Why Coordination Compounds Are Coloured | 配合物呈现颜色的原因

📚 Why Coordination Compounds Are Coloured | 配合物呈现颜色的原因

The striking colours of transition-metal coordination compounds are one of the most visual phenomena in chemistry. When white light passes through a solution containing a complex such as [Cu(H₂O)₆]²⁺, the solution appears blue because certain wavelengths of visible light are absorbed and the remaining light is transmitted. Understanding why this happens requires a careful look at electronic structure, d-orbital splitting, and the interaction between metal ions and ligands.

过渡金属配合物的鲜艳颜色是化学中最直观的现象之一。当白光穿过含有如 [Cu(H₂O)₆]²⁺ 这类配合物的溶液时，溶液呈现蓝色，因为某些波长的可见光被吸收，其余光被透过。要理解其原因，需要仔细考察电子结构、d 轨道分裂以及金属离子与配体之间的相互作用。

1. Electronic Transitions in d Orbitals | d 轨道中的电子跃迁

Coordination compounds of transition metals often possess partially filled d orbitals. When visible light strikes the complex, an electron can be excited from a lower-energy d orbital to a higher-energy d orbital. This process is called a d-d transition. The energy difference between the two sets of d orbitals must match the energy of a photon of visible light for colour to be observed.

过渡金属配合物通常具有部分填充的 d 轨道。当可见光照射配合物时，电子可以从较低能量的 d 轨道跃迁到较高能量的 d 轨道，这一过程称为 d-d 跃迁。两组 d 轨道之间的能量差必须与可见光光子的能量相匹配，才能产生颜色。

ΔE = hν = hc / λ

The equation above links the energy gap ΔE to the wavelength λ of absorbed light. If ΔE falls in the range 1.7–3.1 V, which corresponds to visible light, the compound will appear coloured.

上述方程将能隙 ΔE 与吸收光的波长 λ 联系起来。如果 ΔE 落在 1.7–3.1 eV 范围内，即对应可见光，那么化合物就会呈现颜色。

2. Degenerate d Orbitals in a Free Ion | 自由离子中的简并 d 轨道

In an isolated transition-metal ion, all five d orbitals (d_xy, d_xz, d_yz, d_x²−y², d_z²) have the same energy. They are said to be degenerate. Because they are equal in energy, no visible light can be absorbed by a d-d transition in a free ion — there is no energy gap to overcome.

在孤立的过渡金属离子中，五个 d 轨道（d_xy、d_xz、d_yz、d_x²−y²、d_z²）具有相同的能量，称为简并轨道。由于能量相等，在自由离子中不存在能隙，因此无法通过 d-d 跃迁吸收可见光。

This explains why gaseous metal ions do not show intense visible colour from d-d transitions alone. It is only when ligands approach and break the symmetry that the d orbitals become split.

这解释了为什么单独的气态金属离子不会因 d-d 跃迁而呈现强烈的可见颜色。只有当配体接近并打破对称性时，d 轨道才会发生分裂。

3. Octahedral Crystal Field Splitting | 八面体晶体场分裂

In an octahedral complex, six ligands approach along the x, y, and z axes. The d orbitals pointing along these axes — d_x²−y² and d_z² — experience stronger repulsion from the ligand electrons. These orbitals are raised in energy and are called the e_g set.

在八面体配合物中，六个配体沿 x、y、z 轴方向接近。指向这些轴向的 d 轨道——d_x²−y² 和 d_z²——受到配体电子更强的排斥作用，因此能量升高，称为 e_g 轨道组。

The three orbitals that point between the axes — d_xy, d_xz, d_yz — are less affected. They are lowered in energy and are called the t_2g set. The energy gap between the two sets is the crystal field splitting parameter, Δ_oct.

指向轴间区域的三个轨道——d_xy、d_xz、d_yz——受到的影响较小，能量降低，称为 t_2g 轨道组。两组轨道之间的能量差就是晶体场分裂参数 Δ_oct。

Δ_oct = Energy(e_g) − Energy(t_2g)

For most first-row transition metal complexes, Δ_oct corresponds to the energy of visible-light photons. When an electron is promoted from t_2g to e_g, a specific wavelength of light is absorbed.

对于大多数第一过渡系金属配合物，Δ_oct 对应于可见光光子的能量。当电子从 t_2g 跃迁到 e_g 时，会吸收特定波长的光。

4. Ligand Strength and the Spectrochemical Series | 配体强度与光谱化学序列

Different ligands cause different amounts of d-orbital splitting. Strong-field ligands, such as CN⁻ and CO, create a large Δ_oct. Weak-field ligands, such as I⁻ and Br⁻, create a small Δ_oct. The order of ligands by their splitting ability is known as the spectrochemical series.

不同的配体引起的 d 轨道分裂程度不同。强场配体如 CN⁻ 和 CO 产生较大的 Δ_oct；弱场配体如 I⁻ 和 Br⁻ 产生较小的 Δ_oct。按分裂能力排列的顺序称为光谱化学序列。

Weak field: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O
弱场：I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O

Intermediate: NH₃ < ethylenediamine < NO₂⁻
中等：NH₃ < 乙二胺 < NO₂⁻

Strong field: CN⁻ ≈ CO
强场：CN⁻ ≈ CO

Because Δ_oct changes with the ligand, the wavelength of light absorbed also changes. This directly alters the perceived colour of the coordination compound.

