Name: isomer shapes in coordination complexes
Uploaded: 2020-09-24T15:06:53.000Z
Duration: 2 min 52 s
Channel: JoVE

配位化合物的立体异构体 是具有相同化学式的分子，它的配体与金属原子的连接方式相同，但配体分子围绕中心原子的 排列不同。根据分子结构，配位化合物的立体异构体分为 几何异构体或光学异构体。几何异构体具有完全不同的分子几何形状 的立体异构体，它们不同的物理和化学性质存在。顺反异构体是几何异构体的一个例子。它们出现在带有两组相同配体的 正方形平面络合物中，或 MA₂B₂，或出现在带有一组 4 个相同配体和一对其他配体的 八面体络合物中，或 MA₄B₂。例如，在称为顺铂的正方形平面络合物二氨二氯合铂(II)的 顺式异构体中，氯化物配体彼此相邻并位于 分子的同一侧。在反式异构体中，也就是反铂中，氯化物配体位于分子的相对侧。具有两组三个相同配体的八面体络合物，或 MA₃B₃，可表现出另一种几何异构现象，称为面经式 异构。例如，在三氨三氯合钴(III)的面式异构体中，三个胺配体和三个氯配体 位于分子的相对侧，因此构成了八面体的 相对面。在经式异构体中，三个相同的配体 围绕中心金属原子形成一个弧形。另一类立体异构体定义为光学异构体。像我们的手一样，光学异构体是彼此的镜像，不可重叠。这些异构体，也称为对映异构体，被描述为手性的，它们在物理和化学性质上几乎没有区别。光学异构体的区别在于它们 与平面偏振光的相互作用。偏振光具有在 单个平面内振荡的电场矢量，当偏振光通过对映异构体溶液时，电场矢量旋转一定的量。例如，三(乙二胺)合钴(III)的一种对映异构体 可将平面向右旋转，称为右旋异构体 或 d 光学异构体。另一种对映异构体将平面向左旋转，称为左旋异构体，或 l 光学异构体。

立体异构

<h3>复合物中的异构 </h3>
 同分异构体是不同的化学物种，具有相同的化学公式。 
 过渡金属波群通常以几何同分异构体形式存在，在这种情况下，相同原子通过相同类型的键连接，但它们在空间中的方向不同。 在 CIS 中使用两种不同配体的协调复合物，并从感兴趣的配体同分异构体中转移位置。 例如，八面体 [Co (NH3) 4Cl2] + 离子在 CIS 配置中有两个同分异构体 (图 1) ，两个氯化物配体彼此相邻。 另一个同分异构体是 TRANS 配置，两个氯化物配体直接相互交叉。 
 <img alt="Image1" src="/files/ftp_upload/11459/11459_Image1.svg" style="text-align: center; height: 200px;" />
 图 1. [Co (H2O) 4Cl2]+ 的 CIS 和 Trans 同分异构体包含与同一金属离子相连的相同配体，但空间排列会导致这两种化合物的性质差异很大。 
 物质的不同几何同分异构体是不同的化学化合物。 尽管采用相同的公式，但它们展示的性质有所不同。 例如， [Co (NH3) 4Cl2] NO3 的两个同分异构体的颜色不同； CIS 表单为紫色，而 TRANS 表单为绿色。 此外，这些同分异构体具有不同的偶极点，溶解性和反应性。 作为空间中排列如何影响分子性质的示例，请考虑两个 [Co (NH3)4Cl2] NO3 同分异构体的极性。 请记住，分子或离子的极性由键偶极 (这是由于键原子的电负性差异) 及其空间中的排列决定。 在一个同分异构体中， CIS 氯化配体会导致分子一侧的电子密度高于另一侧，从而使其具有极性。 对于 TRANS 同分异构体，每个配体都直接与相同配体交叉，因此键偶极会被取消，分子为非极性。 
 另一种重要的同分异构体类型是光学同分异构体，或对等器，其中两个对象是彼此的精确镜像，但不能对齐以使所有部件匹配。 这意味着光学同分异构体是不可叠加的镜像。 这方面的一个典型例子是一对手，其中右手和左手是彼此的镜像，但不能叠加。 Optical 同分异构体在有机和生物化学中非常重要，因为生活系统通常包含一个特定的光学同分异构体，而不是另一个。 与几何同分异构体不同，光学同分异构体对具有几乎相同的性质 (沸点，极性，溶解度等)。 光同分异构体只有在影响极化光的方式以及它们与其他光同分异构体的反应方式上有所不同。 对于协调复合物，许多配位化合物 (如 [M(en)3]n+ [ 其中 Mn+ 是中心金属离子 (如铁 (III) 或钴 (II)))) 形成对聚物，如图 2 所示。 这两个同分异构体与其它光学同分异构体的反应会有所不同。 例如， 脱氧核糖核酸（DNA） 氦是光学同分异构体，自然形成的形式 (右手 脱氧核糖核酸（DNA）) 将仅绑定到 [M(en)3]n+ 的一个同分异构体，而不是另一个。 
 <img alt="Image2" src="/files/ftp_upload/11459/11459_Image_2.svg" style="text-align: center; height: 200px;" />
 图 2. 复数 [M(en)3]n+ (Mn+ = 金属离子， en = 乙二胺) 有一个不可叠加的镜像。 
 [Co (en) 2Cl2]+ 离子显示几何异构 (CIS/Ctrans) ，其 CIS 同分异构体以一对光学同分异构体的形式存在 (图 3)。 
 <img alt="Image3" src="/files/ftp_upload/11459/11459_Image_3.svg" style="text-align: center; height: 200px;" />
 图 3. 存在三种异构形式的 [Co (en) 2Cl2] +。 当氯处于 180° 角时形成的 Trans 同分异构体与性质 CIS 同分异构体有很大不同。 CIS 同分异构体的镜像图像形成一对光学同分异构体，其行为与其他对等反应不同。 
 本文改编自 <a href="https://openstax.org/books/chemistry-2e/pages/19-2-coordination-chemistry-of-transition-metals"> Openstax, 化学 2e, 第19.2章节：过渡金属的配位化学。 </a>

