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In the rational synthesis of novel materials the oxalate anion has appeared as a useful bridging ligand in constructing a diversity of homo and heterometallic compounds with attractive new architectures and interesting properties. A very frequently used approach for the synthesis of polynuclear oxalate-bridged species consists in using stable mononuclear anionic oxalate complexes as ligands toward the second metal ion - the approach known as "complexes as ligands" or "building-blocks chemistry".

Solvatomorphism has been found between two series of complexes of the composition [M(bpy)_3]_2[NbO(C_2O_4)_3]Cl•nH_2O [M = Fe^2+(1, 2), Co^2+(3, 4), Ni^2+(5, 6), Cu^2+(7) and Zn^2+(8, 9); bpy=2,2'-bipyridine)], crystallizing in the monoclinic space group P2_1/c[3, 5, 8 (n=11)] or in the orthorhombic space group P2_12_12_1[2, 4, 6, 7 (n= 12)]. All the structures contain two symmetry independent [M(bpy)_3]^2+ cations, one [NbO(C_2O_4)_3]^3- anion, one Cl– anion, and crystal water molecules. The cations possess a trigonally distorted octahedral geometry, with an additional tetragonal distortion in 7. Analysis of crystal packing reveals a specific type of supramolecular contact comprising four bipyridine ligands from two neighbouring [M(bpy)_3]^2+ cations – quadruple aryl embrace (QAE) contact. The contact is realized by the alignment of two molecular two-fold rotation axes, preserving the parallel orientation of the molecular three-fold rotation axes. The resulting two-dimensional honeycomb lattices of [M(bpy)_3]^2+ cations are placed between the hydrogen bonding layers made of [NbO(C_2O_4)_3]^3– and Cl– anions and the majority of crystal water molecules. The temperature-dependent magnetic susceptibility measurements (1.8–300 K) show a significant orbital angular momentum contribution for 3 and 4 (high-spin Co^2+), the influence of zero-field splitting for 5 and 6 (Ni^2+) and a substantially paramagnetic Curie behaviour for the Cu^2+ compound (7).

In the rational approaches to the design and synthesis of various metal-organic coordination networks with specific chemical and physical properties, a key role belongs to the nature of the metal ion and the geometry, flexibility and coordination preferences of the bridging ligands. The oxalate C_2O_4^2– anion, having rich binding facilities and capabilities of mediating exchange interaction between the metal centres, has appeared as a very useful bridging ligand in the synthesis of a variety of homo- and heterometallic compounds with attractive new architectures and interesting magnetic properties. As a continuation of our research work on polynuclear transition metal complexes, two novel oxalate-bridged species have been synhesized: a homodinuclear Cu^IICu^II complex, [Cu(bpy)Cl(µ -C_2O_4)Cu(bpy)Cl] (1) (bpy = 2, 2'-bipyridine) and a heterometallic Ni, Nb compound, containing homodinuclear Ni^IINi^II units, [{; Ni(bpy)_2}; _2(μ -C_2O_4)]_3[NbO(C_2O_4)_3]_2• 24H_2O (2). The new compounds have been characterized by means of chemical and TG/DTA analysis, infrared spectroscopy, magnetic susceptibility measurements and by single crystal X-ray diffraction method. Each copper(II) atom in 1 displays a square-pyramidal coordination, whereas nickel(II) atoms in 2 are placed in a slightly distorted octahedral environment. The analysis of magnetic susceptibility data (1.8– 290 K) reveals the existence of antiferromagnetic exchange interaction between the metal centres in oxalate-bridged units of 1 and 2.

