📚 A-Level AQA A2 Inorganic Chemistry: Transition Metals and Aqueous Ions | A-Level AQA A2 无机化学：过渡金属与水合离子
This A2 Inorganic Chemistry revision guide covers the key AQA International A-level topics on transition metals, complex ions, ligand substitution, isomerism, colour, variable oxidation states, catalysis and the characteristic reactions of metal aqua ions in aqueous solution. Each section gives the essential definitions, equations and observations you need for a topic test or exam question.
本 A2 无机化学复习指南涵盖 AQA International A-level 的关键主题：过渡金属、配离子、配体取代、异构现象、颜色、可变氧化态、催化作用以及金属水合离子在水溶液中的特征反应。每个小节都给出了主题测试或考试题目所需的基本定义、方程式和实验现象。
1. Transition Metal Definition and Electron Configurations | 过渡金属定义与电子构型
A transition metal is a d-block element that forms at least one stable ion with a partially filled d subshell. This definition excludes scandium and zinc: Sc forms only Sc³⁺ with a 3d⁰ configuration, while Zn forms only Zn²⁺ with a 3d¹⁰ configuration, so neither has a partially filled d subshell in its common ion.
过渡金属是指能形成至少一种具有部分填充 d 亚层的稳定离子的 d 区元素。这个定义排除了钪和锌：Sc 只形成 3d⁰ 构型的 Sc³⁺，而 Zn 只形成 3d¹⁰ 构型的 Zn²⁺，因此它们在常见离子中都没有部分填充的 d 亚层。
Chromium and copper show unusual electron configurations because a half-filled or completely filled 3d subshell gives extra stability. The ground-state configurations are Cr: [Ar] 3d⁵ 4s¹ and Cu: [Ar] 3d¹⁰ 4s¹, not the expected 3d⁴ 4s² and 3d⁹ 4s². When transition metals form positive ions, the 4s electrons are removed before the 3d electrons.
铬和铜表现出特殊的电子构型，因为半充满或全充满的 3d 亚层具有额外稳定性。基态构型为 Cr：[Ar] 3d⁵ 4s¹，Cu：[Ar] 3d¹⁰ 4s¹，而不是预期的 3d⁴ 4s² 和 3d⁹ 4s²。当过渡金属形成正离子时，4s 电子先于 3d 电子失去。
For example, iron has the electron configuration Fe: [Ar] 3d⁶ 4s². Loss of two 4s electrons gives Fe²⁺: [Ar] 3d⁶, and further loss of one 3d electron gives Fe³⁺: [Ar] 3d⁵.
例如，铁的电子构型为 Fe：[Ar] 3d⁶ 4s²。失去两个 4s 电子得到 Fe²⁺：[Ar] 3d⁶，再失去一个 3d 电子得到 Fe³⁺：[Ar] 3d⁵。
2. General Physical and Chemical Properties | 一般物理与化学性质
Transition metals typically have high melting and boiling points, high density and good electrical conductivity because the 3d and 4s electrons can participate in strong metallic bonding. They are much harder and denser than Group 1 and Group 2 metals such as sodium and magnesium.
过渡金属通常具有高熔点、高沸点、高密度和良好的导电性，因为 3d 和 4s 电子可以参与较强的金属键。它们比第 1 族和第 2 族金属如钠和镁更硬、密度更大。
Chemically, transition metals show variable oxidation states, form coloured compounds, act as catalysts, and readily form complex ions with ligands. These properties arise from the availability of partially filled d orbitals and the small energy differences between d-orbital arrangements.
在化学性质上，过渡金属表现出可变氧化态、形成有色化合物、可作催化剂，并且容易与配体形成配离子。这些性质来源于部分填充的 d 轨道以及不同 d 轨道排布之间的能量差很小。
3. Complex Formation and Ligands | 配合物形成与配体
A complex ion consists of a central metal ion surrounded by ligands. A ligand is a molecule or negative ion that donates a lone pair of electrons to the metal ion, forming a coordinate bond. Ligands therefore act as Lewis bases, while the central metal ion acts as a Lewis acid.
