A LevelChemistryCIETopic Questions28. Chemistry of Transition Elements28.4 Stereoisomerism in Transition Element Complexes

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First teaching 2023

First exams 2025

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Stereoisomerism in Transition Element Complexes (CIE A Level Chemistry)

Topic Questions

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1a2 marks

Carboplatin and oxaliplatin are both platinum(II) diamines, similar to cisplatin, which are widely used in chemotherapy to treat a wide variety of cancers. However, their use is often plagued with severe toxicity and the development of resistance, which leads to disease progression. Recently, oxoplatin, iproplatin, ormaplatin and satraplatin are Pt complexes that have been used clinically or in clinical trials. 

The central platinum ions in the four complexes shown in Fig. 1.1 all have the same shape and oxidation number.

 

platins

Fig. 1.1


Give the following information for the complexes.

oxidation number of the platinum ion ..................................................

shape of the complexes ..................................................

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1b4 marks

One isomer of the [Pt(NH3)2Cl2(OH)2] complex oxoplatin is shown in part (a) Fig. 1.1.

Complete three-dimensional diagrams to show the shape of the other oxoplatin stereoisomers and indicate the chiral one(s).

screenshot-2023-06-01-130550

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1c1 mark

Platinum complexes can be considered prodrugs that are intracellularly activated to kill cancer cells.

Cis-platin can be produced by the reduction of the prodrug cis, trans, cis-[PtCl2(OCOCH3)2(NH3)2], which has a similar structure to satraplatin.

Draw the structure of cis, trans, cis-[PtCl2(OCOCH3)2(NH3)2].

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1a2 marks

Cobalt is a transition element with atomic number 27.

Complete the electronic configurations of a Co atom and a Co2+ ion.

 
Co atom   1s2 2s2 2p6  ..................................
 
Co2+ ion   1s2 2s2 2p6  ..................................

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1b2 marks

Co2+ ions form a linear complex with Cl– ions, which are monodentate ligands.

 

Draw the structure of this complex and include its overall charge.

structure




charge ......................................

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1c3 marks

Co2+ ions exist as [Co(H2O)6]2+ complex ions in an aqueous solution.

 

Complete a three-dimensional diagram to show the shape of this complex.

 

Name its shape.

 

Label and state the value of two bond angles.

 
6-3-3c-m-blank-octahedral-co-a
 
name of shape ..................................................

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1d4 marks

When NH3 (aq) is added to Co2+ (aq), dropwise at first and then in excess, two chemical reactions occur as shown.

 
[Co(H2O)6]2+ rightwards arrow with 1. space dropwise space NH subscript 3 space open parentheses aq close parentheses on top A rightwards arrow with 2. space excess space NH subscript 3 space stretchy left parenthesis aq stretchy right parenthesis on top B
 

For each reaction, describe what you would see and write an equation.

 
reaction 1
 
   observation ................................................................................
 
   equation ................................................................................
 
reaction 2
 
   observation ................................................................................
 
   equation ................................................................................ 

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1e5 marks

EDTA4– is a hexadentate ligand. When a solution of EDTA4– is added to a solution containing [Co(H2O)6]2+ a new complex is formed. The formula of this complex is [CoEDTA]2–.

 
i)
Explain what is meant by the term hexadentate ligand.
 
[2]
 
ii)
Name the type of reaction occurring here.
 
[1]
 
iii)
Write an expression for the stability constant, Kstab, of [CoEDTA]2– in this reaction.
 
[1]
 
iv)
The numerical value of the Kstab of [CoEDTA]2– is 7.7 x 104 mol–6 dm18 at 298K.
 
State what this tells us about the [CoEDTA]2– complex ion.
 
[1] 

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2a4 marks
The chloride ion is a monodentate ligand. When concentrated hydrochloric acid is added to [Co(H2O)6]2+ (aq) ions the water ligands are replaced.

i)
Suggest an equation for the reaction of [Co(H2O)6]2+ (aq) with concentrated hydrochloric acid.
 
[2]
 
iii)
Draw the three-dimensional diagram to show the complex formed in the reaction between [Co(H2O)6]2+ (aq) and HCl.
 
[2]

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2b5 marks

[Co(NH3)4Cl2]+ is an example of an octahedral complex which exhibits cis-trans isomerism.

i)
Deduce the oxidation state of cobalt in this complex. Explain your answer.
 
[2]
 
ii)
Draw structures of the two [Co(NH3)4Cl2]+ isomers clearly identifying each isomer.
 
[3]

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2c3 marks

Explain the origin of colour in a transition element complex such as [Co(NH3)4Cl2]+.

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3a1 mark

Bidentate ligands form two coordinate bonds to a central metal atom or ion in a complex.

Suggest the meaning of bidentate ligand.

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3b3 marks

The product of the reaction between [Cu(H2O)6]2+ and 1,2-diaminoethane exists as a pair of optical isomers.

Draw three‑dimensional structures for the two optical isomers.

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3c2 marks

1,2-diaminoethane is an example of a compound which is a bidentate ligand. Full ligand substitution occurs when 1,2-diaminoethane is added to [Cu(H2O)6]2+.

Write an equation for the reaction of [Cu(H2O)6]2+ with 1,2-diaminoethane.

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