由于 Δ_oct 随配体变化，吸收光的波长也随之变化，这会直接改变配合物呈现的颜色。

5. Complementary Colours and the Colour Wheel | 互补色与色环

A solution appears coloured because it transmits or reflects the wavelengths that are not absorbed. The colour observed is the complementary colour of the absorbed light. The colour wheel is a convenient tool for predicting the observed colour.

溶液呈现颜色，是因为它透射或反射了未被吸收的波长。观察到的颜色是被吸收光的互补色。色环是预测观察颜色的便捷工具。

For example, if a complex absorbs orange light (about 600 nm), it will appear blue. If it absorbs green light (about 520 nm), it will appear red or purple.

例如，如果配合物吸收橙光（约 600 nm），它将呈现蓝色；如果吸收绿光（约 520 nm），它将呈现红色或紫色。

Absorbed colour → Observed colour: Red→Green, Orange→Blue, Yellow→Violet, Green→Red, Blue→Orange, Violet→Yellow

This complementary-colour relationship is central to understanding why a small change in ligand or metal ion can cause a dramatic change in colour.

这种互补色关系是理解为什么配体或金属离子的微小变化能引起颜色巨大变化的核心。

6. How Δ_oct Affects Absorbed Wavelength | Δ_oct 如何影响吸收波长

A larger Δ_oct means a greater energy gap. According to ΔE = hc / λ, a larger ΔE corresponds to a shorter wavelength of absorbed light. Thus strong-field ligands shift absorption to higher energy (shorter wavelength) light.

Δ_oct 越大意味着能隙越大。根据 ΔE = hc / λ，ΔE 越大，吸收光的波长越短。因此强场配体会使吸收向更高能量（更短波长）的光移动。

Weak ligand (small Δ_oct): absorbs red/long wavelength light; appears green or blue.
弱配体（小 Δ_oct）：吸收红光/长波长光；呈现绿色或蓝色。

Strong ligand (large Δ_oct): absorbs violet/short wavelength light; appears yellow or orange.
强配体（大 Δ_oct）：吸收紫光/短波长光；呈现黄色或橙色。

This trend is observed when comparing [Ni(H₂O)₆]²⁺, which is green, with [Ni(en)₃]²⁺, which is violet. The chelating ligand en creates a larger Δ_oct, shifting the absorption to shorter wavelengths.

比较 [Ni(H₂O)₆]²⁺（绿色）和 [Ni(en)₃]²⁺（紫色）可以观察到这一趋势。螯合配体 en 产生更大的 Δ_oct，使吸收向更短波长移动。

7. Effect of Oxidation State on Colour | 氧化态对颜色的影响

The oxidation state of the central metal ion influences Δ_oct because a higher positive charge draws the ligands closer and increases the electrostatic interaction. Higher oxidation states therefore tend to produce larger crystal field splitting.

中心金属离子的氧化态影响 Δ_oct，因为更高的正电荷使配体靠得更近，增强了静电相互作用。因此，更高的氧化态通常产生更大的晶体场分裂。

Fe²⁺ complexes are often pale green or pale blue.
Fe²⁺ 配合物通常呈淡绿色或淡蓝色。

Fe³⁺ complexes are often yellow or orange.
Fe³⁺ 配合物通常呈黄色或橙色。

This is because Fe³⁺ has a higher charge density, leading to a larger Δ_oct and absorption at shorter wavelengths. The same metal ion in different oxidation states can thus produce completely different colours.

这是因为 Fe³⁺ 具有更高的电荷密度，导致更大的 Δ_oct 和更短波长的吸收。同一金属离子处于不同氧化态时，可能产生完全不同的颜色。

8. Effect of Coordination Number and Geometry | 配位数与几何构型的影响

Coordination number and geometry also affect d-orbital splitting. An octahedral complex (coordination number 6) has a different splitting pattern from a tetrahedral complex (coordination number 4).

配位数和几何构型也会影响 d 轨道分裂。八面体配合物（配位数 6）的分裂模式与四面体配合物（配位数 4）不同。

In a tetrahedral complex, the splitting parameter Δ_tetr is approximately 4/9 of Δ_oct for the same ligands and metal ion. As a result, tetrahedral complexes often absorb lower-energy light and appear more intensely coloured in the visible region.

在四面体配合物中，对于相同的配体和金属离子，分裂参数 Δ_tetr 约为 Δ_oct 的 4/9。因此，四面体配合物通常吸收较低能量的光，并在可见光区呈现更强烈的颜色。

Δ_tetr ≈ (4/9) Δ_oct

Square-planar complexes, such as those of Pt²⁺ and Ni²⁺, have their own unique splitting patterns, which also produce distinctive colours.

