Stereoisomerism in Transition Element Complexes (Cambridge (CIE) A Level Chemistry)

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  ..................................

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 ......................................

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.

 name of shape ..................................................

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⁺  A  B

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

  reaction 1

   observation ................................................................................

   equation ................................................................................

reaction 2

   observation ................................................................................

   equation ................................................................................ 

EDTA^4– is a hexadentate ligand. When a solution of EDTA^4– 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. 

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ii) Name the type of reaction occurring here. 

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iii) Write an expression for the stability constant, K_stab, of [CoEDTA]^2– in this reaction. 

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iv) The numerical value of the K_stab of [CoEDTA]^2– is 7.7 x 10⁴ mol^–6 dm¹⁸ at 298K. 

State what this tells us about the [CoEDTA]^2– complex ion. 

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