This thesis concerns the synthesis and characterisation of new di(pyrazolyl)pyridine and di(pyrazolyl)pyrazine derivatives, and their iron(II) complexes which exhibit spin crossover behaviour. The examination of their magnetic behaviour using NMR spectroscopy, X-ray crystallography and magnetic measurements is described. Chapter One introduces the background behind the spin crossover phenomenon, especially in the case of iron(H) compounds. The methods of identification and the applications of spin crossover are discussed. Finally the aims of the research are also detailed. Chapters Two and Three discuss the synthesis and characterisation of previously unreported 2,6-di(4-R-pyrazol-I-yl)pyrazine compounds (R ~ CI, Br, I, Me, NO" NH" CCPh). The synthesis of their iron(ll) complexes as BF, and CIO, salts is also detailed, and their magnetic behaviour is discussed and compared to previous 2,6- bis(pyrazol-l-yl)pyridine complexes. The crystal structures of the complexes are presented. Chapter Four is a study of novel di(pyrazol-l-yl)pyridine derivatives substituted at the pyrazolyl or pyridyl 4-positions, and their iron(Il) complexes as before. Magnetochemical data for three complexes are presented, together with crystallographic data and a discussion relating structure to magnetic behaviour. Chapter Five focuses on the study of di(pyrazolyl)pyridine derivatives substituted at the pyridyl 4-positions by azo, ethynyl or styryl groups, and their iron(ll) complexes. Examination of their magnetic behaviour and crystal structures demonstrates the difficulty of predicting the spin state of an iron(lI) complex.