Transition Metals & Complexes

Revise Transition Metals & Complexes for Chemistry 5 (IAL) WCH15 (A2 Level) — revision notes and instant AI marking.

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Edexcel IAL Chemistry · Inorganic

Transition Metals & Complexes

The Big Idea: Transition metal ions are magnets for electron pairs — molecules and ions called ligands dock onto them to form complexes. This bonding splits the metal's d-orbitals just enough that visible light gets absorbed, which is why transition metal compounds are colourful, why they can swap one ligand for another, and why some (like cisplatin) can be turned into life-saving drugs.

d-block chemistry Ligands & complexes Colour theory Isomerism Medicinal chemistry

Summary — The Whole Chapter at a Glance

A transition metal is an element that forms at least one stable ion with a partially filled d-subshell. This is why Sc and Zn are excluded, even though they sit in the d-block.
Transition metals lose their 4s electrons before 3d electrons when forming ions — always. Cr and Cu have "anomalous" ground-state configurations ([Ar]3d⁵4s¹ and [Ar]3d¹⁰4s¹) because a half-full or full d-subshell is extra stable.
Because the d-subshell can lose different numbers of electrons, transition metals show variable oxidation states (shown with Roman numerals).
A complex = central metal ion + ligands attached by dative covalent (co-ordinate) bonds, where the ligand donates both electrons.
Ligands are classed by how many dative bonds they can form: monodentate (1), bidentate (2), multidentate (many, e.g. EDTA⁴⁻ = 6).
The coordination number = total number of dative bonds to the central ion. This decides the shape: 6 → octahedral (90°), 4 → tetrahedral (109.5°) or square planar (90°).
Colour arises because ligands split the five degenerate 3d orbitals into two energy levels. An electron absorbs visible light to jump the gap (d–d transition); the colour you see is the complementary colour of what's absorbed.
Colour changes when you change the ligand, oxidation state, or coordination number — all four factors alter the size of the splitting energy, ΔE.
Ligand exchange (substitution) is reversible and swaps one ligand type for another, often changing colour, and sometimes changing coordination number/shape if the ligand sizes differ a lot.
Bidentate/multidentate ligands replacing monodentate ones is the chelate effect — favourable because it's driven by a big positive entropy change (more product particles than reactant particles).
Transition metal complexes can show cis-trans (geometric) isomerism and optical isomerism, even without a C=C double bond.
Cisplatin is a square planar complex used in cancer treatment; it works by ligand-exchanging its Cl⁻ ligands for water, then binding to DNA bases and blocking replication.
Haemoglobin uses an Fe²⁺ centre in a square-planar-like haem group to transport O₂; CO poisoning happens because CO binds more strongly than O₂ to the same site.

1. Transition Metals — Electron Configuration & Oxidation States

What actually makes something a "transition metal"?

Here's the precise definition, and it matters because exam questions love to test the exception cases: a transition metal is an element that forms at least one stable ion with an incomplete (partially filled) d-subshell.

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