This thesis is an account of the synthesis of novel iron(II) and ruthenium(II) complexes using 2,6-di(1H-pyrazol-3-yl)pyridine based ligands and their efficacy as spin crossover complexes and transfer hydrogenation catalysts respectively. Chapter 1 is an introduction to the spin crossover and transfer hydrogenation detailing significant literature examples, important influencing factors and applications. Chapter 2 details the synthetic pathways towards all novel ligands used in this work, including numerous attempted pathways and problems which were overcome. Chapter 3 describes the properties of a number of iron(II) complexes made using 2,6-di(alkylpyrazol-3-yl)pyridine ligands. It is shown here that subtle steric and electronic factors about the iron(II) coordination sphere hinder spin crossover. Chapter 4 is a comparison of spin crossover properties of iron(II) complexes containing 2,6-di(1H-pyrazol-3-yl)pyridine ligands bearing a number of different substituents on different positions. The spin crossover behaviour of these complexes is rationalised based on crystal structures, powder x-ray diffraction and magnetic susceptibility measurements. Chapter 5 is an explanation of the differing magnetic susceptibilities of a number of solid solutions of the complex [FeL2]X2 (L = 2,6-di(1H-5-methylpyrazol-3-yl)pyridine). Chapter 6 shows the effectiveness of a number of ruthenium(II) pincer type complexes as catalysts for the transfer hydrogenation of acetophenone derivatives. Chapter 7 is an account of all the synthetic procedures used in this work and includes the full analysis of all novel compounds synthesised.