The design and synthesis of new materials based on polynuclear transition metal complexes has been a very stimulating challenge for chemists nowadays, as the need for the materials that have more diversified and more sophisticated properties is constantly growing. A generally accepted strategy in synthesizing new materials consists in assembling (paramagnetic) metal centres in such a way that the resulting solid lattices, with their components properly arranged, exhibit desired physical characteristics: mechanical, optical, electric and especially, magnetic ones. The magnetic properties (ferro-, ferri- and antiferro-) of such systems depend mostly on the nature of the interacting metal ions and the bridging ligands, as well as on the whole structural architecture that can be created by the supramolecular arrangements of the building blocks [1, 2]. In designing homo- and heteropolynuclear complexes a very essential role belongs to the oxalate ion, for its ability to adopt bis(chelating) coordination mode as well as to mediate magnetic interactions between paramagnetic metal centres, these properties representing the basis for the intensive magneto-structural investigations of the oxalate-bridged compounds. A very frequently used approach for the synthesis of polynuclear oxalate-bridged species consists in using stable mononuclear anionic oxalate complexes as ligands toward the second metal ion, the approach known as -complexes as ligands- or -building-blocks chemistry-. In relation to the current magneto-structural studies on the homo- and heterometallic polynuclear transition-metal complexes in our laboratory, the heterotrinuclear oxalate-bridged complex [/Cu(mu-C_2O_4)(bpy)_2/_2Cr(C_2O_4)]NO_3 x H_2O (1) (bpy = 2, 2'-bipyridine) has been synthesized using the mononuclear anionic oxalate ligand [Cr(C_2O_4)_3]^3- as the building block [2]. Analysis of magnetic properties (between 1.8 and 290 K) for this compound showed the ferromagnetic exchange interaction between the Cu^II and Cr^III ions (J = +1.20 cm^-1). Two novel heterometallic complexes [/Cu(bpy)_2/_2(mu-C_2O_4)][Cu(bpy)_2(mu-C_2O_4)NbO(C_2O_4)_2]_2 x 0.5bpy x 7H_2O (2) and [/Ni(bpy)_2/_2(mu-C_2O_4)]_3[NbO(C_2O_4)_3]_2 x 24H_2O (3), with the oxalate-bridged dinuclear Cu^IICu^II and Cu^IINb^V units in 2 and the oxalate-bridged dinuclear Ni^IINi^II units in 3, have been prepared using [NbO(C_2O_4)_3]^3- as the initial reagent [3]. The magnetic susceptibility measurements (between 1.8 and 290 K) showed the ferromagnetic exchange interaction between the copper(II) atoms in the Cu^IICu^II unit in 2 (J = +7.9 cm^-1), and the antiferromagnetic interaction between the nickel(II) atoms in the Ni^IINi^II unit in 3 (J = -35.7 cm^-1). Except for the magnetic susceptibility and single crystal X-ray diffraction analysis, the novel compounds have been characterized by means of infrared spectroscopy and thermogravimetric measurements.

Nine new complex salts of the composition [M(bpy)_3]_2[NbO(C_2O_4)_3]Cl x nH_2O [M = Fe^2+ (1, 2), Co^2+ (3, 4), Ni^2+ (5, 6), Cu^2+ (7) and Zn^2+ (8, 9) ; bpy = 2, 2'-bipyridine ; n = 11 or 12) have been isolated from the reactions of [M(bpy)_3]Cl_2 x nH_2O and [NbO(C_2O_4)_3]^3– . The crystal structures of seven of the compounds (2– 8) have been determined. The compounds make two isostructural sets of pseudopolymorphs crystallizing in the centrosymmetric monoclinic space group P21/c (3, 5, 8) or in the noncentrosymmetric orthorhombic space group P212121 (2, 4, 6, 7). All the structures contain two symmetry independent [M(bpy)_3]^2+ cations (A and B), one [NbO(C_2O_4)_3]^3– anion and one Cl^– anion, and 11 or 12 crystal water molecules (monoclinic and orthorhombic compounds, respectively). The interatomic Fe-N distances in 2 are typical for diamagnetic low-spin Fe^2+ species, the same as are the Co-N distances in 3 and 4 typical for high-spin Co^2+ species. By analysis of the crystal packing in all the structures a specific type of supramolecular contacts between the [M(bpy)_3]^2+ cations has been identified. The contact comprises four bipyridine ligand molecules from two neighbouring [M(bpy)_3]^2+ cations and is termed the quadruple aryl embrace (QAE) contact. This contact is realized by the alignment of two molecular two-fold rotation axes. Influence of aggregation of [M(bpy)_3]^2+ cations via QAE contacts on magnetic properties of these compounds, as well as the influence of trigonal distortion of octahedral coordinations around metal atoms (driven by didendate bpy ligands), has been presented and analyzed.

