The ligand bipyridine consists of two pyridine rings. Pyridine, C₅H₅N, and benzene, C₆H₆, have similar planar, cyclic structures.

1. (a)The 3d orbitals in an isolated Fe2+ ion are degenerate. Complete the diagram to show the splitting of the 3d orbital energy levels in an isolated Fe2+ ion and when Fe2+ forms an octahedral complex. energy isolated Fe2+ ion Fe2+ in an octahedral complex[2]
2. (b)(i)Bipyridine, bipy, is a bidentate ligand. [Figure 4.1] Explain what is meant by bidentate ligand.[2]
3. (b)(ii)The complex [Fe(bipy)3]²+ exists as two stereoisomers. Complete the three-dimensional diagrams to show the two stereoisomers of [Fe(bipy)3]2+. State the type of stereoisomerism shown. Use N N to represent bipy in your diagrams. Fe type of stereoisomerism[3]
4. (c)Standard electrode potentials can be used to compare the stability of different complex ions for a given transition element. Table 4.1 lists electrode potentials for some electrode reactions for Fe3+/Fe2+ systems. Table 4.1 electrode reaction E°/V [Fe(H2O)6]3+ + e¯ ⇌ [Fe(H2O)6]2+ +0.77 [Fe(CN)6]3- + e¯ ⇌ [Fe(CN)6]4- +0.36 [Fe(bipy)3]³+ + e¯ ⇌ [Fe(bipy)3]2+ +0.96 Use relevant data from Table 4.1 to state which iron(III) complex is hardest to reduce. Explain your choice. iron(III) complex explanation[1]
5. (d)By reference to the hybridisation of the carbon atoms and the nitrogen atom, and orbital overlap, suggest how the σ and π bonds are formed in a pyridine molecule.[3]
6. (e)Pyridine reacts with Cl₂ in the presence of AlCl₃ as shown in Fig. 4.3. [Figure 4.3 showing reaction scheme] The mechanism of this reaction is similar to that of the chlorination of benzene. AlCl₃ reacts with chlorine to generate an electrophile, Cl⁺. Complete the diagram to show the mechanism for the reaction of pyridine with Cl⁺. Include all relevant charges, dipoles, lone pairs of electrons and curly arrows as appropriate. N Cl+[3]

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