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*Revision Materials and Past Papers* 1 Atomic Structure 2 Amounts of Substance 3 Bonding 4 Energetics 5 Kinetics 6 Chemical Equilibria & Kc 7 Redox Equations 8 Thermodynamics 9 Rate Equations 10 Kp (Equilibrium Constant) 11 Electrode Potentials & Cells 12 Acids and Bases 13 Periodicity 14 Group 2: Alkaline Earth Metals 15 Group 7: The Halogens 16 Period 3 Elements & Oxides 17 Transition Metals 18 Reactions of Ions in Aqueous Solution 19 Intro to Organic Chemistry 20 Alkanes 21 Halogenoalkanes 22 Alkenes 23 Alcohols 24 Organic Analysis 25 Optical Isomerism 26 Aldehydes & Ketones 27 Carboxylic Acids & Derivatives 28 Aromatic Chemistry 29 Amines 30 Polymers 31 Amino Acids, Proteins & DNA 32 Organic Synthesis 33 NMR Spectroscopy 34 Chromatography RP1–RP12 Required Practicals

2.5 Transition Metals (A-level only)

2.5.1 General Properties of Transition Metals 2.5.2 Substitution Reactions 2.5.3 Shapes of Complex Ions 2.5.4 Formation of Coloured Ions 2.5.5 Variable Oxidation States 2.5.6 Catalysts

Shapes of Complex Ions

Specification Reference Inorganic chemistry, Transition metals 3.2.5.3

Quick Notes

  • Transition metal ions commonly form octahedral complexes with small ligands like H2O and NH3.
  • Octahedral complexes can show:
    • Cis-trans isomerism with monodentate ligands.
    AQA A-Level Chemistry cis-trans isomerism illustrated for an octahedral complex
    • Optical isomerism with bidentate ligands.
    AQA A-Level Chemistry optical isomerism illustrated for a complex containing bidentate ligands
  • Tetrahedral complexes are commonly formed with larger ligands such as Cl−.
  • Square planar complexes also exist and can display cis-trans isomerism.
    • Cisplatin is an example of a square planar complex that exists in the cis form.
  • Ag+ forms a linear complex, [Ag(NH3)2]+, which is used in Tollens' reagent for testing aldehydes.

Full Notes

The shape of a complex is determined by the co-ordination number and type of ligands bonded to a central metal atom or ion. The most common shapes are octahedral, tetrahedral, square planar and linear.

AQA A-Level Chemistry overview of complex ion shapes: octahedral, tetrahedral, square planar, and linear

Octahedral Complexes

Most common shape for transition metal complexes is octahedral.

Simple examples are formed when six small ligands such as H2O (water) and NH3 (ammonia) surround a central metal ion.

Example Octahedral complexes:[Cu(H2O)6]2+ and[Cr(NH3)6]3+

AQA A-Level Chemistry diagram of an octahedral complex with six small ligands around a metal ion

Cis-trans isomerism (E–Z isomerism)

Octahedral complexes can exhibit cis-trans isomerism.

AQA A-Level Chemistry cis-trans arrangement of two identical ligands in an octahedral complex

Occurs when two identical ligands are arranged either next to each other (cis) or opposite each other (trans).

For Example: [Cu(NH3)4(H2O)2]2+
AQA A-Level Chemistry octahedral complex showing cis and trans positions of identical ligands Trans form: H2O ligands are opposite each other.
Cis form: Both H2O ligands are next to each other.

Optical isomerism

Optical isomerism occurs when a complex has bidentate ligands (e.g., ethane-1,2-diamine).

The isomers are mirror images of each other (non-superimposable).

AQA A-Level Chemistry mirror-image pair of octahedral complexes with bidentate ligands

For Example [Cu(C2O4)3]4−

AQA A-Level Chemistry optical isomers of a tris(oxalato)copper complex

Tetrahedral Complexes

Tetrahedral complexes are formed when four ligands surround a central metal ion, most commonly with large anions like chloride ions, Cl−.

Example [CuCl4]2−

AQA A-Level Chemistry tetrahedral [CuCl4]2− complex diagram

Square Planar Complexes

Square planar complexes are formed when four ligands surround a central metal ion in the same plane.

Cis-trans isomerism occurs in some square planar complexes when there are two different types of ligand.

Common in platinum(II) and nickel(II) complexes.

Example Cisplatin ([Pt(NH3)2Cl2])

AQA A-Level Chemistry cis versus trans isomers for Cisplatin square planar platinum complex

Cis form: Both Cl− ligands are next to each other.
Trans form: Cl− ligands are opposite each other.
Only the cis form is used in cancer treatment.

Linear Complexes

Formed when two ligands surround a central metal ion.

Example Tollens' reagent ([Ag(NH3)2]+)
Used to test for aldehydes (produces a silver mirror).

AQA A-Level Chemistry linear silver ammine complex used in Tollens' test

Summary Table

Shape Co-ordination number Typical ligands Common isomerism Example complex
Octahedral 6 H2O, NH3 (small, neutral) Cis–trans; Optical (with bidentate) [Cu(H2O)6]2+, [Cr(NH3)6]3+
Tetrahedral 4 Cl− (larger anion) Usually none [CuCl4]2−
Square planar 4 Pt(II), Ni(II) complexes Cis–trans [Pt(NH3)2Cl2] (cisplatin)
Linear 2 NH3 (ammine) None [Ag(NH3)2]+ (Tollens’ reagent)