Name: crystal field splitting in non-octahedral shapes
Uploaded: 2020-09-24T15:06:53.000Z
Duration: 2 min 46 s
Channel: JoVE

晶体场理论可以用来模拟四面体和 正方形平面过渡金属络合物，类似于该理论在八面体络合物中 的应用。例如，为了模拟四面体 四氯镍酸盐(II)离子，每个胺配体被一个负点电荷取代，从而产生一个四面体晶体场。由于这个场的影响，dxy、dyz 和 dxz 轨道的能量 高于 dx²-y² 和 dz² 轨道。这归因于四面体晶体场 与 dxy、dyz 和 dxz 轨道之间 更强的相互作用。较高能量轨道具有 t₂ 对称性，称为 t₂ 集，较低能量轨道具有 e 对称性，组成 e 集。与八面体络合物中 d 轨道的分裂相比，四面体络合物中 d 轨道的 相对能量是反向的，晶体场分裂能Δtet 较低。在正方形平面络合物中，例如四氰合镍(II)离子，所有配体都位于 xy 平面。这里，通过用负点电荷取代 氰化物配体获得 正方形平面晶体场。在这个场的影响下，金属离子的 d 轨道 分裂成四个不同的能级。这里，dx²-y² 轨道是能量最高的轨道，它的波瓣直接指向配体电荷。dxy 轨道的能量次之，波瓣与配体电荷位于同一平面。dz² 轨道的能量更低，这是由于 dz² 轨道与 xy 平面中的 晶体场之间的重叠很小。能量最低的轨道集 dxz 和 dyz 与 晶体场的相互作用 相对最小。正方形平面络合物中的 晶体分裂能Δsp 定义为 最高能量轨道 dx²-y² 与 最低能量轨道 dyz 和 dxz 之间的能量差。假设所有络合物都有相同的金属离子和配体分子，则Δtet、Δsp 和Δoct 的比为 0.44:1.7:1。

晶体场理论 - 四面体和平面四边形络合物

Crystal Field Theory - Tetrahedral and Square Planar Complexes

四面体络合物 
 晶体场理论 (CFT) 适用于几何结构中不是八面体的分子。 在八面体络合物， dx2−Y2 和 dz2 轨道的凸缘直接指向配体。 对于四面体络合物， d 轨道仍保持不变，但轴之间只有四条配体。 轨道没有一个直接指向四面体配体。 但是， dx2−Y2 和 dz2 轨道 (沿笛卡尔坐标轴) 与配体小于 the dxy ， dxz 和 dyz 轨道重叠。 通过与八面体大小写进行类比，可以预测四面体晶体场中 d 轨道的能量图，如图 1 所示。 为了避免混淆，八面体例如组变成四面体 e 组，八面体 t2g 组变成 T2 组。 
 <img alt="Image1" src="/files/ftp_upload/11462/11462_Image_1.png" style="text-align: center; height: 350px;" />
 图 1. 在八面体和四面体晶体场下分割金属离子的 d 轨道。 与八面体晶体场相比，四面体晶体场中的分割模式是反转的。 八面体复合体的晶体分割能量 (或 Δoct) 大于四面体复合体的晶体分割能量， Δtet。 
 由于 CFT 基于静电互斥，与配体更近的轨道将在能量中受到破坏，并相对于另一组轨道进行升高。 分割是用于八面体络合物的小于，因为重叠较少，因此晶体场分割能量 (或 Δtet) 通常很小。 
 平面四边形络合物 
 另一个常见的几何结构是平面四边形。 可以将平面四边形几何结构视为一个八面体结构，并删除一对横拉线。 假定移除的配体位于 z 轴上。 这会改变 d 轨道的分布，因为 z 轴上或附近的轨道变得更加稳定， x 轴或 y 轴上或附近的 变得更加不稳定。 这会导致八面体 t2g 和 EG 设置拆分，并提供更复杂的拆分模式 (图 2)。 
 <img alt="Image2" src="/files/ftp_upload/11462/11462_Image_2.png" style="text-align: center; height: 350px;" />
 图 2. 在平面四边形晶体场中分割 t2g 集和 eg 集轨道。 分割平面四边形络合物 of 能量 (或 Δsp) 的晶体场大于 Δoct。 
 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">配位化合物的光谱和磁性</a><a href="https://openstax.org/books/chemistry-2e/pages/19-3-spectroscopic-and-magnetic-properties-of-coordination-compounds"></a>。

四面体配合物  
晶体场理论（CFT）可适用于八面体以外几何构型的分子。在八面体配合物中，d轨道的叶瓣……

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Transition Metals and Coordination Complexes

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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 orbital energy diagram is therefore split in the opposite way from the octahedral case. The octahedral e g set becomes a tetrahedral e set, and the octahedral t 2 g set becomes a t 2 set. The splitting energy, called Δ tet, is usually small because the overlap is less than in octahedral complexes.

Square planar complexes can be viewed as octahedral structures with a pair of trans ligands removed. The removed ligands are assumed to be on the z-axis. This changes d orbital stability. Orbitals on or near the z-axis become more stable, while orbitals on or near the x- or y-axes become less stable. The octahedral t 2 g and e g sets split into a more complex pattern, and the crystal field splitting energy, Δ sp, is larger than Δ oct.

Crystal Field Splitting in Non-Octahedral Shapes

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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

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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...

crystal field theory - tetrahedral and square planar complexes

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

Explore how crystal field theory explains d orbital splitting in tetrahedral and square planar complexes, including inverted splitting and larger square planar splitting.

Crystal Field Splitting in Non-Octahedral Shapes | JoVE K12

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Crystal Field Splitting in Non-Octahedral Shapes

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