Name: octahedral d-orbital splitting in crystal fields
Uploaded: 2020-09-24T15:06:53.000Z
Duration: 2 min 58 s
Channel: JoVE

晶体场理论基于 过渡金属离子与配体分子之间的静电相互作用 描述了过渡金属络合物的 电子结构。它用来解释过渡金属络合物的某些性质，例如磁性和颜色。通过将配体近似为负点电荷，并计算由这些电荷引起的净静电场或晶体场，可以模拟过渡金属络合物中 配体分子与金属离子之间的 静电相互作用。然后，通过检查晶体场 对过渡金属离子价态轨道能量的影响，来描述过渡金属络合物的 电子结构。例如，当对八面体络合物 六氨合钴(III)建模时，每个胺配体被一个负点电荷取代，从而产生一个八面体晶体场。在这个场的影响下，Co(III)离子的五个 d 轨道的 能量不再相同。这里，dx²−y² 和 dz² 轨道 比 dxy、dyz 和 dxz 轨道具有更高的能量。这归因于 d 轨道的取向。dx²−y² 和 dz² 轨道的波瓣 直接指向配体，因此，这些轨道中的电子会受到配体电荷 更强的排斥。较高能量的轨道具有 e_g 对称性，称为 e_g 轨道集，而较低能量的轨道具有 t_2g 对称性，构成 t_2g 轨道集。两个集合之间的能量 称为晶体场分裂能，用符号Δoct 表示。Δoct 的大小取决于金属离子和 配体分子之间的净静电相互作用。羰基等配体会产生很强的晶体场，导致Δoct 的值较大。这种配体称为强场配体。相比之下，碘化物等配体的Δoct 值较小，称为弱场配体。在光谱化学系列中列出了配体引起Δoct 值 增加的能力。金属离子上电荷的增加 也增加了络合物内的净静电相互作用，从而导致Δoct 值更高。

晶体场理论 - 八面体络合物

Crystal Field Theory - Octahedral Complexes

<h3>晶体场理论 </h3>
 为了解释观察到的过渡金属复合物 (如颜色) 的行为，开发了一种模型，其中涉及来自配体的电子与中央金属原子未杂交 d 轨道中的电子之间的静电交互。 这种静电模型是晶体场理论 (CFT)。 它有助于了解，解释和预测配位化合物 of 过渡金属的颜色，磁性行为和某些结构。 
 CFT 侧重于在非金属配体键上的协调复合物中的中心金属离子上的非粘接电子。 像价键理论一样， CFT 只讲述了部分复杂事件的行为。 CFT 以纯净形式忽略配体和金属离子之间的任何共价键合。 配体和金属均被视为无限小点电荷。 
 所有电子都是负的，因此从配体中捐赠的电子将排斥中心金属的电子。 考虑八面体中未杂交 d 轨道中电子的行为。 如图 1 所示， Five d 轨道由叶形区域组成，并在空间中排列。 在一个八面体中，沿轴的六条配体配位。 
 <img alt="Image1" src="/files/ftp_upload/11461/11461_Image_1.png" style="text-align: center; height: 200px;" />
 图 1. 此处显示了五 d 轨道的方向特性。 阴影部分表示轨道的相位。 沿八面体络合物轴的配体 (L) 配位。 为了清晰起见， dx2−Y2 轨道中遗漏了配体，因此可以显示轴标签。 
 在气相中一个皮肤不全的金属离子中，电子根据 洪德规则（Hund's rules） 在五个轨道中分布，因为轨道都有相同的能量。 在过渡金属波群中，轨道原子的 d 过渡金属的能量会受到配体类型和分子几何结构的影响。 如果配体分子均匀地分布在金属离子周围，则会产生球形晶体场。 这个球形晶体场通过一个相等的数量来提高金属离子的 d 轨道的能量 (图 2)。 当向八面体几何结构中的金属离子配体配位时， d 轨道的能量就不再相同。 
 <img alt="Image2" src="/files/ftp_upload/11461/11461_Image_2.png" style="text-align: center; height: 350px;" />
 图 2. 球状晶体场中金属离子的 d 轨道与未结肤色的游离金属离子的 d 轨道相比不稳定 (在能量中较高)。 在八面体络合物中，由于配体与直接指向的 d 轨道的交互作用更强，因此 EG 轨道与 t2g 轨道相比更加不稳定 (能量中的值更高)。 
 在八面体络合物，五个 d 轨道中的两个叶片 dx2−Y2 和 dz2 轨道指向配体 (图 1)。 这两个轨道称为 EG 轨道 (符号指轨道的对称性)。 另外三个轨道，即 dxy ， dxz 和 dyz 轨道，在配体之间有指向的凸角，称为 t2g 轨道 (该符号也是指轨道的对称性)。 当六条配体沿八面体轴接近金属离子时，它们的点电荷会在金属离子的 d 轨道中重新产生电子。 但是，轨道 (dx2-Y2 轨道) 中的电子与配体之间的脉冲是大于 t2g 轨道 (dxy ， dxz 和 dyz 轨道) 中电子与配体之间的脉冲。 这是因为 EG 轨道的凸缘直接指向配体，而 t2g 轨道的凸缘则指向配体之间。 因此，八面体中金属离子的轨道等电子的潜在能量高于 t2g 轨道中电子的潜在能量。 能量的差异可能如图 2 所示。 
 EG 和 t2g 轨道之间的能量差异称为晶体分割，由 Δoct 符号表示，其中 oct 代表八面体。  Δoct 的大小取决于许多因素，包括围绕中心金属离子的六条配体的性质，金属上的电荷，以及金属使用的是 3D ， 4D 还是 5d 轨道。 不同的配体产生不同的晶体场碎片。 配体产生的晶体场分裂量不断增加，以光谱化学系列表示，其简短版本如下： 
 <img alt="Image3" src="/files/ftp_upload/11461/11461_Image_3.png" style="text-align: center; height: 100px;" />
 本文改编自 <a href="https://openstax.org/books/chemistry-2e/pages/19-3-spectroscopic-and-magnetic-properties-of-coordination-compounds"> Openstax, 化学 2e, 第19.3节： </a><a href="https://openstax.org/books/chemistry-2e/pages/19-3-spectroscopic-and-magnetic-properties-of-coordination-compounds">S</a><a href="https://openstax.org/books/chemistry-2e/pages/19-3-spectroscopic-and-magnetic-properties-of-coordination-compounds"> 配位化合物的光谱和磁性</a>。