配离子由中心金属离子和围绕它的配体组成。配体是提供孤对电子给金属离子形成配位键的分子或负离子。因此配体作为路易斯碱，中心金属离子作为路易斯酸。
Common monodentate ligands are water H₂O:, ammonia :NH₃, chloride :Cl⁻ and cyanide :CN⁻. Each monodentate ligand forms one coordinate bond to the metal ion. Examples of complex ions are [Cu(H₂O)₆]²⁺, [Fe(CN)₆]³⁻ and [CoCl₄]²⁻.
常见的单齿配体有水 H₂O:、氨 :NH₃、氯离子 :Cl⁻ 和氰根 :CN⁻。每个单齿配体与金属离子形成一个配位键。配离子的例子有 [Cu(H₂O)₆]²⁺、[Fe(CN)₆]³⁻ 和 [CoCl₄]²⁻。
Bidentate ligands such as ethane-1,2-diamine NH₂CH₂CH₂NH₂ and ethanedioate C₂O₄²⁻ can form two coordinate bonds per ligand. They form more stable complexes through the chelate effect, which is mainly entropy driven.
双齿配体如乙二胺 NH₂CH₂CH₂NH₂ 和乙二酸根 C₂O₄²⁻ 每个配体能形成两个配位键。它们通过螯合效应形成更稳定的配合物，该效应主要由熵驱动。
4. Coordination Number and Shapes | 配位数与空间构型
The coordination number is the number of coordinate bonds from ligands to the central metal ion. The most common coordination number is six, giving an octahedral shape with bond angles of 90°. Examples include [Cu(H₂O)₆]²⁺ and [Fe(CN)₆]³⁻.
配位数是指配体与中心金属离子形成的配位键数目。最常见的配位数是 6，形成八面体构型，键角为 90°。例子有 [Cu(H₂O)₆]²⁺ 和 [Fe(CN)₆]³⁻。
A coordination number of four can give either a tetrahedral shape with bond angles of 109.5°, as in [CoCl₄]²⁻, or a square planar shape with bond angles of 90°, as in cisplatin [Pt(NH₃)₂Cl₂]. A coordination number of two gives a linear shape, as in [Ag(NH₃)₂]⁺ with a bond angle of 180°.
配位数为 4 时，既可以形成键角为 109.5° 的四面体构型，如 [CoCl₄]²⁻，也可以形成键角为 90° 的平面正方形构型，如顺铂 [Pt(NH₃)₂Cl₂]。配位数为 2 时形成直线形，如 [Ag(NH₃)₂]⁺，键角为 180°。
5. Isomerism in Complex Ions | 配离子的异构现象
Square planar complexes of the type [Pt(NH₃)₂Cl₂] show cis-trans isomerism. In the cis isomer, the two identical ligands are adjacent; in the trans isomer, they are opposite each other. Cisplatin is the cis isomer and is an important anticancer drug, while the trans isomer does not show the same biological activity.
[Pt(NH₃)₂Cl₂] 型平面正方形配合物表现出顺反异构。在顺式异构体中，两个相同配体相邻；在反式异构体中，它们相对。顺铂是顺式异构体，是一种重要的抗癌药物，而反式异构体不具有相同的生物活性。
Octahedral complexes with four monodentate ligands of one type and two of another, such as [Co(NH₃)₄Cl₂]⁺, also show cis-trans isomerism. Octahedral complexes with three bidentate ligands, such as [Ni(NH₂CH₂CH₂NH₂)₃]²⁺, can show optical isomerism because the two forms are non-superimposable mirror images.
含四个同种单齿配体和两个另一种配体的八面体配合物，如 [Co(NH₃)₄Cl₂]⁺，也表现出顺反异构。含三个双齿配体的八面体配合物，如 [Ni(NH₂CH₂CH₂NH₂)₃]²⁺，可以表现出旋光异构，因为两种形式是互为不能重叠的镜像。
6. Colour and d-d Transitions | 颜色与 d-d 跃迁
In an isolated transition metal ion, the five d orbitals have the same energy. When ligands surround the ion in an octahedral complex, the d orbitals split into two groups: a higher-energy set and a lower-energy set. The energy gap between these sets is called ΔE.
