Name: metal ligand bonds in coordination complexes
Uploaded: 2020-09-24T15:06:53.000Z
Duration: 2 min 42 s
Channel: JoVE

配位錯体は、配位共有結合に関与する金属軌道に応じて、八面体、四面体、正方形平面などの異なる形状を示します。
価電子結合理論(VBT)は、金属のs、p、d軌道の混成に基づいてこれを説明しています。ハイブリダイゼーションは、充填された配位子軌道に配位できる等価エネルギーの空軌道を提供します。
八面体錯体、ヘキサフルオロコバルト(III)を考えてみましょう。Co3+イオンは、3d6 軌道と空いている 4s、4p、および 4d 軌道を部分的に充填しています。 
錯体を形成するために、金属の6つの空軌道、4s、4p、および2つの4dは、八面体の角を指している6つのsp3d2軌道に混成されます。ハイブリッド軌道は、6つのフッ化物基からの孤立電子対を受け入れて常磁性複合体を形成します。
ヘキサミンコバルト(III)では、アミノ基によってコバルトの3日電子が再配列し、空いている2つの3日軌道が作られます。 
これらは4s軌道と4p軌道と結合して6つのd2sp3混成軌道を生成し、それが6つのアンモニア基に協調して結合して反磁性錯体を形成します。 
次に、四面体錯体、テトラクロロニッケル酸について考えてみましょう。Ni2+ イオンは 3d8 の配置をしています。空の 4s 軌道と 4p 軌道は混成して、四面体の角を指す 4 つの sp3 軌道が得られます。 
ここでは、各塩化物基から1つずつ、4対の電子が混成軌道を占めて常磁性錯体を形成します。 
最後に、テトラクロロ白金酸のような正方形の平面複合体を調べます。Pt2+イオンはd8構成です。
塩化物基は、金属の3d電子を再配置させ、空軌道を作り出します。空いている軌道は、4s 軌道と 2 つの 4p 軌道と組み合わされて、正方形の角に向けられた 4 つの dsp2 ハイブリッド軌道が得られます。
塩化物配位子から電子対を受け入れると、反磁性錯体が形成されます。 
配位錯体は色や磁気的振る舞いが異なりますが、VBTでは、その電子スペクトルや温度による磁気的振る舞いの変化は説明できません。

原子価結合理論

Valence Bond Theory

配位化合物や錯体は、構成する金属原子・イオン・配位子によって、さまざまな色や形、磁気的挙動を示します。このような配位錯体の結合や構造を説明するために、ライナス・ポーリングは「混成」と「原子軌道の重なり」という概念を用いて、「原子価結合理論」（VBT）を提唱しました。VBTによると、中心となる金属原子やイオン（ルイス酸）が混成して、適切なエネルギーの空軌道を得ます。これらの軌道は、満たされた配位子軌道（ルイス塩基）から電子対を受け取り、金属-配位子間の配位共有結合を形成します。混成の種類と混成軌道の数によって、錯体の形状が決まります。
 <table style="margin-left: auto; margin-right: auto;">
 <tbody>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">幾何配置</td>
 <td style="color: black; border: 1px solid black; text-align: center;"> Hybridization  </td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">直線状</td>
 <td style="color: black; border: 1px solid black; text-align: center;">sp</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">四面体型</td>
 <td style="color: black; border: 1px solid black; text-align: center;">sp3</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;"> 平面四角形 </td>
 <td style="color: black; border: 1px solid black; text-align: center;">dsp2</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">八面体型</td>
 <td style="color: black; border: 1px solid black; text-align: center;"> d2sp3 or sp3d2 </td>
 </tr>
 </tbody>
 </table>
 四面体型錯体では、金属上の3つの空のp軌道と1つの空のs軌道が混成して4つのsp3混成軌道が形成されます。これが満たされた配位子の軌道と重なり、共有結合を形成します。同様に、八面体型錯体の場合は、中心の金属イオン上の空いている原子軌道を混ぜ合わせることで6つの混成軌道が作られる（d2sp3またはsp3d2混成化）。直線状の錯体の場合、1つのsと1つのp軌道が重なり、2つのsp混成軌道が形成されます。
 Inner and Outer Orbital Complexes
 近接する配位子場の強さは、中心金属イオン上の原子軌道の混成に影響を与えます。例えば、[Co(NH3)6]3+のような八面体型錯体の例を考えたいです。Co3+イオンは、3d軌道に6個の電子を持ち、4sおよび4p軌道が空いています。強い配位子場を形成するNH3配位子が入ると、不対になっている3d電子が再配置され、他の3d電子と電子対を形成します。これにより、2つの空の3d軌道が作られ、それが1つの4sと3つの4p軌道と組み合わさって、6つの等価なd2sp3混成軌道が形成されます。この6つの混成軌道は、アンモニア配位子の満たされた原子軌道と重なり、八面体の複合体を形成します。金属の内側のd（3d）軌道が混成に関与しているので、[Co(NH3)6]3+は内軌道錯体です。不対電子が存在しないため、反磁性を示し、低スピン錯体とも呼ばれます。
 <img alt="Image1" src="/files/ftp_upload/11460/11460_Image_1.png" style="text-align: center; height: 100px;" />
       [Co(F)6]3+のような別の八面体型錯体では、フッ化物イオンの配位子場が弱いため、金属の3d6電子は再配列しません。混成のための空軌道を提供するために、一番外側の空の4d軌道のうちの2つが、1つの4sと3つの4p軌道と結合して、6つの空の混成軌道を形成します。一番外側のd軌道を使うので、この混成はsp3d2混成と呼ばれ、この錯体は外軌道錯体と呼ばれます。また、不対電子が存在すると常磁性体となるため、これらの錯体は高スピン錯体とも呼ばれます。
 <img alt="Image2" src="/files/ftp_upload/11460/11460_Image_2.png" style="text-align: center; height: 100px;" />
 高スピンまたは外軌道錯体は、低スピンまたは内軌道錯体に比べて、（sp3d2 軌道のエネルギーが高いために）より不安定で安定性が低いです。