晶体场理论  
为解释过渡金属配合物的观测行为（如颜色），需采用一种涉及……之间的静电相互作用的模型

crystal-field-theory-octahedral-complexes

Education

教育

JoVE Core

Chemistry

化学

Transition Metals and Coordination Complexes

过渡金属与配位化合物

transition-metals-and-coordination-complexes

KEY TERMS AND CONCEPTS

关键术语与概念

octahedral d-orbital splitting in crystal fields

Crystal field theory explains how ligands affect the d orbitals in transition metal complexes. This electrostatic model helps predict the colors, magnetic behavior, and some structures of coordination compounds. It focuses on the nonbonding electrons on the central metal ion, not on the metal-ligand bonds.

In crystal field theory, the metal ion and ligands are treated as very small point charges. Because electrons are negative, electrons donated by the ligands repel the electrons in the metal’s unhybridized d orbitals. In its pure form, this model ignores covalent bonding between ligands and metal ions, so it gives only part of the full picture.

The five d orbitals have different shapes and directions in space. In an octahedral complex, six ligands approach the metal ion along the axes. In a free metal ion in the gas phase, the d orbitals have the same energy, so electrons fill them according to Hund’s rule.

When ligands are arranged evenly around a metal ion, they create a spherical crystal field. This field raises the energy of all the d orbitals by the same amount. The picture changes in an octahedral geometry, where the d orbitals no longer all stay at the same energy.

In an octahedral complex, the d x2-y2 and d z2 orbitals point directly toward the ligands. These are called the e g orbitals. The d xy, d xz, and d yz orbitals point between the ligands, and these are called the t 2g orbitals. Because the e g orbitals face the ligands more directly, they feel stronger repulsion and have higher energy than the t 2g orbitals.

The energy gap between the e g and t 2g orbitals is called crystal field splitting and is written as Δ oct. The size of Δ oct depends on the ligands around the metal ion, the charge on the metal, and whether the metal uses 3 d, 4 d, or 5 d orbitals. Different ligands produce different splitting values, which are organized in the spectrochemical series.

