Ligands, complex ions and their shapes

Ligands and complexes (spec 17.4 to 17.6, 17.10)

• A ligand is a molecule or ion with a lone pair of electrons that forms a dative (coordinate) covalent bond to a central metal ion.
• A complex ion is a central metal ion surrounded by ligands joined by dative bonds.
• The coordination number is the number of dative bonds from the ligands to the central metal ion.

The charge of a complex is the metal ion's charge plus the ligands' charges: [Fe(H2O)6]3+, [Cr(OH)6]3-.

Monodentate ligands and octahedral shape (spec 17.12, 17.13)

H2O, NH3, OH- and Cl- are monodentate: each forms one dative bond. Small ligands such as H2O, OH- and NH3 usually give six-coordinate, octahedral complexes with 90° bond angles: [Cu(H2O)6]2+, [Co(NH3)6]2+.

Tetrahedral and square planar (spec 17.14, 17.15)

• Larger ligands such as Cl- fit only four around the metal, giving tetrahedral complexes (109.5°): [CuCl4]2-, [CoCl4]2-.
• Some four-coordinate complexes are square planar (90°). Cisplatin, cis-[Pt(NH3)2Cl2], is a square planar complex used as an anticancer drug: it binds to DNA in cancer cells and stops them dividing. It is supplied as the single cis isomer, because the trans isomer is not effective.

Worked example

For [CoCl4]2-, give the oxidation number of cobalt, the coordination number and the shape.

1. Charge: x + 4(-1) = -2, so x = +2.
2. Four Cl- ligands each form one dative bond.

Answer: Co +2; coordination number 4; tetrahedral

Check yourself

1. What is a ligand?
   • A positive ion at the centre of a complex
   • A molecule that accepts electrons from a metal
   • A molecule or ion with a lone pair that forms a dative covalent bond to a metal ion
   • Any negative ion in solution
   Show answer

   A molecule or ion with a lone pair that forms a dative covalent bond to a metal ion. Ligands donate electron pairs to the central metal ion.

2. What is the coordination number of cobalt in [CoCl4]2-?
   Show answer

   4. Four monodentate Cl- ligands form four dative bonds.

3. What is the shape of [Cu(H2O)6]2+?
   • Tetrahedral
   • Square planar
   • Octahedral
   • Linear
   Show answer

   Octahedral. Six ligands around a metal ion give an octahedral shape.

4. Why does Cl- usually form four-coordinate complexes with Cu2+ while H2O forms six-coordinate ones?
   • Chloride ions have no lone pairs
   • Chloride ions are larger, so only four fit around the metal ion
   • Water is a negative ion
   • Chloride forms double bonds
   Show answer

   Chloride ions are larger, so only four fit around the metal ion. Size limits how many ligands fit.

5. What is the shape of cisplatin?
   • Tetrahedral
   • Octahedral
   • Trigonal planar
   • Square planar
   Show answer

   Square planar. Pt(II) with four ligands in one plane.

6. What is the oxidation number of chromium in [Cr(OH)6]3-?
   Show answer

   +3. x + 6(-1) = -3, so x = +3.

7. Why is cisplatin given as the cis isomer only?
   • The trans isomer is more expensive
   • The cis isomer is octahedral
   • The trans isomer contains no platinum
   • Only the cis isomer binds effectively to DNA in cancer cells
   Show answer

   Only the cis isomer binds effectively to DNA in cancer cells. The two Cl- ligands must be next to each other to bind to DNA.

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Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson Edexcel IAL Chemistry (2018) scheme of work

Last updated 7 November 2026. Spotted a mistake? Tell us.