Due to a wide variety of modes in which metal ions (complexes) and ligands can combine forming new assemblies of extended structures with new properties, the polynuclear transition metal complex compounds attract a continuously growing scientific interest. Under the proposed project two kinds of polynuclear metal systems will be investigated: polynuclear high-spin transition metal complexes and hexanuclear halide clusters of niobium and tantalum. The hypothesis is that the magnetic exchange interaction between two relatively distant (~ 5 Å) paramagnetic metal centres in polynuclear metal systems can be controlled by a careful selection of bridging ligand, interacting metal ions and other constituents. The synthetic procedures for getting new polynuclear complex systems of interesting magnetic (and other) properties will be established. The tris(oxalato) [M(C₂O₄)₃]^3– (M = Cr, Fe, Mn) and [MO(C₂O₄)3]^3- (M = Nb or Ta) complexes as ligands towards other paramagnetic transition metal ions (Co, Ni, Cu, rare earth elements) will be used. In order to obtain more complex systems other bridging ligands (like CN group) will be introduced, as well. The new compounds will be characterized in detail, with the emphasis on the correlation of structural and magnetic properties. The influence of ions (spin value) and ligands (bridging and terminal) upon the nature and magnitude of magnetic exchange interaction between two paramagnetic centres will be studied. Within the hexanuclear halide clusters of niobium and tantalum, [(M₆X₁₂)L₆]^n± (X = Cl, Br; n = 2, 3, 4) a great number of new cluster species with interesting properties, such as those exhibiting semiconducting properties or having two or more differently charged cluster entities in the same formula unit have been prepared in our laboratory. Under this project the investigation of up to date scarcely studied magnetic properties of paramagnetic M6 clusters will dominate. The aim is to extend the fundamental knowledge on magnetic properties of these nanodimensional clusters, i.e. to explain some basic principles of these particles, like magnetic moment, g-factor, mechanism and magnitude of magnetic exchange interaction. The approach that will be used has not been applied so far to any similar nanodimensional group of atoms (one unpaired electron delocalized over the whole M6 unit). The results of the research work will be verified through the scientific papers published in the world relevant scientific journals.