Oksalatna skupina, C_2O_4^2-, ima vrlo važnu ulogu u dizajniranju homo- i heterovišenuklearnih kompleksnih spojeva, zbog njenog dikelatnog načina koordiniranja, te mogućnosti posredovanja u magnetskim interakcijama između paramagnetskih metalnih centara [1]. Za razliku od homometalnih, heterometalne sustave s premosnom oksalatnom skupinom znatno je teže dobiti, budući da tijekom priprave često nastaju smjese homometalnih kompleksa [2]. U sklopu naših istraživanja višenuklearnih kompleksnih spojeva prijelaznih metala, pripravljen je novi heterodimetalni sustav [Cu/Zn(bpy)Cl(mu-C_2O_4)Zn/Cu(bpy)Cl] (bpy = 2, 2'-bipiridin), čija su svojstva opisana temeljem kemijske i termogravimetrijske (TG/DTA) analize te infracrvene spektroskopije, a struktura određena metodom rentgenske difrakcije na monokristalu.

U ciljanom pristupu dobivanju različitih anorgansko-organskih koordinacijskih polimera kao materijala koji bi imali poželjne kemijske i fizikalne značajke, priroda metalnog iona te geometrija, savitljivost i koordinacijske sklonosti premosnih liganada imaju izuzetno značajnu ulogu. Višenuklearni koordinacijski sustavi temeljeni na spojevima prijelaznih metala s premosnim oksalatnim skupinama područje su intenzivnog istraživanja, jer predstavljaju vrlo djelotvoran put za pripravu funkcionalnih materijala specifičnih magnetskih svojstava. U ovom će radu biti prikazan jedan novi heterometalni kompleksni sustav s dinuklearnom jedinkom Ni^IINi^II u kojoj su niklovi(II) atomi premošteni oksalatnom skupinom, empirijske formule [Ni_2(mu-C_2O_4)(bpy)_4]_3[NbO(C_2O_4)_3]_2 x 7H_2O (bpy = 2, 2'-bipiridin). Spoj je opisan temeljem kemijske i termogravimetrijske analize (TG/DTA), infracrvene spektroskopije, mjerenja magnetske susceptibilnosti, te određivanjem kristalne i molekulske strukture.

The design of molecule-based magnetic materials has become one of the most stimulating challenges for chemists nowadays [1]. In synthesizing homo- and heteropolynuclear complexes a very essential role belongs to the oxalate ion, (C_2O_4)^2-, for its ability to adopt bis(chelating) coordination mode as well as to mediate magnetic interactions between paramagnetic metal centres. On that account, stable mononuclear anionic oxalate complexes are often used as ligands toward the second metal ion. Related to our current magneto-structural studies on the polynuclear transition metal complexes, the heterometallic compound [(Cu(mi-C_2O_4)(bpy)_2)_2Cr(C_2O_4)]NO_3xH_2O (bpy = 2, 2'-bipyridine) has been synthesized using the [Cr(C_2O_4)_3]^3- anion as the building block. The structural data of the complex have been correlated with magnetic and spectroscopic measurements. The central [Cr(C_2O_4)_3]^3- unit acts as a bridge between two copper(II) ions. Analysis of magnetic data (1.8-290 K) showed the ferromagnetic exchange interaction between Cu(II) and Cr(III). [1] Kahn O.: Molecular Magnetism, Wiley-VCH, New York, 1993.

Two new heterometallic complexes [[Cu(mu-C_2O_4)(bpy)_2]_2Cr(C_2O_4)]NO_3 center dot H2O (1) and [Cu(bpy)_3]_2[Cr(C_2O_4)_3]NO_3 center dot 9H_2O (2) (bpy = 2, 2'-bipyridine) have been synthesized. Their crystal structures were determined and magnetic and spectroscopic (IR, UV/Vis) characterization accomplished. In 1, the central [Cr(C_2O_4)_3]^3- unit acts as a bridge between two copper(II) ions, each of them having a distorted octahedral geometry. The apical positions are occupied by one oxygen atom belonging to the bridging oxalate anion [Cu-O2 = 2.6439(18) Angstrom] and one nitrogen atom from one of the bpy ligands [Cu-N1 = 2.1079(18) Angstrom]. The N_3O equatorial environment consists of three bpy-nitrogen atoms [Cu-N = 1.995 Angstrom (mean value)] and the second oxygen atom from the bridging oxalate anion [Cu-O1 = 2.1168(15) Angstrom]. The chromium(III) atom is in a slightly distorted octahedral environment of four oxygen atoms from two bridging oxalate groups [Cr-O = 1.988 Angstrom (mean value)] and two oxygen atoms from the terminally bonded bidentate oxalate group [Cr-O = 1.9454(16) Angstrom]. Complex 2 consists of two mononuclear [Cu(bpy)_3]^2+ cations, one mononuclear [Cr(C_2O_4)_3]^3- anion, one (NO_3)^- anion and nine molecules of water of crystallisation. The copper(II) ions in two crystallographically independent [Cu(bpy)_3]^2+ cations are in a tetragonally distorted octahedral coordination environment of six N atoms from the bpy ligands. The chromium(III) ion in [Cr(C_2O_4)_3]^3- has a slightly distorted octahedral environment of oxygen atoms from three oxalate groups [with the Cr-O bond lengths ranging from 1.964(3) to 1.982(3) Angstrom]. Analysis of the magnetic properties (1.8-290 K) for 1 showed a ferromagnetic exchange interaction between Cu^II and Cr^III with J = +1.20 cm^-1 [using H_(INT) = -J(S_(Cu1)S_Cr + S_(Cu2)S_Cr)].

