Magneto-structural studies on polynuclear complexes, aimed at understanding the structural and chemical factors that govern the exchange coupling between paramagnetic centers, are of continuing interest. One of the best illustrative examples corresponds to the di-µ-hydroxo-dicopper(II) complexes where the angle at the hydroxo bridge is the main factor governing the nature and magnitude of the intramolecular magnetic coupling. This book presents a systematic study of complex formation between gadolinium(III) and carboxylate-containing ligands which investigates the factors that govern the magnetic coupling in the digadolinium(III) units.