;D of high energy levelx2-y2d，dz2, called eg(g is subscript) track.The difference between these two energy levels is calledCrystal field splitting energyΔ ，ligandfield strengthThe bigger, the splitEmergyThe larger.D electrons are filled in these two groups of orbits according to the relative size of Δ and pairing energy (P), forming a strong field low spin andWeak fieldhighSpin structure。In the complexes with different configurations, the energy level splitting of the d orbital of the central ion is different.A series of properties such as structure, spectrum, stability and magnetism of coordination compounds were successfully explained.
Orbital theory
Coordination compoundmolecular orbitalThe theory deals with the bonding between metal ions and ligands with the viewpoints and methods of molecular orbital theory.The description of the state of coordination compound molecule is mainly MValence shell electronwave functionThe Molecular Orbital ψ L Composition of ψ M and Ligand LDelocalizationMolecular orbital: ψ=cM ψ M+∑ cL ψ L.In order to effectively form molecular orbitals, symmetry matching, maximum overlap of orbitals and similar energy levels should be met.
Coordination field theory is the development of crystal field theory. Its essence is the molecular orbital theory of coordination compounds.The central metal under the action of the electric field generated by the treatment of the ligandAtomic orbitalWhen the energy level changesMolecular orbital theory methodMainly, using similar atomic orbitalslinear combinationetc.Mathematical methodAccording to the symmetry of the ligand field, we simplify it and absorb the results of crystal field theory to clarify the structure and properties of coordination compounds.
Stabilization energy
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Because of the existence of the coordination field, the d orbital energy level of the central ion is split, and most d electrons tend to be at lower energy levels, the system is relatively stable.This indicates that due to the existence of the ligandcharge distributionIt is no longer spherical symmetric, but producesElectric dipole momentThe interaction between the electric dipole moment and the ligand produces an additional energy, calledStabilizing energy of coordination fieldAnd make the complex stable.It can be seen from Figure 1 and the overview diagram that an electron in the t2g orbit contributes to the stabilization energy of the system while in the eg orbit contributes
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Therefore, the octahedral field coordination field stabilization energy (Table 2) is calculated by the following two formulas: therefore, in the case of strong field, the electron tends to be at a lower energy level, so that the central ions d4, d5, d6, d7 can form a more stableLow spin complexHowever, the pair electrons with opposite spin still have repulsive effect, so the actual calculated value of stabilization energy should also deduct this value called pairing energy.Stabilizing energy is generally measured by spectrum or estimated approximately by point charge model, which can be further exploredcomplex compoundOfthermodynamicsandreaction kineticsnature.ginger- Teller distortion If a non-linear molecule is in orbitDegeneracy, the molecule will deformDegeneracyElimination.This shows that the additional stabilization caused by Young Teller distortion can stabilize the complex.Some complexes are not strictly regular polyhedral coordination orCoordination atomstayBalance positionVibration deviates from the regular polyhedron coordination, resulting in static distortion.The distorted octahedron has two configurations, that is, it is elongated or shortened along the fourth axis.
Molecular orbital theory
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If the central ion andCoordination fieldThe covalent complexes with significant overlap should be reflected more trulyChemical bondThe molecular orbital theory must be adopted for properties.
Figure 3 Molecular orbital combination form
For ML6Octahedral complex, one s, three p and two d atomic orbitals of the central ion andσ orbitCombine to form 6 σBonding orbitalAnd 6 σ * antibonding orbitals.transition metalD electron of is in t2gNonbonding orbitalOr partly in weak anti bonding e orbitals.In addition, when the ligand may further provideP OrbitWhen π bonding and π * anti bonding orbits are formed with the central ion t2g orbit (Figure 3), the s, p, d orbits of the central ion and the σ orbits of the ligand are classified into single, double and triple degenerate orbits a1g, eg, t1u according to the octahedral symmetry.The atomic orbitals of the central ion with the same symmetry and the σ orbitals of the ligand are combined into molecular orbitals, and their molecular orbital combination form and energy level order are shown in Figure 3.If thevalence electronPress aboveEnergy level diagramFill in in order to get theElectronic configuration。The contribution of metal ion atomic orbitals and ligand orbitals to their molecular orbital composition is not necessarily the same.If the contribution of the ligand orbital is large, the electrons occupying the molecular orbital mainly reflect the nature of the ligand;Conversely, it reflects the properties of metal ions.
If the electrons of the molecular orbital mentioned above have transitions, the charge transfer spectrum will appearElectronic spectrum；The transition of electrons from molecular orbitals mainly characterized by ligands to molecular orbitals mainly characterized by metal ions or vice versa, which is equivalent to the transfer of charges from ligands to metal ions or vice versa, occursCharge transfer spectrum。For example, ML6 complex may have three charge transfer spectral bandsElectron transition：
① The ligand and metal ions only form σ bond, and there may be σ → t2g and σ → e transitions, which can be associated with the metal reduction band.
② Ligand and metal ion form σ sumpi bondThere may be two types of transitions σ → t2g and σ → e, and π→ t2g and π→ e, which are also equivalent to metal reduction zones.
③ The ligand has unoccupied anti bond orbit π *, and the metal ions feed back electrons to the ligand. In addition to the possible σ → t2g and σ → e transitions, there are also t2g → π * or e → π * transitions, which is equivalent to the metalOxidation zone。 Obviously, the molecular orbital theory is very successful in explaining the charge transfer spectrum.Molecular orbital theory is dealing withComplex structureAnd itsphysical property、chemical propertyIt is slightly higher than the crystal field theory.Since the 1950s, a lot of molecular orbital theoretical calculations have been carried out for the complexes, and the non empiricalSelf consistent fieldAb initio sumxAlpha method and semi empiricalTotally abbreviated differential overlap、Slight differential overlapetc.Quantum chemical calculation method。
With the increasing synthesis of inorganic and organic complexes and the study of various structures and properties, the coordination field theory has been developed continuously and has become an important modernChemical bond theoryOne, yesTheoretical physicsandTheoretical chemistryAn important branch of.It is used to explain the relationship between structure and performance of coordination compounds, catalysisreaction mechanism, working principle of laser material andcrystalIts physical properties are widely used.
