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

結晶場理論は、四面体および正方形の平面遷移金属錯体を、八面体錯体におけるこの理論の適用と同様の方法でモデル化するために使用できます。
たとえば、四面体テトラクロロニッケル酸(II)イオンをモデル化するには、各塩化物配位子を負の点電荷に置き換えて、四面体結晶場を生成します。
この場の影響により、dxy、dyz、およびdxz軌道は、dx2−y2およびdz2軌道よりもエネルギーが高くなります。これは、四面体結晶場とdxy、dyz、およびdxz軌道との間の相互作用が強いことに起因しています。 
高エネルギー軌道はt2対称性を持ち、t2 集合と呼ばれ、低エネルギー軌道は e 対称性を持ち、e セットを構成します。
八面体錯体におけるd軌道の分裂と比較して、四面体錯体における軌道の相対エネルギーは逆転し、結晶場分裂エネルギー、すなわちΔテトは低くなる。
テトラシアノニッケル酸(II)イオンのような正方形の平面錯体では、すべての配位子がxy平面にあります。ここでは、シアン化物配位子を負の点電荷に置き換えることにより、正方形の平面結晶場が得られます。
この磁場の影響下で、金属イオンのd軌道は4つの異なるエネルギー準位に分割されます。
ここで、dx2−y2軌道は最高エネルギー軌道であり、配位子電荷を直接指すローブがあります。dxy軌道は次にエネルギーがあり、ローブは配位子の電荷と同じ平面にあります。 
dz2軌道は、dz2軌道とxy平面の結晶場との間にわずかな重なりがあるため、エネルギーがさらに低い。軌道の最も低いエネルギーセットであるdxzとdyzは、結晶場との相互作用が比較的最小限です。
正方形平面錯体における結晶場分割エネルギー(Δsp)は、最高エネルギー軌道d x2−y2と最低エネルギー軌道d yzおよびdxzとの間のエネルギー差として定義されます。
すべての錯体で同じ金属イオン分子と配位子分子を仮定すると、Δtet、Δsp、および Δoct の比率は 0.44:1.7:1 です。

結晶場理論 - 四面体及び平面四辺型錯体

Crystal Field Theory - Tetrahedral and Square Planar Complexes

四面体型錯体
 結晶場理論（CFT）は、八面体以外の幾何学的構造を持つ分子にも適用できます。八面体型錯体では、dx2−y2とdz2の軌道のローブが直接配位子の方を向いています。四面体型錯体の場合、d軌道はそのままですが、軸の間に4つの配位子だけが配置されています。どの軌道も四面体の配位子の方を直接向いていません。しかし、dx2− y2とdz2の軌道（直交軸に沿った軌道）は、dxy、dxz、dyzの軌道よりも配位子との重なりが少ません。八面体の場合との類似性から、四面体結晶場におけるd軌道のエネルギー図は、Figure 1のように予測できます。混乱を避けるため、八面体のeg表記は四面体のe表記となり、八面体のt2g表記はt2表記となります。
 <img alt="Image1" src="/files/ftp_upload/11462/11462_Image_1.png" style="text-align: center; height: 350px;" />
 Figure 1.八面体結晶場と四面体結晶場での金属イオンのd軌道の分裂の様子。八面体結晶場と比較して、四面体結晶場での分裂パターンは反転しています。八面体型錯体の結晶場分割エネルギー（ Δoct ）は、四面体型錯体の結晶場分割エネルギー（ Δtet ）よりも大きいです。
 CFTは静電反発に基づいているため、配位子に近い軌道は不安定になり、他の軌道に比べてエネルギーが上昇します。重なりが少ないので八面体型錯体よりも結晶場分裂は小さく、結晶場の分裂エネルギー、つまり Δtetは通常小さくなります。
 平面四角形型錯体
 もう1つの一般的な幾何学形状は平面四角形です。正方形の平面構造は、一対のtrans配位子を取り除いた八面体型構造と考えることができます。除去された配位子はz軸上にあると仮定します。これにより、d軌道の分布が変化し、z軸上またはその近くの軌道はより安定化し、xまたはy軸上またはその近くの軌道はより不安定化します。この結果、八面体型におけるt2gとegの縮退が分裂し、より複雑な分裂パターンが得られます。(Figure 2)
 <img alt="Image2" src="/files/ftp_upload/11462/11462_Image_2.png" style="text-align: center; height: 350px;" />
 Figure 2. 平面四角形型の結晶場における軌道のt2g軌道とeg軌道の分裂。平面四角形型錯体の結晶場分裂エネルギー（ Δsp ）は、 Δoct よりも大きいです。
 上記の文章は以下から引用しました。<a href="https://openstax.org/books/chemistry-2e/pages/19-3-spectroscopic-and-magnetic-properties-of-coordination-compounds">Openstax, Chemistry 2e, Section 19.3: </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">pectroscopic and Magnetic Properties of Coordination Compounds</a>.

四面体複合体
結晶場理論(CFT)は、八面体以外の形状の分子に適用できます。八面体複合体では、...

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Transition Metals and Coordination Complexes

遷移金属と配位錯体

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KEY TERMS AND CONCEPTS

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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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