Complex Ions

A complex ion consists of a central metal ion surrounded by ligands. Ligands are molecules or ions that donate a lone pair of electrons to the metal ion, forming a coordinate (dative covalent) bond. The study of complex ions is a central topic in transition metal chemistry and appears extensively at A-Level.

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Key Terminology

• Central metal ion: The transition metal ion at the centre of the complex (e.g., Fe³⁺, Cu²⁺, Co²⁺)
• Ligand: A species (molecule or ion) with at least one lone pair of electrons that it donates to the central metal ion
• Coordination number: The total number of coordinate bonds formed between the ligands and the central metal ion
• Complex ion: The central metal ion together with its ligands, usually carrying an overall charge

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Types of Ligand

Ligands are classified by the number of coordinate bonds they can form with the central metal ion:

Monodentate Ligands

These donate one lone pair and form one coordinate bond per ligand:

Ligand	Formula	Lone Pair Donor	Charge
Water	H₂O	O	0
Ammonia	NH₃	N	0
Chloride	Cl⁻	Cl	−1
Cyanide	CN⁻	C	−1
Hydroxide	OH⁻	O	−1
Thiocyanate	SCN⁻	S or N	−1
Carbon monoxide	CO	C	0

Exam note: Thiocyanate (SCN⁻) is an ambidentate ligand — it can bond through either S or N. When it bonds through S, it is thiocyanato; through N, it is isothiocyanato. At A-Level, just know it can bond through either end.

Bidentate Ligands

These donate two lone pairs from different atoms and form two coordinate bonds per ligand:

Ligand	Formula	Donor Atoms
Ethane-1,2-diamine (en)	H₂NCH₂CH₂NH₂	Two N atoms
Ethanedioate (oxalate)	C₂O₄²⁻	Two O atoms

The name "bidentate" means "two-toothed" — the ligand grips the metal ion at two points, like a crab's claw.

Multidentate (Polydentate) Ligands

These donate more than two lone pairs and form multiple coordinate bonds:

• EDTA⁴⁻ (ethylenediaminetetraacetate) is a hexadentate ligand — it forms six coordinate bonds with a single metal ion (through two N atoms and four O atoms)
• Haemoglobin contains a porphyrin ring that acts as a tetradentate ligand, coordinating to Fe²⁺ through four nitrogen atoms

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Shapes of Complex Ions

The shape of a complex depends on the coordination number:

Coordination Number 6 — Octahedral

Six ligands arranged around the metal ion at 90° bond angles. This is the most common geometry.

Examples:

• [Fe(H₂O)₆]²⁺ (hexaaquairon(II)) — pale green
• [Cr(NH₃)₆]³⁺ (hexaamminechromium(III)) — yellow
• [Co(en)₃]²⁺ (three bidentate en ligands, coordination number still 6)
• [Fe(EDTA)]²⁻ (one hexadentate EDTA, coordination number 6)
• [Cu(NH₃)₄(H₂O)₂]²⁺ (mixed ligands, still octahedral with coordination number 6)

Coordination Number 4 — Tetrahedral

Four ligands arranged at approximately 109.5° bond angles. Common with larger ligands such as Cl⁻.

Examples:

• [CoCl₄]²⁻ (tetrachlorocobaltate(II)) — blue
• [CuCl₄]²⁻ (tetrachlorocuprate(II)) — yellow-green
• [FeCl₄]⁻ (tetrachloroferrate(III)) — yellow

Coordination Number 4 — Square Planar

Four ligands arranged in a plane at 90° bond angles. Less common but important for certain metals (especially Pt²⁺ and Ni²⁺ with strong-field ligands).

Examples:

• [Pt(NH₃)₂Cl₂] (cisplatin — an anti-cancer drug)
• [Ni(CN)₄]²⁻

Coordination Number 2 — Linear

Two ligands arranged at 180°.

Examples:

• [Ag(NH₃)₂]⁺ (diamminesilver(I), formed in Tollens' reagent)
• [CuCl₂]⁻

flowchart TD
    A["Coordination Number"] --> B{"Number?"}
    B -->|"6"| C["Octahedral<br>90° bond angles<br>e.g. [Fe(H₂O)₆]²⁺"]
    B -->|"4"| D{"Ligand type?"}
    D -->|"Large ligands (Cl⁻)"| E["Tetrahedral<br>109.5° bond angles<br>e.g. [CoCl₄]²⁻"]
    D -->|"Strong-field, Pt/Ni"| F["Square Planar<br>90° bond angles<br>e.g. [Pt(NH₃)₂Cl₂]"]
    B -->|"2"| G["Linear<br>180° bond angle<br>e.g. [Ag(NH₃)₂]⁺"]

