The thesis describes the synthesis, characterisation and magnetic properties of some novel well-defined paramagnetic polymetallic arrays based on the 3d metal ions Fe(III), Ni(II) and Cu(II). Chapter 1 begins with an overview of modern applications of paramagnetic assemblies and then examines synthetic strategies that can be employed for their preparation before identifying the aims and objectives set for the thesis. In Chapter 2, the reactivity of a ligand blend consisting of a combination of two different bifunctional 2-pyridine alcohols [drawn from 2-{(CH­2)nOH}-6-X-C5H3N] (X = H or halide; n = 0 -- 3) towards M(OAc)2.xH2O (M = Ni, Cu) in the presence of triethylamine is systematically examined. In Chapter 3, neutral and charged {M2}, {M3} and {M4} (M = Ni or Cu) species are accessible on treatment of M(OAc)2.xH2O with a ligand blend consisting of benzilic acid and [2-{(CH2)nOH}-6-X-C5H3N] (X = H, or halide; n = 0 -- 3). A study of the role of the base employed (NEt3 vs. NaOMe) on product type has also been investigated. The presence of very weak magnetic exchange interactions is a feature of the open dicuboidal dianion {Ni4}2- prepared. In Chapter 4, isophthalic acid and [2-{(CH2)nOH}-6-X-C5H3N] (X = H or halide; n = 1 -- 3) are used as components of the ligand blend in combination with M(OAc)2.xH2O affording polynuclear mixed-ligand {M2} (M = Ni, Cu) complexes. In contrast, the combination of isophthalic acid and an aryl-substituted formamidine leads to the neutral spheroidal {Cu24} clusters (nanoballs) and in one case to a 1-D {Cun} polymer; strong antiferromagnetic behaviour is a feature of the nanoball. In Chapters 5 and 6, a selection of discrete polynuclear iron complexes [e.g., {Fe2}, {Fe4}, {Fe6}, {Fe8}, {Fe10} and {Fe11}] containing mixed-ligand sets have been prepared. In Chapter 7, full details of the experimental procedures along with spectroscopic, analytical and X-ray data (ca. 50 structures) are reported.