Edexcel International A Level Chemistry: Transition metals: electronic structure, complexes and colour (YCH11) – Revision Notes

For full explanations and longer worked examples, read the study guide first.

These notes condense outcomes 17.1 to 17.17 of Topic 17: Transition Metals and their Chemistry, in the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry specification (YCH11), Issue 1, September 2017. All of it is IA2 content, assessed in Unit 5: Transition Metals and Organic Nitrogen Chemistry. The rest of Topic 17 (vanadium, chromium, hydroxide and ammonia reactions, catalysts) is in the redox and catalysis revision notes.

Links: Edexcel chemistry hub, printable checklist, practice questions for this unit, and the free diagnostics.

Definitions to learn

Term	Definition
Transition metal (17.1)	A d-block element with at least one stable ion whose d-orbitals are incompletely filled
Ligand (17.4)	A molecule or ion that bonds to a metal ion by donating a lone pair
Dative (coordinate) covalent bond (17.5)	A shared pair of electrons where the ligand’s donor atom supplies both electrons
Complex ion (17.6)	A metal ion at the centre with ligands bonded all round it
Coordination number (17.10)	How many dative bonds the metal ion receives in total
Monodentate (17.12)	Forms one dative bond: H₂O, OH⁻, NH₃ (also Cl⁻)
Bidentate (17.16)	Forms two dative bonds to the same ion: NH₂CH₂CH₂NH₂
Hexadentate (17.16)	Forms six dative bonds: EDTA⁴⁻
Polydentate (17.17)	Forms several dative bonds: the ligand around iron(II) in haemoglobin

17.2 Configurations in steps

1. Count electrons beyond argon: Z − 18.
2. Atom: put two in 4s and the rest in 3d. Write 3d first.
3. Apply the two exceptions: Cr [Ar] 3d⁵ 4s¹, Cu [Ar] 3d¹⁰ 4s¹.
4. Ion: remove 4s electrons first, then 3d.

Element	Atom	Common ion(s)
Sc (21)	[Ar] 3d¹ 4s²	Sc³⁺ [Ar]
Ti (22)	[Ar] 3d² 4s²	Ti³⁺ [Ar] 3d¹
V (23)	[Ar] 3d³ 4s²	V²⁺ [Ar] 3d³, V³⁺ [Ar] 3d²
Cr (24)	[Ar] 3d⁵ 4s¹	Cr³⁺ [Ar] 3d³
Mn (25)	[Ar] 3d⁵ 4s²	Mn²⁺ [Ar] 3d⁵
Fe (26)	[Ar] 3d⁶ 4s²	Fe²⁺ [Ar] 3d⁶, Fe³⁺ [Ar] 3d⁵
Co (27)	[Ar] 3d⁷ 4s²	Co²⁺ [Ar] 3d⁷
Ni (28)	[Ar] 3d⁸ 4s²	Ni²⁺ [Ar] 3d⁸
Cu (29)	[Ar] 3d¹⁰ 4s¹	Cu⁺ [Ar] 3d¹⁰, Cu²⁺ [Ar] 3d⁹
Zn (30)	[Ar] 3d¹⁰ 4s²	Zn²⁺ [Ar] 3d¹⁰

Not transition metals: Sc (only ion is d⁰) and Zn (only ion is d¹⁰).

17.3 Variable oxidation number in one paragraph

The 4s and 3d sub-shells are close in energy, so electrons from both can be lost or used in bonding. Successive ionisation energies climb gradually, with no sudden large jump until the 3d electrons run out. The energy cost of a higher charge can be paid back by lattice energy, hydration or covalent bonding. Result: several stable oxidation numbers, for example manganese from +2 up to +7.

Reading any complex: method in steps

1. List the ligands and their charges (H₂O, NH₃, NH₂CH₂CH₂NH₂ = 0; OH⁻, Cl⁻ = −1; EDTA⁴⁻ = −4).
2. Count dative bonds: monodentate × 1, bidentate × 2, hexadentate × 6. That is the coordination number.
3. Oxidation number of metal = overall charge − sum of ligand charges.
4. Shape from coordination number and ligand type (table below).

Worked reminder. [Cr(NH₂CH₂CH₂NH₂)₂Cl₂]⁺: bonds = 2 × 2 + 2 × 1 = 6, so octahedral. Oxidation number = +1 − 2(−1) = +3.

17.13–17.15 Shapes

Shape	Coordination number	Bond angle	When	Example
Octahedral	6	90°	small ligands: H₂O, OH⁻, NH₃	[Fe(H₂O)₆]³⁺
Tetrahedral	4	109.5°	relatively large ligands such as Cl⁻	[CoCl₄]²⁻
Square planar	4	90°	e.g. platinum(II)	cis-platin, [Pt(NH₃)₂Cl₂]

Why octahedral: six bonding pairs repel equally and get as far apart as possible.

Cis-platin: the cis isomer of square planar [Pt(NH₃)₂Cl₂], Pt is +2. Cl–Pt–Cl is 90° in cis and 180° in trans. It is a cancer drug and is supplied as a single isomer, not mixed with the trans form.

17.7–17.9 Colour: method in steps

1. Ligands split the 3d orbitals into two energy levels, gap ΔE.
2. A d electron absorbs visible light of frequency f where ΔE = hf, and is promoted.
3. Those frequencies are removed from white light.
4. The transmitted (or reflected) light is the colour seen.