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Naming Complexes

Complex ion naming follows systematic rules:

1. Number of ligands — use Greek prefixes: di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6)
2. Ligand names — water = aqua, ammonia = ammine, chloride = chloro, cyanide = cyano, hydroxide = hydroxo
3. Metal name — use the element name for cations, but add the suffix "-ate" for anions (often using the Latin root)
4. Oxidation state — given in Roman numerals in parentheses

Complex	Name	Working for Oxidation State
[Cu(H₂O)₆]²⁺	Hexaaquacopper(II)	Cu + 6(0) = +2, so Cu = +2
[CoCl₄]²⁻	Tetrachlorocobaltate(II)	Co + 4(−1) = −2, so Co = +2
[Cr(NH₃)₆]³⁺	Hexaamminechromium(III)	Cr + 6(0) = +3, so Cr = +3
[Fe(CN)₆]⁴⁻	Hexacyanoferrate(II)	Fe + 6(−1) = −4, so Fe = +2
[Ag(NH₃)₂]⁺	Diamminesilver(I)	Ag + 2(0) = +1, so Ag = +1

Key detail: For anionic complexes, the metal takes the Latin name with an "-ate" suffix: iron → ferrate, copper → cuprate, cobalt → cobaltate, chromium → chromate. For cationic or neutral complexes, the standard English name is used.

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Isomerism in Complexes: Cis-Trans Isomerism

Square planar and octahedral complexes with two different types of ligand can show cis-trans (geometric) isomerism.

In the square planar complex [Pt(NH₃)₂Cl₂]:

• Cis isomer: the two Cl ligands are adjacent (on the same side) — this is cisplatin, used in cancer treatment
• Trans isomer: the two Cl ligands are opposite each other — this is transplatin, which is much less effective as a drug

The two isomers have different physical and biological properties despite having the same molecular formula. Cisplatin works by binding to DNA and preventing cell division; the spatial arrangement of the chloride ligands is critical for this binding.

Optical Isomerism in Complexes

Octahedral complexes with three bidentate ligands (e.g., [Ni(en)₃]²⁺) can show optical isomerism. The two mirror-image forms are non-superimposable and rotate plane-polarised light in opposite directions. This type of isomerism is tested at A-Level.

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The Chelate Effect

When bidentate or multidentate ligands replace monodentate ligands, the resulting chelated complex is more stable. This is called the chelate effect.

For example: [Ni(H₂O)₆]²⁺ + 3en → [Ni(en)₃]²⁺ + 6H₂O

Three bidentate en ligands replace six monodentate water ligands. The coordination number remains 6, but the chelated complex is significantly more stable.

Thermodynamic Explanation (Entropy)

The chelate effect is primarily an entropy-driven phenomenon. In the reaction above:

• Reactant side: 1 complex ion + 3 en molecules = 4 species
• Product side: 1 complex ion + 6 water molecules = 7 species

The number of species in solution increases from 4 to 7, so disorder (entropy) increases. The positive entropy change (ΔS > 0) makes ΔG more negative, favouring the chelated product.

This can be quantified: ΔG = ΔH − TΔS. Even if ΔH is approximately zero (similar bond strengths), the large positive TΔS term ensures ΔG is negative.

Biological and Medical Importance

• EDTA is used in medicine to treat heavy metal poisoning — it chelates toxic metal ions (such as Pb²⁺) and removes them from the body. EDTA is hexadentate, wrapping around the metal ion with six bonds, making the complex extremely stable.
• Haemoglobin uses a chelating porphyrin ring to hold Fe²⁺ in place for oxygen transport. Carbon monoxide (CO) can also bind to this Fe²⁺, forming a very stable chelated complex that prevents oxygen binding — this is why CO is so toxic.

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Worked Example: Naming and Calculating Oxidation State

Question: Name the complex [Cr(NH₃)₄Cl₂]⁺ and determine the oxidation state of chromium.

Answer:

• NH₃ is neutral (charge = 0), there are 4 of them: total = 0
• Cl⁻ has charge −1, there are 2 of them: total = −2
• Overall charge = +1
• Cr + 0 + (−2) = +1, so Cr = +3
• Name: tetraamminedichlorochromium(III) ion
• (Ligands are listed in alphabetical order: ammine before chloro)

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Summary

Complex ions consist of a central metal ion bonded to ligands via coordinate bonds. Ligands can be monodentate, bidentate, or multidentate. Common shapes include octahedral (6), tetrahedral (4), square planar (4), and linear (2). The chelate effect makes complexes with polydentate ligands more stable, driven by an entropy increase. Naming follows systematic rules with ligand prefixes, ligand names in alphabetical order, metal name (with "-ate" for anions), and oxidation state in Roman numerals.