配合物中的异构现象  
异构体是具有相同化学式但结构不同的化学物种。  
过渡金属配合物常以几何异构体形式存在……

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Chemistry

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Transition Metals and Coordination Complexes

过渡金属与配位化合物

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KEY TERMS AND CONCEPTS

关键术语与概念

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 has two geometric isomers. In the cis form, the two chloride ligands are next to each other. In the trans form, the chlorides are directly across from one another.

These geometric isomers can behave very differently. The two isomers of [Co(NH3)4Cl2]NO3 have different colors. The cis form is violet, and the trans form is green. They also differ in dipole moment, solubility, and reactivity. Polarity depends on bond dipoles and how they are arranged in space. In the cis isomer, the chloride ligands create more electron density on one side of the ion, so it is polar. In the trans isomer, matching ligands are opposite each other, so the dipoles cancel and the ion is nonpolar.

Another important category is optical isomerism, also called enantiomerism. Enantiomers are mirror images that cannot be superposed, like right and left hands. These optical isomers are important in organic chemistry and biochemistry because living systems often use one optical isomer but not the other. Unlike geometric isomers, enantiomers usually have nearly identical boiling points, polarity, and solubility. They differ in how they affect polarized light and how they react with other optical isomers.

Many coordination compounds also form enantiomers. A complex such as [M(en)3]n+, where M is a metal ion like iron(III) or cobalt(II) and en is ethylenediamine, can exist as a pair of non-superposable mirror images. These forms react differently with other optical isomers. For example, DNA helices are optical isomers, and natural right-handed DNA binds to only one isomer of [M(en)3]n+.

Some complexes show both geometric and optical isomerism. The [Co(en)2Cl2]+ ion has cis and trans forms. Its trans isomer has the chlorines 180° apart and has very different properties from the cis form. The cis isomer also exists as a pair of optical isomers. These mirror-image forms behave the same in most ways, except when they react with other enantiomers.

Isomer Shapes in Coordination Complexes

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 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-octahedral-complexes

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-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

Explore cis, trans, and optical isomers in coordination complexes. See how ligand arrangement changes color, polarity, and reactivity in transition metal compounds.

Isomer Shapes in Coordination Complexes | JoVE K12

chemistry

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Isomer Shapes in Coordination Complexes

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