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The structural and magnetic properties of the two novel compounds: [(Cu(phen)_2)_2(mu-C_2O_4)][Cu(phen)_2(mu-C_2O_4)NbO(C_2O_4)_2]_2 center dot 8H_2O (1) and [(Cu(bpy)_2)_2(mu-C_2O_4)][Cu(bpy)_2(mu-C_2O_4)NbO(C_2O_4)_2]_2 center dot 0.5bpy center dot 7H_2O (2) (phen = 1, 10-phenanthroline, bpy = 2, 2'-bipyridine) was presented. The molecular structures of both compounds consist of a discrete Cu^IICu^II oxalate-bridged dinuclear [(Cu(L)_2)_2(mu-C_2O_4)]^2+ cations (A units) and two Cu^IINb^V oxalate-bridged dinuclear [Cu(L)_2(mu-C_2O_4)NbO(C_2O_4)_2]^- anions (B units) (L = phen, bpy). In 1 a crystallographic inversion centre is located in the middle of the oxalate bridge of the A unit, whereas in 2 an analogous inversion centre is missing. In the A units the copper(II) atoms adopt a tetragonally elongated octahedral coordination with the equatorial planes being perpendicular to the mean planes of the oxalate bridge and parallel to each other. In both structures, similar one-dimensional structural motifs are generated through the ligand stacking interactions, with a difference that one free bipyridine molecule, present in 2, intercalates into one of the motifs. It is shown that the phenanthroline ligand, due to its ability of stacking through the central aromatic ring, causes longer intermolecular Cu...Cu distances than the bipyridine ligand. Structural data are correlated with the magnetic susceptibility measurements (1.8 - 290 K) which show the ferromagnetic exchange interaction between the copper(II) atoms in the A units of both compounds, with J = + 5.9 cm^-1 and + 7.9 cm^-1 for 1 and 2, respectively (J is the magnetic exchange parameter in the isotropic spin Hamiltonian H_(INT) = -JS1xS2).

The isostructural, trigonal clusters of the composition [Nb6Cl12(H2O)(6)](OH)(2) . 1/4CH(3)OH . 6H(2)O 1 and [Nb6Cl12(H2O)(6)](OH)(2) 6H(2)O 2 have been isolated from alkaline solutions of [Nb6Cl12](2+) cluster in alcohol/water (1:3) in the presence of NMe(4)OH, while the presence of NEt(4)OH yielded NEt(4)[Nb6Cl12(H3O2)(6/2)]. 3H(2)O 3. Compound 1 crystallizes in trigonal space group R(3) over barm$ with a = 21.166(5)Angstrom and c = 23.812(4)Angstrom, gamma = 120 degrees, U = 9238.6(3) Angstrom(3), Z = 12. Two crystallographically independent cluster octahedra (I and II) have been found in the unit cell of 1 in the ratio 3:1. The Nb-O distances of 2.211(11)Angstrom and 2.217(16)Angstrom for I and 2.238(19)Angstrom for II indicate six co-ordinated water molecules. The cluster units of I are connected by hydrogen bonds of 2.49(1)Angstrom into a two-dimensional framework. Compound 3 is monoclinic, space group P2(1)/n, a = 10.085(5) Angstrom, b = 11.446(4)Angstrom, c = 14.971(6)Angstrom, beta = 102.95(6)degrees, U = 1684.2(1)Angstrom(3), Z = 2. The short Nb-O distances at 2.167(6), 2.177(8) and 2.187(7)Angstrom and considerable reduction of the Nb-O stretching frequency in the IR spectra support the presence of H3O2- (co-ordinated water molecule from one cluster unit and co-ordinated OH- group from the neighbouring cluster unit) bridges. Each cluster unit is joined to the six neighbouring units by intermolecular hydrogen bonds of distances 2.540(11)Angstrom and 2.594(10)Angstrom in a three-dimensional network.

Conditions have been established for the preparation of three different hydrated chloroniobium cluster hydroxide compounds from alkaline solutions: triclinic [Nb6Cl12(OH)(2)(H2O)(4)].4H(2)O 1 and two cubic compounds whose structures are uncertain with regard to the co-ordination of OH- ions. Nb6Cl12(OH)(2)(H2O)(14) 2 and Nb6Cl12(H2O)(14)(OMe)(2) 3. Based upon structural data 2 and 3 may contain either the cation [Nb6Cl12(H2O)(6)](2+)or the neutral cluster [Nb6Cl12(OH)(2)(H2O)(4)] containing hydroxo ligands. In 1 distinct distances of 2.147(5) for Nb-OH, and 2.250(5) and 2.289(5) Angstrom for Nb-OH2 were found. In 2 and 3 only distances of 2.19(1) and 2.193(7) Angstrom, respectively, were found for the Nb-O bonds of co-ordinated water (or hydroxide-water averaged). Compound 1 is triclinic, space group P (1) over bar, a = 8.930(3), b = 9.195(2), c = 9.118(1) Angstrom, alpha = 104.37(2), beta = 99.51(2) and gamma = 116.99(2)degrees, U = 611.9(3) Angstrom(3) Z = 1, R = 0.036. Compound 2 is cubic, space group Fm (3) over bar m, a = 14.59(2) Angstrom, U = 3105.7(1) Angstrom(3) Z = 4, R = 0.028. Compound 3 is also cubic, space group Fm (3) over bar m, a = 14.627(1) Angstrom, U = 3129.3(1) Angstrom(3), Z = 4. R = 0.031. IR spectra of 1 show strong O-H and Nb-O stretching bands that reflect the presence of co-ordinated OH- groups. In 2 and 3 the IR spectra are less definitive about possible co-ordination of hydroxide.

The proton NMR and specific conductance studies of the [(M_6X_12)(H_2O)_4X_2]• 4H_2O (M = Nb, Ta ; X = Cl, Br) clusters in methanol or water-methanol solutions have been performed. The data indicate a lower degree of dissociation for chloride outer ligands than for bromide.

Structural and magnetic investigations of the series Sr_2Ln_(1-x)Ce_xMCu_20_(8+delta), Ln = Pr, Nd, Sm, Eu, Gd ; M = Nb, Ta, which can be derived from YBa_2Cu_3O_(7-delta) by replacement of the CuO4 chains with MO6 octahedra, have been performed. Substitution of the rare earth metal atoms by cerium atoms in this series causes the appearance of a second tetragonal phase. For x = 0.5 only this second tetragonal phase exists. It has a structure as that found for the superconducting compound Sr_2Nd_(1.5)Ce_(0.5)NbCu_2O_(10-delta). Between the semiconducting Sr_2Ln_(0.5)Ce_(0.5)MCu_2O_(8+delta) and superconducting Sr_2Ln_(1.5)Ce_(0.5)MCu_2O_(10-delta) compounds, keeping the Ce content constant (Ce = 0.5), the rare earth stoichiometry can be varied without changing the type of the crystal structure, with superconductivity occurring for Ln>0.9, except for Ln = Pr.