在孤立的过渡金属离子中，五个 d 轨道能量相同。当配体在八面体配合物中围绕离子时，d 轨道分裂成两组：一组能量较高，一组能量较低。这两组之间的能量差称为 ΔE。
Visible light can promote an electron from the lower-energy d orbitals to the higher-energy d orbitals. This is called a d-d transition. The frequency absorbed is related to ΔE by the equation:
可见光可使电子从能量较低的 d 轨道跃迁到能量较高的 d 轨道。这称为 d-d 跃迁。吸收的频率与 ΔE 的关系为：
ΔE = hν
where h is the Planck constant and ν is the frequency of light absorbed. The colour observed is the complementary colour of the light absorbed. For example, [Cu(H₂O)₆]²⁺ absorbs mainly red-orange light and therefore appears blue.
其中 h 是普朗克常数，ν 是吸收光的频率。观察到的颜色是所吸收光的互补色。例如，[Cu(H₂O)₆]²⁺ 主要吸收红橙色光，因此呈现蓝色。
The size of ΔE, and therefore the colour, depends on the identity of the metal ion, its oxidation state and the type of ligand. A change in ligand or oxidation state changes the colour because it changes ΔE.
ΔE 的大小以及因此产生的颜色取决于金属离子的种类、氧化态和配体类型。配体或氧化态的改变会改变颜色，因为这会改变 ΔE。
7. Variable Oxidation States and Redox | 可变氧化态与氧化还原
Transition metals show a wide range of oxidation states because the 3d and 4s electrons can be lost in different numbers. Common examples are iron in +2 and +3 states, chromium in +3 and +6 states, and manganese in +2, +4, +6 and +7 states.
过渡金属表现出多种氧化态，因为 3d 和 4s 电子可以以不同数目失去。常见的例子有铁 +2 和 +3 态，铬 +3 和 +6 态，以及锰 +2、+4、+6 和 +7 态。
Manganate(VII), MnO₄⁻, is a powerful oxidising agent in acidic solution and is widely used in redox titrations. It oxidises Fe²⁺ to Fe³⁺ while being reduced to Mn²⁺:
高锰酸根 MnO₄⁻ 在酸性溶液中是强氧化剂，广泛用于氧化还原滴定。它将 Fe²⁺ 氧化为 Fe³⁺，自身被还原为 Mn²⁺：
MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺
The endpoint of a manganate(VII) titration is detected by the first permanent pale pink colour from excess MnO₄⁻. Dichromate(VI), Cr₂O₇²⁻, can also oxidise Fe²⁺ in acidic solution, changing from orange to green as Cr³⁺ is formed.
高锰酸根滴定的终点通过过量的 MnO₄⁻ 产生的首次持续淡粉红色来检测。重铬酸根 Cr₂O₇²⁻ 也能在酸性溶液中氧化 Fe²⁺，溶液由橙色变为绿色，因为生成了 Cr³⁺。
8. Catalytic Behaviour | 催化行为
Transition metals and their compounds are important heterogeneous catalysts because they can use variable oxidation states to adsorb reactants, weaken bonds and provide an alternative reaction pathway with lower activation energy. Iron is the catalyst in the Haber process for ammonia synthesis, and vanadium(V) oxide V₂O₅ catalyses the contact process oxidation of SO₂ to SO₃.
过渡金属及其化合物是重要的非均相催化剂，因为它们可以利用可变氧化态吸附反应物、削弱化学键并提供活化能较低的另一条反应路径。铁是合成氨哈伯法中的催化剂，五氧化二钒 V₂O₅ 催化接触法中 SO₂ 氧化为 SO₃。
Nickel catalyses the hydrogenation of alkenes, and manganese(IV) oxide catalyses the decomposition of hydrogen peroxide. Homogeneous catalysis also occurs when Fe²⁺ and Fe³⁺ ions catalyse the reaction between iodide ions and peroxodisulfate ions, S₂O₈²⁻, by cycling between oxidation states.
