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CAIE A-Level Chemistry A2 28.4 Stereoisomerism in Transition Element Complexes Questions

Practise drawing cis/trans and optical isomers of complexes and deducing polarity from three-dimensional symmetry.

Syllabus
2028–2030
Course
Chemistry 9701
Level
A2

Exam points

  • draw all distinct cis/trans and optical isomers while preserving the complex geometry and ligand denticity
  • identify enantiomers as non-superimposable mirror images and distinguish them from geometrical isomers
  • deduce polarity by deciding whether individual bond dipoles cancel in the three-dimensional arrangement

Covered syllabus points

28.4.1—Types of stereoisomerism shown by complexes28.4.2—The overall polarity of complexes such

Question 1

[Maximum number: 4]

Titanium is a transition element in Period 4. It is commonly found as TiO2\mathrm{TiO}_{2} in minerals.

Question (a)

(a)

Some reactions of TiO2\mathrm{TiO}_{2} are shown in Fig. 3.1.

The anion, acac −^{-}, is a bidentate ligand.

Fig. 3.1

Fig. 3.1

[ 4 ]

Question (i)

(i)

Ti(acac)2Cl2\mathrm{Ti}(\mathrm{acac})_{2} \mathrm{C} l_{2} shows both optical and geometrical (cis/trans) isomerism.
Ti(acac)2Cl2\mathrm{Ti}(\mathrm{acac})_{2} \mathrm{C} l_{2} exists as three stereoisomers.
The structure of one stereoisomer of Ti(acac)2Cl2\mathrm{Ti}(\mathrm{acac})_{2} \mathrm{Cl}_{2} is shown in Fig. 3.2.

Fig. 3.2

Fig. 3.2

Complete the structures of the other two stereoisomers of Ti(acac)2Cl2\mathrm{Ti}(\mathrm{acac})_{2} \mathrm{C} l_{2}.

Figure for Question (i) — CAIE A-Level Chemistry A2
Figure for Question (i) — CAIE A-Level Chemistry A2
[ 2 ]

Question (ii)

(ii)

The acac- anion is symmetrical.

Deduce which, if any, of stereoisomers 1, 2 and 3 in (d)(iv) are polar. Explain your answer.

[ 2 ]

Question 2

[Maximum number: 2]

An aqueous solution of copper(II) sulfate is a blue colour due to the presence of [Cu(H2O)6]2+\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+} complex ions.

Question (a)

(a)

If an excess of ammonia is added to a solution of [Cu(H2O)6]2+\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+} a deep blue solution containing [Cu(NH3)4(H2O)2]2+\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+} complex ions is formed.

[ 2 ]

Question (i)

(i)

There are two possible stereoisomers with the formula [Cu(NH3)4(H2O)2]2+\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}.

Complete the diagrams to show the two stereoisomers in the boxes below.

isomer 1

isomer 1

isomer 2

isomer 2

[ 1 ]

Question (ii)

(ii)

Use your answer in (b)(i) to deduce whether each of these isomers is polar or non-polar. polarity of isomer 1
polarity of isomer 2

[ 1 ]

Question 3

[Maximum number: 4]

Copper is a transition element and has atomic number 29.

Question (a)

(a)

Nickel forms the complex ion [Ni(en)3]2+\left[\mathrm{Ni}(e n)_{3}\right]^{2+} in which it is surrounded octahedrally by six nitrogen atoms.

[ 4 ]

Question (i)

(i)

Name the type of stereoisomerism displayed by [Ni(en)3]2+\left[\mathrm{Ni}(e n)_{3}\right]^{2+}.

[ 1 ]

Question (ii)

(ii)

Draw three-dimensional diagrams to show the two stereoisomers of [Ni(en)3]2+\left[\mathrm{Ni}(e n)_{3}\right]^{2+}.

[ 3 ]
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