5.2Unit 5. Transition Metals and Organic Nitrogen Chemistry

Transition Metals & Complexes — IAL Edexcel Modular (Chemistry) Revision Notes & Topical Questions

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5.2 Transition Metals and Complexes — IAL EDEXCEL MODULAR (CHEMISTRY) notes: Describe transition metal properties, complex ions, ligands, colour, shapes and isomerism.Transition metal — an element with an incomplete d-subshell that forms at least one stable ion with an incomplete…

Filling order
Exceptions
Forming ions
Distinctive properties
Variable oxidation states
Co-ordination number

Section 1 of 12Transition metals and their electron configurations

Transition metal

Transition metal an element with an incomplete d-subshell that forms at least one stable ion with an incomplete d-subshell.
Scandium forms only Sc³⁺ ([Ar] 3d⁰) and zinc forms only Zn²⁺ ([Ar] 3d¹⁰), so neither is a transition metal.
The first transition series is therefore titanium to copper.

Filling order

4s fills before 3d, and [Ar] represents 1s to 3p.

Exceptions

Exceptions an electron moves from 4s to 3d to give a half-full or full d-subshell, which is more stable.
Cr is [Ar] 3d⁵ 4s¹, not [Ar] 3d⁴ 4s².
Cu is [Ar] 3d¹⁰ 4s¹, not [Ar] 3d⁹ 4s².

Forming ions

4s electrons are lost first, because once the orbitals are occupied electron repulsion raises 4s above 3d.
This is why +2 is a common oxidation state.

Section 2 of 12General properties and oxidation states

1Distinctive properties

Distinctive properties variable oxidation states, formation of complex ions, coloured compounds and catalytic behaviour.

2Variable oxidation states

Variable oxidation states transition elements form more than one positive ion because 4s and 3d electrons are close in energy.
Roman numerals show the oxidation state, for example Fe(II) is Fe²⁺ and Fe(III) is Fe³⁺.
Sodium needs no numeral because it only forms Na⁺.

Section 3 of 12Complex ions and ligands

Complex

Complex a central metal ion surrounded by ligands.

Ligand

Ligand a molecule or ion that donates a lone pair to form a dative covalent (co-ordinate) bond to the metal ion.
It is a Lewis base (electron pair donor) and is the same species as a nucleophile.
It has a negative charge or a lone pair.

Co-ordination number

Co-ordination number the number of dative bonds to the central metal ion.

Monodentate ligands

Monodentate ligands form one dative bond through a single lone pair.
H₂O, NH₃, Cl^- and CN^- are examples.

Bidentate ligands

Bidentate ligands form two dative bonds through two atoms with lone pairs.
1,2-diaminoethane (en), H₂NCH₂CH₂NH₂, and ethanedioate (ox), C₂O₄^2-.

Multidentate ligands

Multidentate ligands form more than two dative bonds.
EDTA^4- is hexadentate and forms six dative bonds.

Section 4 of 12Representing and naming complexes

Square brackets

Square brackets group the metal ion and its ligands, with the overall charge shown outside.
The overall charge is the sum of the oxidation states of the metal and all ligands.
With neutral ligands the charge equals the oxidation state of the metal.

Cation names

Cation names number prefix, ligand name, metal name, then oxidation number in brackets.
Prefixes di, tetra and hexa are used for 2, 4 and 6.
Hexaaquairon(II) and hexaamminecobalt(II).

Anion names

Anion names the metal name ends in -ate and may use a Latin stem.
Tetrachlorocuprate(II) and tetracyanonickelate(II).

Ammonia as a ligand

Ammonia as a ligand is named ammine, spelt with a double m unlike the amine functional group.

Section 5 of 12Colour in transition metal complexes

Origin of colour

Origin of colour a compound absorbs some visible frequencies and the colour seen is made of the frequencies not absorbed.
The colour seen is complementary to the colour absorbed, which lies opposite on the colour wheel.

d-orbital splitting

d-orbital splitting ligands split the five d orbitals into two sets of slightly different energy.
An electron in the lower set absorbs visible light and is promoted to the higher set.
The energy absorbed, ΔE, is proportional to frequency and inversely proportional to wavelength.

Colourless ions

Zn²⁺ (full 3d) and Sc³⁺ (empty 3d) have no electrons to promote.

Factors affecting ΔE

Factors affecting ΔE the ligand, the metal ion's identity and nuclear charge, the oxidation state and the shape of the complex.

Section 6 of 12Why colours change

Cause	Example
Different ligand	[Cu(H₂O)₆]²⁺ is light blue and [Cu(NH₃)₄(H₂O)₂]²⁺ is dark blue
Different oxidation state	[Fe(H₂O)₆]²⁺ is green and [Fe(H₂O)₆]³⁺ is orange
Different co-ordination number	[Cu(H₂O)₆]²⁺ is light blue and [CuCl₄]^2- is yellow

Why colours change

Ligand charge density

Ligand charge density a higher charge density gives stronger interaction, greater splitting and absorption of higher-energy light.

Oxidation number

Oxidation number a higher oxidation number also interacts more strongly with the ligands.

Co-ordination number change

Co-ordination number change alters colour through the change of ligand and geometry together.

Section 7 of 12Ligand exchange and the chelate effect

Ligand exchange and the chelate effect

Ligand exchange (substitution)

Ligand exchange (substitution) one ligand is replaced by another, partially or completely, to form a more stable complex.
Ligands of similar size (H₂O, NH₃) keep the same co-ordination number and geometry.
A larger ligand such as Cl^- changes the co-ordination number.