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A-Level Deep Dive: Complex Ions

Spec mapping

Edexcel 9CH0 specification Topic 15 — Transition Metals, sub-topic 15.2 covers ligands as electron-pair donors (Lewis bases), monodentate / bidentate / polydentate ligands (H₂O, NH₃, Cl⁻, CN⁻, OH⁻; en (1,2-diaminoethane); EDTA⁴⁻), coordination number, common shapes (octahedral, tetrahedral, square planar), and ligand-exchange reactions in aqueous transition-metal chemistry (refer to the official specification document for exact wording). Examined in Paper 1 (9CH0/01) and Paper 3 (9CH0/03) through CP16. Synoptic links to Topic 2 (Bonding and Structure: VSEPR) for shapes, Topic 13 (Energetics II) for chelate effect (entropy), and to lesson 7 (colour) and lesson 9 (qualitative analysis).

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Worked example with full mark scheme

Question (8 marks):

(a) Define a ligand and explain the difference between monodentate, bidentate and polydentate ligands. Give one example of each. (3)

(b) When excess concentrated NH₃ is added stepwise to aqueous CuSO₄, the pale blue solution becomes deep blue. Write the equations for the ligand-exchange steps and explain the colour change in terms of the species present. (3)

(c) Predict the shape of [Ni(CN)₄]²⁻ and [NiCl₄]²⁻, justifying any difference. (2)

Solution with mark scheme:

(a) Ligand = species (atom, ion or molecule) that donates a lone pair of electrons into an empty orbital of a transition-metal cation, forming a dative covalent (coordinate) bond.

B1 — definition.

• Monodentate = donates one pair (one bond): H₂O, NH₃, Cl⁻, CN⁻, OH⁻.
• Bidentate = donates two pairs (two bonds, one ligand): 1,2-diaminoethane (en, H₂NCH₂CH₂NH₂); ethanedioate (oxalate, C₂O₄²⁻).
• Polydentate (hexadentate) = donates ≥3 pairs: EDTA⁴⁻ (donates 6).

B1 — three types correctly distinguished.

B1 — at least one valid example for each.

(b) Step 1 — initial complex.

[Cu(H₂O)₆]²⁺(aq) — pale blue (octahedral, six water ligands).

Step 2 — partial ligand exchange.

[Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O

Ammonia replaces only the four equatorial water ligands; the two axial waters remain (the axial bonds are longer due to Jahn–Teller distortion of d⁹).

M1 — equation correct, four NH₃ in, four H₂O out.

M1 — colour change: pale blue → deep royal blue. The deeper colour arises because NH₃ produces a larger ligand-field splitting Δ_oct than H₂O (NH₃ is higher in the spectrochemical series), so the d–d absorption shifts to higher energy (shorter wavelength), making the complementary colour more intensely blue.

A1 — note the formation of pale blue intermediate gelatinous Cu(OH)₂ precipitate at low NH₃ concentration before redissolving in excess as [Cu(NH₃)₄(H₂O)₂]²⁺.

(c) [Ni(CN)₄]²⁻ — square planar. CN⁻ is a strong-field ligand (high in spectrochemical series); the d⁸ configuration of Ni²⁺ in a strong field undergoes pairing with two electrons in the higher-energy d_x²−y² orbital depopulated, favouring square planar.

[NiCl₄]²⁻ — tetrahedral. Cl⁻ is a weak-field ligand; ligand-field stabilisation does not favour square planar over tetrahedral; the larger Cl⁻ also creates steric pressure favouring tetrahedral 4-coordinate geometry.

M1 — both shapes correct.

A1 — justified by ligand-field strength (CN⁻ vs Cl⁻ in spectrochemical series).

Total: 8 marks (M3 A2 B3).

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Specimen question modelled on the Edexcel 9CH0 Paper 1 format

Question (6 marks): Excess concentrated HCl is added to a solution containing [Cu(H₂O)₆]²⁺.

(a) Write the equation for the ligand-exchange reaction. (1)

(b) Predict the shape and coordination number of the product. State and explain the colour change. (3)

(c) Discuss why the chloride product has a different coordination number from the original aqua complex. (2)

Mark scheme decomposition by AO: