Transition elements form complexes. (a) Molybdenum, Mo, forms an octahedral complex consisting of one Mo atom surrounded by carbon monoxide, CO, molecules. CO is a monodentate ligand. Iron forms an octahedral complex consisting of one Fe³⁺ and a number of cyanide, CN⁻, ions. CN⁻ is a monodentate ligand. [Figure not explicitly referenced, but implied for drawing in (a)(iii)]

1. (a)(i)Define the term monodentate ligand.[1]
2. (a)(ii)Complete the table by stating the formulae and charges of the complexes described. molybdenum complex formula charge iron(III) complex[2]
3. (a)(iii)Draw a three-dimensional diagram to show the shape of this iron(III) complex. Label one 180° bond angle on your diagram. [Figure Fe][1]
4. (b)(i)An excess of aqueous ammonia is added to dilute copper(II) sulfate solution. A dark blue complex, [Cu(NH₃)₄(H₂O)₂]²⁺, is formed. Write an ionic equation for this reaction.[1]
5. (b)(ii)Explain the origin of colour in copper(II) complexes.[4]
6. (c)(i)An excess of concentrated hydrochloric acid is added to the dark blue solution of [Cu(NH₃)₄(H₂O)₂]²⁺. A new complex, Z, is formed. The colour of the solution changes. Write an equation for the formation of Z from the solution of [Cu(NH₃)₄(H₂O)₂]²⁺. Include the formula and charge of Z.[2]
7. (c)(ii)Name the type of reaction when Z forms from [Cu(NH₃)₄(H₂O)₂]²⁺.[1]
8. (c)(iii)State the geometry of Z.[1]
9. (c)(iv)State the colour of a solution of Z.[1]
10. (c)(v)Explain why the colour of a solution of [Cu(NH₃)₄(H₂O)₂]²⁺ is different from the colour of a solution of Z. You should refer to the energies of the orbitals involved in your answer.[1]

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