Copper(II) with chloride

[Cu(H₂O)₆]²⁺ + 4Cl^- ⇌ [CuCl₄]^2- + 6H₂O.
Adding concentrated HCl turns blue to green (mixture) and then yellow; the reaction is reversible.
The co-ordination number falls from 6 to 4 and the charge changes from +2 to -2 but copper stays +2.

Cobalt(II) with chloride

[Co(H₂O)₆]²⁺ + 4Cl^- ⇌ [CoCl₄]^2- + 6H₂O gives a blue solution.

Dissolving a chloride salt in water

Dissolving a chloride salt in water gives the aqua complex, not the chloro complex.

Chelate effect

Chelate effect replacing monodentate ligands with bidentate or multidentate ligands is favourable because ΔG is negative.
ΔG = ΔH - TΔS,
- and the enthalpy change is small.
  What the symbols mean: ΔG = Gibbs free energy change (kJ mol⁻¹; feasible when ≤ 0) · ΔH = enthalpy change (kJ mol⁻¹) · T = temperature (K) · ΔS = entropy change (J K⁻¹ mol⁻¹, divide by 1000 to use kJ)
The number of particles increases, so ΔS is positive and the reaction is entropy driven.
[Co(H₂O)₆]²⁺ + EDTA^4- → [CoEDTA]^2- + 6H₂O turns two reactant species into seven product species.

Section 8 of 12Shapes of complexes

Octahedral

Octahedral six co-ordinate bonds with bond angles of 90°.
Six monodentate ligands such as H₂O, NH₃, OH^- and SCN^-.
Three bidentate ligands such as en or ethanedioate.
One hexadentate ligand such as EDTA^4-.

Tetrahedral

Tetrahedral four co-ordinate bonds with bond angles of 109.5°.
Large ligands such as Cl^- cannot fit six around the metal, so four chloride ions give a tetrahedral complex.

Square planar

Square planar four co-ordinate bonds with bond angles of 90°, as in cisplatin and some complexes with CN^-.

Predicting shapes

Predicting shapes electron pair repulsion applies, ignoring the 3d electrons and the overall charge and counting only the electron pairs donated by the ligands.

Section 9 of 12Isomerism in complexes

Isomerism in complexes

Cis-trans isomerism

Cis-trans isomerism occurs in square planar and octahedral complexes with two pairs of different ligands.
The cis isomer has the two different ligands adjacent and the trans isomer has them opposite.
[Co(NH₃)₄(H₂O)₂]²⁺, [Ni(en)₂(H₂O)₂]²⁺ and cisplatin / transplatin.

Optical isomerism

Optical isomerism occurs in octahedral complexes with bidentate ligands.
The two forms are non-superimposable mirror images with no plane of symmetry.
They rotate the plane of polarised light in opposite directions.
[Ni(en)₃]²⁺ and [Ni(en)₂(H₂O)₂]²⁺ are examples.

Change of oxidation state of the metal

Fe²⁺ complexes are oxidised to Fe³⁺.
Pale green [Fe(H₂O)₆]²⁺ slowly oxidises in air to yellow [Fe(H₂O)₆]³⁺.
Chlorine oxidises yellow K₄[Fe(CN)₆] to red K₃[Fe(CN)₆].

Section 10 of 12Cisplatin and haemoglobin

Cisplatin and haemoglobin

Cisplatin

Cisplatin a square planar cis isomer used against testicular, ovarian, cervical, breast, lung and brain cancers.
Chlorine ligands are exchanged for water in the cell.
Nitrogen atoms on DNA bases then bond to platinum by dative bonds.
DNA is distorted and cannot replicate.
Transplatin cannot be used in treatment.

Side effects

Side effects cisplatin also affects healthy cells that replicate quickly, such as hair follicles, causing hair loss.
Society balances adverse effects against therapeutic value.

Haemoglobin

Haemoglobin contains haem groups with Fe²⁺ at the centre.
Four nitrogen atoms of the haem form dative bonds in a square planar arrangement.
A fifth dative bond comes from the protein (globin).
O₂ binds weakly through a sixth dative bond, so it is carried in blood and released easily.

Carbon monoxide poisoning

CO is a better ligand than O₂, binds more strongly to Fe²⁺ and prevents oxygen transport.
Carboxyhaemoglobin is darker red than oxyhaemoglobin.

Anaemia

Anaemia too little haemoglobin because of blood loss or iron deficiency, treated with iron(II) sulfate tablets.

Section 11 of 12Key terms

Term	Meaning
Transition metal	An element with an incomplete d-subshell that forms at least one stable ion with an incomplete d-subshell.
Ligand	A molecule or ion that donates a lone pair to a metal ion to form a dative covalent bond.
Co-ordination number	The number of dative bonds formed between the ligands and the central metal ion.
Ligand exchange	Replacement of one or more ligands in a complex by other ligands.
Chelate effect	The extra stability gained, through increased entropy, when monodentate ligands are replaced by bidentate or multidentate ligands.
Bidentate ligand	A ligand that forms two dative bonds to the metal ion.
Complementary colour	The colour seen when the opposite colour on the colour wheel is absorbed.
Stereoisomerism in complexes	Cis-trans and optical isomers with the same bonds but different arrangement in space.

Section 12 of 12Exam technique

Calling Sc and Zn transition metals — their ions have an empty or full d-subshell, so they are d-block but not transition metals.

Writing Cr as [Ar] 3d⁴ 4s² and Cu as [Ar] 3d⁹ 4s² — the half-full and full d-subshell arrangements are more stable.

Removing 3d electrons before 4s when forming ions — 4s electrons are lost first.

Saying ligand exchange with Cl⁻ changes the oxidation state — copper stays +2 even though the charge changes from +2 to -2.

Writing amine instead of ammine for NH₃ as a ligand.

Last updated: 13 October 2026