The $3d$ orbitals in an isolated $\text{Fe}^{2+}$ ion are degenerate. Finish the diagram to illustrate how the $3d$ orbital energy levels split in an isolated $\text{Fe}^{2+}$ ion and when $\text{Fe}^{2+}$ forms an octahedral complex.
Bipyridine, bipy, is a bidentate ligand (Fig. 4.1). Explain what is meant by bidentate ligand.
The complex $[\text{Fe(bipy)}_3]^{2+}$ exists as two stereoisomers. Complete the three-dimensional diagrams to show the two stereoisomers of $[\text{Fe(bipy)}_3]^{2+}$. State the type of stereoisomerism shown. Use the given $\text{N}\!\!\_\!\!\_\!\!\text{N}$ symbol to represent bipy in your diagrams.
Standard electrode potentials may be used to compare the stability of different complex ions for a particular transition element. Table 4.1 gives electrode potentials for some electrode reactions for $\text{Fe}^{3+}/\text{Fe}^{2+}$ systems. Use relevant data from Table 4.1 to state which iron(III) complex is hardest to reduce. Explain your choice.
The ligand bipyridine is made up of two pyridine rings. Pyridine, $\text{C}_5\text{H}_5\text{N}$, and benzene, $\text{C}_6\text{H}_6$, have comparable planar, cyclic structures. By referring to the hybridisation of the carbon atoms and the nitrogen atom, and to orbital overlap, suggest how the $\sigma$ and $\pi$ bonds are formed in a pyridine molecule.
Pyridine reacts with $\text{Cl}_2$ in the presence of $\text{AlCl}_3$ as shown in Fig. 4.3. The mechanism of this reaction is similar to the chlorination of benzene. $\text{AlCl}_3$ reacts with chlorine to form an electrophile, $\text{Cl}^+$. Complete the diagram to show the mechanism for the reaction of pyridine with $\text{Cl}^+$. Include all relevant charges, dipoles, lone pairs of electrons and curly arrows where needed.