Octahedral d-Orbital Splitting in Crystal Fields

octahedral-d-orbital-splitting-in-crystal-fields

properties of transition metals

Transition Metals in the Periodic Table

Transition metals are the d-block elements and the inner transition metals in the f-block. They are defined by partially filled d orbitals. In the periodic table, the transition elements are in groups 3 through 11, and the inner transition metals appear in the two rows below the main body of the table. The d-block is often grouped into transition series. The first transition series runs from Sc through Cu. The second series runs from Y through Ag. The third series includes La and Hf through...

coordination compounds and nomenclature

How Transition Metal Complexes Are Named

Transition metal complexes are named by following rules for ligands, metal ions, and oxidation state. These compounds form when a central metal ion or atom, often a transition metal or inner transition metal, accepts electron pairs from ligands. A ligand is a Lewis base that can be an atom, molecule, or ion with one or more lone pairs to donate. In these complexes, the bond is a coordinate covalent bond. That means both electrons in the bond come from the donor, such as ammonia, and are...

metal-ligand bonds

Coordination Complexes and Chelation

Coordination complexes form when a metal ion bonds to ligands. These compounds appear in familiar examples such as hemoglobin in blood, chlorophyll in green plants, vitamin B-12, and the catalyst used to make polyethylene. Transition metal ions are especially likely to form these complexes. In a coordination complex, the metal acts as a Lewis acid. The ligand acts as a Lewis base and donates both electrons in the coordinate covalent bond. A ligand can be an atom, molecule, or ion, as long as...

coordination number and geometry

Transition Metal Shapes from Coordination Number

Transition metal complexes are often identified by their coordination number and the shape around the metal ion. The coordination number is the number of donor atoms bonded to the central metal. In many complexes, the most common shapes are octahedral, tetrahedral, and square planar. Coordination number and molecular geometry are linked in a clear pattern. A coordination number of 2 gives a linear shape, as in [Ag(NH3)2]+. A coordination number of 3 gives a trigonal planar shape, as in...

structural isomerism

Linkage and Ionization Isomers

Isomers are different chemical species that share the same chemical formula. In coordination compounds, structural isomerism includes linkage isomers and coordination-sphere isomers. These two forms show how the same atoms can be arranged in different ways around a metal center. Linkage isomers form when a ligand can attach to a transition metal through two different atoms. The cyanide ligand, CN−, can bind through carbon or nitrogen. The thiocyanate ligand, SCN−, can bind through sulfur or...

stereoisomerism

Isomer Shapes in Coordination Complexes

Isomer shapes in coordination complexes can change a compound’s properties even when the chemical formula stays the same. Isomers are different chemical species with the same formula. In transition metal complexes, the atoms stay connected through the same types of bonds, but their arrangement in space can differ. One common form is geometric isomerism. In these complexes, ligands (the ions or molecules attached to the metal) can sit in cis or trans positions. The octahedral [Co(NH3)4Cl2]+ ion...

valence bond theory

Metal Ligand Bonds in Coordination Complexes

Coordination complexes form metal-ligand bonds through valence bond theory. In this model, Linus Pauling explained bonding with hybridization and overlap of atomic orbitals. The central metal atom or ion acts as a Lewis acid. It uses empty orbitals of suitable energy to accept electron pairs from ligands, which act as Lewis bases. These coordinate covalent bonds help explain the color, geometry, and magnetic behavior seen in coordination compounds. The type of hybridization helps determine the...

crystal field theory - octahedral complexes

Crystal field theory explains how ligands affect the d orbitals in transition metal complexes. This electrostatic model helps predict the colors, magnetic behavior, and some structures of coordination compounds. It focuses on the nonbonding electrons on the central metal ion, not on the metal-ligand bonds. In crystal field theory, the metal ion and ligands are treated as very small point charges. Because electrons are negative, electrons donated by the ligands repel the electrons in the...

crystal field theory - tetrahedral and square planar complexes

crystal-field-theory-tetrahedral-and-square-planar-complexes

Crystal Field Splitting in Non-Octahedral Shapes

Crystal field theory also explains d orbital splitting in tetrahedral and square planar complexes. These shapes do not follow the same pattern as an octahedral complex, so the d orbitals respond to ligand placement in different ways. In tetrahedral complexes, the four ligands sit between the axes. None of the d orbitals points directly at the ligands, but the d x2−y2 and d z2 orbitals lie along the Cartesian axes and overlap less with the ligands than the d xy, d xz, and d yz orbitals. The d...

colors and magnetism

Color and Spin in Coordination Complexes

Color in coordination complexes comes from how metal electrons absorb light. When atoms or molecules absorb light at the right frequency, electrons move to higher-energy orbitals. For many main-group atoms and molecules, the absorbed light is ultraviolet and cannot be seen. In coordination compounds, the energy gap between d orbitals often matches visible light, so the compound can absorb and emit colors that the human eye detects. Small changes in orbital energy can cause large shifts in the...

key terms and concepts

Learn how octahedral ligands split d orbitals into e g and t 2g levels, and how crystal field splitting affects color, magnetism, and structure.

Octahedral d-Orbital Splitting in Crystal Fields | JoVE K12

chemistry

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Octahedral d-Orbital Splitting in Crystal Fields

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