配位化合物および錯体は、金属原子/イオンおよび配位子に応じて、異なる色、形状、および磁気挙動を示します...

Education

JoVE Core

Chemistry

化学

Transition Metals and Coordination Complexes

遷移金属と配位錯体

transition-metals-and-coordination-complexes

KEY TERMS AND CONCEPTS

主要用語と概念

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 shape of the complex. A linear complex uses sp hybridization. A tetrahedral complex uses sp3 hybridization. A square planar complex uses dsp2 hybridization. Octahedral complexes use either d2sp3 or sp3d2 hybridization. In a tetrahedral complex, three vacant p orbitals and one vacant s orbital on the metal mix to make four sp3 hybrid orbitals. These overlap with filled ligand orbitals to form the bond. In a linear complex, one s orbital and one p orbital overlap to form two sp hybrid orbitals. Octahedral complexes form six hybrid orbitals from the vacant orbitals on the metal ion.

Ligand strength can change which orbitals the metal uses. A strong field ligand can force electrons to pair up, while a weak field ligand may not. The octahedral complex [Co(NH3)6]3+ is a good example. The Co3+ ion has six electrons in the 3d orbitals and vacant 4s and 4p orbitals. The NH3 ligands are strong field ligands, so the unpaired 3d electrons rearrange and pair. This creates two vacant 3d orbitals. These join with one 4s orbital and three 4p orbitals to make six d2sp3 hybrid orbitals. The ammonia ligands then overlap with these orbitals to form the octahedral complex.

Because the inner 3d orbitals take part in hybridization, [Co(NH3)6]3+ is an inner orbital complex. It has no unpaired electrons, so it is diamagnetic. It is also called a low spin complex. A similar octahedral complex, [Co(F)6]3+, behaves differently because fluoride is a weak field ligand. In that case, the 3d6 electrons do not rearrange. Two outer 4d orbitals combine with one 4s and three 4p orbitals to form six sp3d2 hybrid orbitals. Since outer d orbitals are used, this is an outer orbital complex. The unpaired electrons make it paramagnetic, so it is a high spin complex. High spin, or outer orbital, complexes are also more labile and less stable than low spin, inner orbital complexes.

Metal Ligand Bonds in Coordination Complexes

metal-ligand-bonds-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

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

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

Learn how valence bond theory explains metal-ligand bonding, hybridization, complex shape, and the difference between inner and outer orbital complexes.

Metal Ligand Bonds in Coordination Complexes | JoVE K12

chemistry

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Metal Ligand Bonds in Coordination Complexes

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