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).

Oksalatna skupina, C_2O_4^2-, u koordinacijskoj kemiji poznata je kao polidentatni ligand. Mogućnost elektronskog međudjelovanja (fero- ili antiferomagnetskog) između homo- ili heteroatoma prijelaznih metala preko premošćujuće oksalatne skupine već je prepoznata. Stoga se oksalatna skupina često koristi za pripravu novih sustava koji bi imali svojstva molekulskih magneta. U tu svrhu priređen je složeni heteronuklearni spoj sastava [Cu_2(mi-C_2O_4)(bpy)_4][NbO(C_2O_4)_2(mi-C_2O_4)Cu(bpy)_2]_2x0, 5(bpy)x7H_2O (bpy = 2, 2'-bipiridin, C_10H_8N_2) s bidentatno i tetradentatno vezanim oksalatnim skupinama. Kompleks kristalizira u triklinskom sustavu, prostorna grupa P-1 (a = 15, 974(1) A ; b = 16, 241(1) A ; c = 21, 436(2) A ; alfa = 70, 23(1)° ; beta = 82, 17(1)° ; gama = 85, 22(1)° ; V = 5180, 5(7) A^3 ; Z = 2), a sastoji se od homodinuklearnog kationa [Cu_2(mi-C_2O_4)(bpy)_4]^2+ i dva heterodinuklearna aniona [NbO(C_2O_4)_2(mi-C_2O_4)Cu(bpy)_2]^-. Razmatraju se strukturna, spektroskopska i magnetska svojstva novog spoja.

Heksanuklearni halogenidni klusteri niobija i tantala poznati su u obliku [M_6(μ -X)_12X_6]^4– (M = Nb, Ta ; X = Br, Cl), gdje dvanaest halogenih atoma (μ -X) premošćuje atome metala smještene u vrhovima oktaedra, dok je preostalih šest halogenih atoma (X) koordinirano u krajnjim oktaedarskim koordinacijskim položajima klusterske jedinke. Ovi se spojevi priređuju redukcijom odgovarajućih pentahalogenida metalnim prahom uz prisutnost halogenida alkalijskih metala, pri temperaturama iznad 700 °C [1]. Pogodnim odabirom stehiometrijskog odnosa reakcijskih komponenata moguća je djelomična ili potpuna izmjena liganada μ -X drugim halogenim elementom. Do sada su ovakvim postupkom priređeni spojevi s jedinkama [M_6(μ -X^a)_9(μ -X^b)_3X^a(b)_6]^4– , [M_6(μ -X^a)_8(μ -X^b)_4X^a(b)_6]^4– i [M_6(μ -X^a)_6(μ -X^b)_6X^a(b)_6]^4– (X^a = Br, X^b = Cl). Klusteri sastava (PyH)_2[M_6(μ -Br)_6(μ -Cl)_6Br_6] (M = Nb, Ta) ranije su bili priređeni egzotermnom izmjenom liganada pri 200 °C u spojevima (PyH)_2[M_6(μ -Br)_12Cl_6] [2]. Istražuju se električna, strukturna i magnetska svojstva novih sustava u različitim oksidacijskim stanjima klusterskih jedinki [M_6(μ -X^a)_9, 8, 6(μ -X^b)_3, 4, 6]^n+ (n = 2, 3, 4).