平面正方形配合物，如 Pt²⁺ 和 Ni²⁺ 的配合物，具有独特的劈裂模式，也会产生各自特有的颜色。

9. Factors That Make a Complex Coloured or Colourless | 使配合物有色或无色的因素

Not all transition-metal complexes are coloured. The presence of colour requires at least one electron in a lower d orbital to be excited into a vacant higher d orbital. If the d subshell is completely empty or completely full, d-d transitions cannot occur.

并非所有过渡金属配合物都有颜色。产生颜色要求至少有一个电子可以从较低 d 轨道被激发到空的较高 d 轨道。如果 d 亚层完全空或完全满，d-d 跃迁就无法发生。

Sc³⁺ has electronic configuration [Ar] 3d⁰ — no d electrons, so its complexes are colourless.
Sc³⁺ 的电子构型为 [Ar] 3d⁰——没有 d 电子，所以其配合物无色。

Zn²⁺ has electronic configuration [Ar] 3d¹⁰ — all d orbitals filled, so its complexes are colourless.
Zn²⁺ 的电子构型为 [Ar] 3d¹⁰——所有 d 轨道均填满，所以其配合物无色。

For d¹ to d⁹ configurations, at least one d-d transition is possible, and if the energy gap corresponds to visible light, the complex will be coloured.

对于 d¹ 到 d⁹ 构型，至少存在一种可能的 d-d 跃迁，只要能隙对应可见光，配合物就会有颜色。

10. Classic Examples | 经典实例

The table below summarises how different ligands and oxidation states affect the colour of copper complexes.

下表总结了不同配体和氧化态如何影响铜配合物的颜色。

Complex	Ligand	Δ_oct trend	Observed colour
[Cu(H₂O)₆]²⁺	H₂O	medium	blue
[Cu(NH₃)₄(H₂O)₂]²⁺	NH₃	larger	deep blue
[CuCl₄]²⁻	Cl⁻	smaller	yellow-green

These examples show that changing a ligand from H₂O to NH₃ increases Δ_oct, shifting the absorption from red-orange light toward green light, so the colour changes from pale blue to deep blue.

这些例子表明，将配体从 H₂O 换成 NH₃ 会增加 Δ_oct，使吸收光从红橙色向绿色移动，因此颜色从淡蓝色变为深蓝色。

11. Charge-Transfer Transitions | 电荷迁移跃迁

Some intensely coloured complexes owe their colour not to d-d transitions but to charge-transfer transitions. In these transitions, an electron moves from a ligand-based orbital to a metal-based orbital, or vice versa. These absorptions are usually very strong and occur at high energies.

一些颜色非常强烈的配合物并非通过 d-d 跃迁显色，而是通过电荷迁移跃迁。在这类跃迁中，电子从配体轨道迁移到金属轨道，或反方向迁移。这些吸收通常非常强，且发生在高能量区域。

For example, the permanganate ion MnO₄⁻ is deep purple because of oxygen-to-manganese charge transfer, despite Mn being in the d⁰ configuration. The dramatic colour is not due to d-d transitions but to electron transfer between the ligand and the metal.

例如，高锰酸根离子 MnO₄⁻ 呈深紫色，是因为氧到锰的电荷迁移，尽管 Mn 为 d⁰ 构型。这种强烈的颜色并非来自 d-d 跃迁，而是来自配体与金属之间的电子转移。

12. Summary and Key Exam Points | 总结与考点

To explain why coordination compounds are coloured, you must connect electronic configuration, ligand field splitting, and the absorption of visible light. The essential idea is that ligands split the degenerate d orbitals into different energy levels, and the energy gap Δ determines which wavelengths are absorbed.

要解释配合物为什么呈现颜色，必须将电子构型、配体场分裂和可见光吸收联系起来。核心思想是配体将简并的 d 轨道分裂为不同能级，能隙 Δ 决定了哪些波长的光被吸收。

Only transition-metal ions with partially filled d subshells can undergo d-d transitions.
只有部分填充 d 亚层的过渡金属离子才能发生 d-d 跃迁。

The magnitude of Δ depends on the metal ion, its oxidation state, the ligand, and the geometry.
Δ 的大小取决于金属离子、其氧化态、配体以及几何构型。

The colour observed is complementary to the light absorbed.
观察到的颜色是被吸收光的互补色。

Strong-field ligands give larger Δ and absorb shorter wavelengths; weak-field ligands give smaller Δ and absorb longer wavelengths.
强场配体产生更大的 Δ，吸收更短波长；弱场配体产生更小的 Δ，吸收更长波长。

d⁰ and d¹⁰ systems do not show d-d transitions and are generally colourless.
d⁰ 和 d¹⁰ 体系不显示 d-d 跃迁，通常无色。

In an IB Chemistry HL exam, be prepared to draw d-orbital splitting diagrams, use the spectrochemical series to rank Δ, and predict colour changes when ligands are substituted. A strong answer always links Δ to the energy/wavelength of visible light and to complementary colours.

在 IB 化学 HL 考试中，要准备好绘制 d 轨道分裂图、使用光谱化学序列排列 Δ，并预测配体被取代时的颜色变化。一个优秀的答案必须将 Δ 与可见光的能量/波长及互补色联系起来。

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Why Coordination Compounds Are Coloured | 配合物呈现颜色的原因