Chapter One gives an overview of molecule-based magnetic materials, and a summary of topics of current interest in this field such as spin crossover (SCO), molecule-based magnets (particularly three room temperature metallo-organic magnets), Metallo-organic frameworks (MOFs), free radical magnets and single molecule magnets (SMMs). There is a brief introduction of key examples and developments in the specific topics, and a perspective at the end. Chapter Two introduces a family of optically pure Fe(II) polymeric chain complexes of formula {FeL2(μ-pz)}∞ and {FeL2(μ-bpy)}∞. [L = bidentate Schiff base ligands obtained from (R)-(+)-α-phenylethanamine and 4-substituted salicylaldehydes]. The structural and magnetic properties of the polymeric products are determined by single crystal X-ray diffraction and SQUID magnetometry. By fitting magnetic data of these complexes with the Bonner- Fisher 1-D chain model, the magnitudes of their magnetic exchanges are rationalised on the basis of substituent electronic properties and bridging ligand identity. Chapter Three describes two Fe(II) coordination polymers containing pyridineconjugated Schiff base isomer ligands. The isomerism of the two ligands leads to a change from trans to cis coordination in the [FeL2] SBU and thus from a tetrahedral diamond-like 3 D network exhibiting a gradual SCO to a 2 D hard magnet. Furthermore, we have also sythesised another four Cu(II) complexes based on these two ligands. Crystallographic studies reveal their structures ranging from 1 D zigzag chains and 2 D mat while their magnetic properties are transformed from ferro- to ferrimagnetic behavior. Chapter Four focuses on three pyrrole-2-ketone bidentate ligands. Three transition metal ions Mn2+, Fe2+ and Co2+ were studied, based on which twelve complexes have been made. The system is structurally diverse, with 1 D, 2 D, monometallic, trimetallic “sandwich” structures and a high nuclearity cluster being observed depending on the use of cations and solvents. Unusual magnetic phenomena are discovered, including a system in which SCO and ferromagnetic coupling are present. Chapter Five details the experimental procedures used to carry out the work in this thesis.