Topic 17 · page 4 of 20

Shapes of complex ions

Six ligands give an octahedral ion, four large ligands give a tetrahedral ion, and a few four-coordinate ions are square planar. Cis-platin is the square planar example to know.

13 min read17.1317.1417.15

Read first: Ligands and complex ions

Six ligands: octahedral 17.13

Water, hydroxide and ammonia are all small, so six of them fit around a metal ion. The six bonding pairs repel one another equally. They get as far apart as possible, which puts them at the corners of an octahedron.

Each ligand has four neighbours at 90° and one ligand directly opposite at 180°.

Four large ligands: tetrahedral 17.14

Chloride ions are relatively large. Only four of them fit around a metal ion, so the coordination number is 4. Four bonding pairs repel equally, giving a tetrahedral shape with bond angles of 109.5°.

[Cu(HX2O)X6]X2++4 ClX−⇌[CuClX4]X2−+6 HX2O \ce{[Cu(H2O)6]^2+ + 4Cl- <=> [CuCl4]^2- + 6H2O}

[CuCl₄]²⁻ and [CoCl₄]²⁻ are the tetrahedral ions to know. The change from six ligands to four is also a change of shape, from octahedral to tetrahedral.

Four ligands in one plane: square planar 17.15

Some four-coordinate ions are not tetrahedral. Their four ligands lie in one plane around the metal ion, with bond angles of 90°. This is a square planar shape.

Three shapes of complex ion drawn with wedges and dashes. Left, an octahedral complex with six ligands and 90 degree angles. Middle, a tetrahedral complex with four chloride ligands and 109.5 degree angles. Right, a square planar complex with four ligands in one plane and 90 degree angles.
Octahedral, tetrahedral and square planar complexes. Wedges point towards you; dashes point away.

The example to know is cis-platin, [Pt(NH₃)₂Cl₂]. Do not try to predict square planar from electron-pair repulsion; learn the example.

Coordination number Shape Bond angle Examples
6 octahedral 90° [Cu(H₂O)₆]²⁺, [Ni(NH₃)₆]²⁺
4 tetrahedral 109.5° [CuCl₄]²⁻, [CoCl₄]²⁻
4 square planar 90° [Pt(NH₃)₂Cl₂]

Cis-platin and its isomer 17.15

Because the four ligands lie in one plane, [Pt(NH₃)₂Cl₂] has two isomers. In the cis isomer the two identical ligands are next to each other (90° apart). In the trans isomer they are opposite each other (180° apart).

Two square planar complexes of platinum. Left, cis-platin, with the two ammonia ligands next to each other and the two chloride ligands next to each other. Right, trans-platin, with the two ammonia ligands opposite each other and the chlorides opposite each other.
Cis-platin and trans-platin. They have the same formula but different arrangements in space.

Cis-platin is used in cancer treatment. It is supplied as the single cis isomer, not as a mixture with the trans form.

Check yourself

  1. Explain why [Ni(H₂O)₆]²⁺ is octahedral. Give the bond angle.
  2. [CoCl₄]²⁻ has four ligands but [Co(H₂O)₆]²⁺ has six. Explain the difference and give the shape and bond angle of [CoCl₄]²⁻.
  3. Draw cis-platin and trans-platin, and label each.
  4. State the shape and the bond angle of cis-platin, and give its medical use.
  5. Why is cis-platin supplied as a single isomer rather than a mixture?
  6. Why could [Zn(NH₃)₂Cl₂] (with Zn tetrahedral) not exist as cis and trans isomers?

Answers cover these until you have tried the questions

  1. The six bonding pairs (dative bonds) round Ni²⁺ repel equally and get as far apart as possible, giving an octahedral shape. The bond angle is 90°.
  2. Chloride ions are relatively large, so only four fit around the metal ion. Water molecules are small enough for six. [CoCl₄]²⁻ is tetrahedral, with bond angles of 109.5°.
  3. Both are square planar, in square brackets. In cis-platin the two NH₃ ligands are adjacent (90° apart) and the two Cl⁻ ligands are adjacent. In trans-platin the two NH₃ are opposite each other (180° apart), and so are the two Cl⁻.
  4. Square planar, bond angle 90°. It is used in cancer treatment.
  5. Only the cis isomer is the form used for treatment (the trans isomer does not work in the same way). The medicine is supplied as the single cis isomer, not in a mixture with the trans form.
  6. In a tetrahedral complex every ligand is next to every other (all bond angles are 109.5°). Swapping two ligands gives the same shape, so only one arrangement exists.