Shapes of complex ions (A-level only) — AQA A-Level Chemistry
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Shapes of complex ions (A-level only) explained
Ligand size decides the shape. Small ligands such as H₂O, NH₃ and CN⁻ pack six around a metal ion to give an octahedral complex with bond angles of 90°.
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Chloride is much bigger, so only four fit, and those four arrange themselves tetrahedrally with bond angles of 109.5°. [CuCl₄]²⁻ and [CoCl₄]²⁻ are the standard examples and both are tetrahedral, not square planar, a confusion that a multiple choice paper has tested directly. Adding concentrated hydrochloric acid to aqueous copper(II) shows the swap: pale blue octahedral [Cu(H₂O)₆]²⁺ becomes yellow tetrahedral [CuCl₄]²⁻, the co-ordination number falling from 6 to 4 and the colour changing as a result.
Square planar complexes are also formed and can display cis–trans isomerism.
Square planar complexes have four ligands in one plane at 90° to each other, and platinum(II) is the classic example. When two of the four ligands differ, as in Pt(NH₃)₂Cl₂, two distinct compounds exist: the cis isomer with the chlorides on adjacent corners, 90° apart, and the trans isomer with them on opposite corners, 180° apart. They are stereoisomers, having the same atoms joined in the same order but arranged differently in space, and they cannot interconvert without breaking bonds. Octahedral complexes do the same: [Cr(NH₃)₄Cl₂]⁺ has cis and trans forms. A mark scheme for drawing them gives one mark for the pair of structures and a separate mark for showing the charge on the complex ion.
Cisplatin is the cis isomer.
Cisplatin is cis-Pt(NH₃)₂Cl₂, a square planar platinum(II) complex with the two ammonia ligands adjacent and the two chloride ligands adjacent. In the body a chloride ligand is replaced by water, and the resulting complex bonds to a nitrogen atom on a guanine base in DNA, then repeats with a second guanine. The DNA strand is locked together, so it cannot unwind and replicate and the cell cannot divide. Geometry is everything here: only in the cis isomer are the two leaving groups close enough to reach two bases on the same strand, so the trans isomer is inactive as a drug. Because cisplatin attacks any rapidly dividing cell it also damages hair follicles and bone marrow, so dose is a balance of benefit against side effects.
Your focus
- Predict the shape and co-ordination number of a complex from the size of its ligand.
- Draw [CuCl₄]²⁻ as a tetrahedral complex and label the 109.5° bond angle.
- Write the equation for the conversion of [Cu(H₂O)₆]²⁺ into [CuCl₄]²⁻ and state the colour change seen.
Show all 9 objectives
- Draw the cis and trans isomers of a square planar complex such as Pt(NH₃)₂Cl₂, including any overall charge.
- Name the type of isomerism shown and justify why it exists in square planar but not in tetrahedral complexes.
- State the bond angle between two ligands in the cis arrangement and in the trans arrangement.
- Draw the structure of cisplatin and distinguish it from the trans isomer.
- Explain how cisplatin prevents cell division through ligand substitution and binding to guanine in DNA.
- Evaluate the use of cisplatin by weighing its action on cancer cells against damage to healthy dividing cells.
Shapes of complex ions (A-level only) exam tips
Marking Points
- one mark for identifying a chloride complex such as [CoCl₄]²⁻ or [CuCl₄]²⁻ as tetrahedral
- one mark for the bond angle, 109.5° in a tetrahedral complex against 90° in an octahedral one
- one mark for attributing the co-ordination number of 4 to the larger size of the chloride ligand
- one mark for a correct substitution equation that includes all six displaced water molecules
- one mark for two correctly drawn isomers, with ligands on adjacent corners for cis and opposite corners for trans
- one mark for showing the overall charge on the complex ion in the drawing
- one mark for naming the isomerism as cis-trans or geometric
- one mark for the bond angles, 90° between cis ligands and 180° between trans ligands
- one mark for drawing cisplatin as square planar with the two chloride ligands adjacent
- one mark for a ligand substitution step in which a chloride is replaced, allowing platinum to bond to a nitrogen atom on guanine in DNA
- one mark for stating that the DNA cannot replicate so the cell cannot divide
- one mark for explaining that the trans isomer is inactive because its leaving groups are too far apart
- one mark for a named side effect linked to damage to healthy rapidly dividing cells
Examiner Tips
- 💡Read the ligand first, because Cl⁻ almost always means four ligands and a tetrahedral shape.
- 💡Give the bond angle whenever you name a shape, since it is often a separate mark.
- 💡Show all six water molecules on the right hand side when chloride replaces them.
- 💡Draw the square with ligands at the corners, not on the edges, so adjacent and opposite are unambiguous.
- 💡Write the charge outside the square bracket every time you draw a complex ion.
- 💡Rotate your second drawing mentally to check that it really is a different isomer.
- 💡Label the ammonia and chloride ligands on your drawing so the cis arrangement is unmistakable.
- 💡Mention DNA replication explicitly, because it stops the cancer does not score.
- 💡If asked to evaluate the use of the drug, give one benefit and one specific side effect.
Common Mistakes
- calling [CoCl₄]²⁻ square planar, which is the distractor used in the multiple choice papers
- quoting 90° as the bond angle in a tetrahedral chloride complex
- keeping six ligands when writing the chloride complex, so giving [CuCl₆]⁴⁻
- explaining the change of shape by the charge on chloride rather than by its size
- drawing a square planar complex as a tetrahedron, which cannot show cis-trans isomerism at all
- omitting the charge on the complex ion in a drawing, which loses a separate mark
- drawing the same isomer twice with the square simply rotated
- calling the two forms optical isomers or structural isomers rather than stereoisomers
- using E-Z nomenclature, which applies to alkenes, not square planar complexes
- drawing cisplatin as tetrahedral, which destroys the whole cis and trans argument
- saying cisplatin kills the cancer cell directly rather than preventing DNA replication
- naming the bonding site as the sugar or the phosphate of DNA instead of a nitrogen atom on a base
- explaining side effects by saying the drug is poisonous rather than by damage to healthy dividing cells