AS & A Level Chemistry 28.1 General physical and chemical properties of the first row of transition elements, titanium to copper: Exam Style Questions Paper 4

Transition metal atoms and transition metal ions form complexes by combining with ligands.
(a) Explain why transition elements form complex ions.
(b) \(Co^{2+}\) ions form complex ion G. Each G ion contains two \(Co^{2+}\) ions, both of which are octahedrally coordinated. Each G ion contains one \(O_2\) molecule, which donates one pair of electrons to each \(Co^{2+}\) ion, and one \(NH_2^–\) ion, which donates one pair of electrons to each \(Co^{2+}\) ion. The remaining ligands are NH₃ molecules.
(i) Deduce the formula of complex ion G. Include its overall charge.
(ii) The d-orbitals of the \(Co^{2+}\) ions present in complex ion G are split. State the number of d-orbitals that are at a higher energy level and the number of d-orbitals that are at a lower energy level in each \(Co^{2+}\) ion.

(iii) \(Co^{2+}\) ions form a different complex ion, M. Each M ion contains two \(Co^{2+}\) ions, both of which are octahedrally coordinated, but the ligands are different from the ligands in G. Explain why G and M have different colours.

(c) Cadmium forms complex ion X, \([Cd(NH_3)_4]^{2+}\). When a solution containing CN⁻ ions is added to an aqueous solution of X, a ligand exchange reaction takes place, forming complex ion Y. Y contains no \(NH_3\) ligands and no \(H_2O\) ligands. Y is in a much higher concentration in the mixture than X. The oxidation state and coordination number of cadmium do not change in this reaction.
(i) Write an ionic equation for this reaction, using the formulae of the complex ions.
(ii) Cadmium forms complex ion Z in the same oxidation state and with the same coordination number as in X. All the ligands in Z are Cl⁻ ions. When NaCl(aq) is added to a solution of X, very little Z forms. Write the three cadmium complexes, X, Y and Z, in order of increasing stability constant, \(K_{stab}\).

(d) Ethanedioate ions, \(C_2O_4^{2–}\), form complexes with transition element ions. The concentration of \(C_2O_4^{2–}\) ions can be found by reaction with acidified \(Cr_2O_7^{2–}\) ions. \(C_2O_4^{2–}\) ions are protonated and form HOOCCOOH molecules which are oxidised by \(Cr_2O_7^{2–}\). The half-equations are shown.

(i) Construct an equation for the reaction between acidified Cr₂O₇²⁻ and HOOCCOOH

(ii) A 25.0 cm³ sample of a solution of Na₂C₂O₄ reacts with exactly 16.20 cm³ of an acidified solution of 0.0500 moldm⁻³ K₂Cr₂O₇. Calculate the concentration of the solution of Na₂C₂O₄.