镍催化烯烃加氢，二氧化锰催化过氧化氢分解。均相催化也发生在 Fe²⁺ 和 Fe³⁺ 离子催化碘离子与过二硫酸根 S₂O₈²⁻ 的反应中，其通过在氧化态之间循环而起作用。
9. Reactions of Hexaaqua Ions in Solution | 六水合离子在溶液中的反应
In aqueous solution, many transition metal ions exist as octahedral hexaaqua complexes. When a few drops of sodium hydroxide solution are added, metal hydroxide precipitates form. Fe²⁺ gives a green precipitate of iron(II) hydroxide, Fe(OH)₂, which turns brown at the surface on standing due to oxidation to Fe(OH)₃.
在水溶液中，许多过渡金属离子以八面体六水合配合物形式存在。加入几滴氢氧化钠溶液时，会生成金属氢氧化物沉淀。Fe²⁺ 生成绿色的氢氧化亚铁 Fe(OH)₂ 沉淀，放置时表面因氧化为 Fe(OH)₃ 而变为棕色。
Fe³⁺ gives a reddish-brown precipitate of iron(III) hydroxide, Fe(OH)₃, and Cu²⁺ gives a pale blue precipitate of copper(II) hydroxide, Cu(OH)₂. These reactions can be written as:
Fe³⁺ 生成红棕色氢氧化铁 Fe(OH)₃ 沉淀，Cu²⁺ 生成淡蓝色氢氧化铜 Cu(OH)₂ 沉淀。这些反应可写为：
[Fe(H₂O)₆]³⁺ + 3OH⁻ → Fe(OH)₃ + 6H₂O
[Cu(H₂O)₆]²⁺ + 2OH⁻ → Cu(OH)₂ + 6H₂O
With excess ammonia, copper(II) hydroxide redissolves to form a deep blue solution of the tetraammine complex, [Cu(NH₃)₄(H₂O)₂]²⁺. With excess NaOH, copper(II) hydroxide does not dissolve, but aluminium hydroxide, which is amphoteric, does dissolve to give [Al(OH)₄]⁻.
加入过量氨水时，氢氧化铜重新溶解，形成深蓝色的四氨合铜配离子 [Cu(NH₃)₄(H₂O)₂]²⁺。加入过量 NaOH 时，氢氧化铜不溶解，但两性的氢氧化铝会溶解，生成 [Al(OH)₄]⁻。
10. Acidity and Amphoteric Hydroxides | 酸性与两性氢氧化物
Hexaaqua metal ions can act as Brønsted-Lowry acids in water. The metal ion polarises one O-H bond in a coordinated water molecule, weakening it and allowing the release of H⁺. The higher the charge density of the metal ion, the stronger the acidity.
六水合金属离子在水中可以作为布朗斯特-劳里酸。金属离子使配位水分子中的 O-H 键极化，削弱该键并释放 H⁺。金属离子的电荷密度越高，酸性越强。
Therefore M³⁺ aqua ions are more acidic than M²⁺ aqua ions. For example, [Fe(H₂O)₆]³⁺ reacts reversibly with water:
因此 M³⁺ 水合离子比 M²⁺ 水合离子酸性更强。例如，[Fe(H₂O)₆]³⁺ 与水发生可逆反应：
[Fe(H₂O)₆]³⁺ + H₂O ⇌ [Fe(H₂O)₅(OH)]²⁺ + H₃O⁺
This acidity explains why FeCl₃ and AlCl₃ solutions have pH values below 7. When carbonate ions are added to M³⁺ aqua ions, the hydrogencarbonate intermediate decomposes and carbon dioxide is released, together with a metal hydroxide precipitate.
这种酸性解释了为什么 FeCl₃ 和 AlCl₃ 溶液的 pH 值小于 7。当向 M³⁺ 水合离子中加入碳酸根时，碳酸氢根中间体分解，释放出二氧化碳，同时生成金属氢氧化物沉淀。
Aluminium hydroxide and chromium(III) hydroxide are amphoteric: they dissolve in excess strong base. Al(OH)₃ dissolves in excess NaOH to form [Al(OH)₄]⁻, and Cr(OH)₃ dissolves to form [Cr(OH)₆]³⁻, giving a green solution.
氢氧化铝和氢氧化铬(III) 具有两性：它们能溶于过量强碱。Al
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