No colour when:

• d⁰ (Sc³⁺, Ti⁴⁺): no electron to promote.
• d¹⁰ (Zn²⁺, Cu⁺): no space in the upper level.
• ΔE corresponds to ultraviolet rather than visible light.

Worked reminder. Why is [Ti(H₂O)₆]³⁺ purple when [Sc(H₂O)₆]³⁺ is colourless? Ti³⁺ is [Ar] 3d¹. The water ligands split its 3d orbitals, and the single d electron absorbs visible light (mainly yellow-green) to jump the gap ΔE. Red and blue light pass through, so the solution looks purple. Sc³⁺ is [Ar] with no d electrons, so nothing can be promoted and no visible light is absorbed. Write the configuration first: it makes the reason for colour, or its absence, easy to state.

17.11 Colour changes: three causes

Cause	Example
Oxidation number changes	Fe²⁺(aq) pale green → Fe³⁺(aq) yellow-brown
Ligand changes	aqua copper(II), pale blue, to the ammine complex [Cu(NH₃)₄(H₂O)₂]²⁺, deep blue
Coordination number changes	[Co(H₂O)₆]²⁺ pink (6) → [CoCl₄]²⁻ blue (4)

Each change alters ΔE, so a different frequency is absorbed.

17.17 Haemoglobin

• Iron(II) complex; the iron is held by a polydentate ligand (and the protein).
• O₂ binds to Fe²⁺ by a dative bond and is released in tissues.
• CO replaces O₂ by ligand exchange and binds far more strongly, so the change is not easily reversed. Less oxygen is carried.
• The haem structure is not assessed.

Must-know distinctions

• d-block element vs transition metal: all transition metals are d-block, but Sc and Zn are d-block and not transition metals.
• Number of ligands vs coordination number: [Ni(EDTA)]²⁻ has one ligand but coordination number 6.
• Colour absorbed vs colour seen: they are complementary.
• Tetrahedral vs square planar: both four-coordinate; 109.5° vs 90°.
• cis vs trans: same ligands on the same side (90°) vs opposite (180°).
• Bidentate vs hexadentate: two donor atoms (both N in NH₂CH₂CH₂NH₂) vs six (2 N and 4 O in EDTA⁴⁻).

Quick self-test

1. Write the configuration of a manganese atom and of Mn³⁺.
2. Write the configuration of Cr²⁺.
3. Explain why copper(I) chloride is white but aqueous copper(II) sulfate is blue.
4. Give the oxidation number of iron and the coordination number in [Fe(H₂O)₄(OH)₂]⁺.
5. Give the oxidation number of vanadium in [V(H₂O)₆]³⁺ and the number of 3d electrons.
6. State the shape and bond angle of [CoCl₄]²⁻.
7. State the coordination number in [Cu(NH₂CH₂CH₂NH₂)₂(H₂O)₂]²⁺.
8. Give the formula and charge of the complex formed when Ni²⁺ bonds to one EDTA⁴⁻.
9. Pale green iron(II) solution turns yellow-brown in air. Which factor in 17.11 explains the change?
10. What is the N–Pt–N angle in trans-[Pt(NH₃)₂Cl₂]?
11. Explain in one sentence why carbon monoxide is toxic.
12. Define a ligand.

Answers

1. Mn [Ar] 3d⁵ 4s²; Mn³⁺ [Ar] 3d⁴ (both 4s electrons lost, then one 3d).
2. [Ar] 3d⁴ (from [Ar] 3d⁵ 4s¹, lose 4s¹ then one 3d).
3. Cu⁺ is 3d¹⁰: no space for a d–d transition. Cu²⁺ is 3d⁹, so visible light can promote an electron.
4. x + 4(0) + 2(−1) = +1, so Fe is +3; coordination number 6.
5. +3; V³⁺ is [Ar] 3d², so 2 d electrons.
6. Tetrahedral, 109.5°.
7. 2 × 2 + 2 × 1 = 6.
8. [Ni(EDTA)]²⁻ (+2 − 4 = −2).
9. A change of oxidation number (+2 to +3).
10. 180° (the NH₃ ligands are opposite each other).
11. CO replaces O₂ on the iron(II) of haemoglobin by ligand exchange and binds more strongly, so less oxygen is carried.
12. A molecule or ion that donates a lone pair to a central metal ion, forming a dative bond.

Where marks are usually lost

• Writing ion configurations with 4s electrons left in, such as Fe³⁺ as [Ar] 3d³ 4s².
• Missing the Cr and Cu exceptions, or applying them to the ions as well.
• Defining a transition metal without the word ion or without incompletely filled d-orbitals.
• Saying the ligand “gives electrons” without saying it donates a lone pair to form a dative bond.
• Counting ligands instead of dative bonds for NH₂CH₂CH₂NH₂ or EDTA⁴⁻ complexes.
• Explaining colour as emission, or naming the absorbed colour as the colour seen.
• Saying “d-orbitals split” without saying it is the ligands that cause the splitting.
• For colourless ions, saying “no d electrons” for Zn²⁺, which actually has ten.
• Giving the trans isomer as cis-platin, or omitting why it is supplied as a single isomer.
• Describing CO poisoning without the words ligand exchange or binds more strongly.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017, published by Pearson Education Limited. Topic 17: Transition Metals and their Chemistry, outcomes 17.1 to 17.17.