Mogućnost elektronskog međudjelovanja homo- ili heteroatoma prijelaznih metala preko premošćujuće oksalatne skupine u posljednje se vrijeme intenzivno istražuje s ciljem dobivanja novih sustava koji bi imali svojstva molekulskih magneta. U tu svrhu priređeni su složeni heteronuklearni spojevi sastava [M_2(bpy)_4(mu-C_2O_4)][NbO(C_2O_4)_2(mu-C_2O_4)M(bpy)_2]_2 centrirana točka 0, 5bpy centrirana točka 7H_2O (M = Cu^2+ (1), Zn^2+ (2), bpy = 2, 2'-bipiridin, C_10H_8N_2). Ovi su kompleksi izostrukturni i kristaliziraju u triklinskom sustavu prostorne grupe P-1 [(1): a = 15, 974(1), b = 16, 241(1), c = 21, 436(2) angstrema, alfa = 70, 23(1)°, beta = 82, 17(1)°, gama = 85, 22(1)°, V = 5180, 5(7) angstrem^3, Z = 2. (2): a = 15, 9060(9), b = 16, 0804(9), c = 21, 589(1) angstrema, alfa = 69, 504 (5)°, beta = 82, 598(5)°, gama = 86, 012(5)°, V = 5127, 5(5) angstrem^3, Z = 2], a sastoje se od homodinuklearnog kationa [M_2(bpy)_4(mu-C_2O_4)]^2+ i dva heterodinuklearna aniona [NbO(C_2O_4)_2(mu-C_2O_4)M(bpy)_2]^-. Razmatraju se strukturna, spektroskopska i magnetska svojstva novih spojeva.

As the first step in our efforts to synthesize polynuclear molecule-based magnetic compounds, we have isolated the new isostructural series of solids [M(bpy)_3]Cl_2[NbO(C_2O_4)_3]Cl&middot ; ; ; 11H_2O (M=Fe^2+, Co^2+, Ni^2+, Zn^2+) formed from aqueous solutions of [M(bpy)_3]Cl_2 and M'_3[NbO(C_2O_4)_3&middot ; ; ; nH_2O (M'=NH_4^+, alkali metal). All compounds crystallize as needle-like monoclinic crystals, space group P2_1/c. This crystal form is metastable and slowly (2-4 weeks) transforms into more stable polyhedra-like orthorombic crystals of the composition [M(bpy)_3]Cl_2[NbO(C_2O_4)_3]Cl&middot ; ; ; 12H_2O, space group P2_12_12_1). Methods used. IR spectroscopy, powder and single-crystal X-ray diffraction, magnetic susceptibility measurements. The crystal structures have been solved for all compounds and pseudopolymorphism found within pairs of complexes from the two series. Magnetic susceptibility measurements show no difference in magnetic behaviour of two pseudopolymorphs.

New cluster methoxides of molibdenum or tungsten of the composition [(M_6X_8^i)X_2^a(OCH_3)_4/2^a-a]• 2CH_3OH (M = Mo, W ; X = Cl, Br), [Mo_6X_8^i(OCH_3)_2^a(OCH_3)_4/2^a-a]• CH_3OH (X = Cl, Br) and [Na(CH_3OH)_5]_2[Mo_6Br_8^i(OCH_3)_6^a] with bridging, mixed bridging-terminal or terminal methoxo groups, respectively, have been prepared. The preparation procedures, spetroscopic and physical properties, including X-ray photoelectron spectra have been correlated. The crystal structure has been solved for [Na(CH_3OH)_5]_2[Mo_6Br_8(OCH_3)_6]. The compound crystallizes in the triclinic crystal system. The structure is built of two [Na(CH_3OH)_5]^+ cations and the [Mo_6Br_8(OCH_3)_6]^2- anion. All six oxygen atoms from coordinated methoxo groups are acceptors of hydrogen of coordinated methanol molecules from the six neighbouring cations making a three-dimensional packing network.

In most magnetic molecular clusters studied in the literature, localized moments of metal ions are coupled by superexchange interactions via ligand entities which are parts of molecular clusters. There is a limited number of studies where magnetic behaviour arises from unfilled molecular orbitals delocalized over the entire metal cores of molecular metal clusters [J. Sinzing et al., Phys. Rev. Lett. 81 (1998) 3211]. This type of magnetism is present in compounds with paramagnetic hexanuclear tantalum halide clusters. Temperature dependent magnetic susceptibility measurements for the compound [Ta_6Cl_12(CH_3OH)_6]Br_3 (I) and temperature dependent magnetic susceptibility anisotropy measurements for the compound [Ta_6Cl_12(CH_3OH)_6]Br_3&middot ; ; 4H_2O (II) can be interpreted as Bonner-Fisher one-dimensional model of interacting magnetic centres [J. C. Bonner, M. E. Fisher, Phys. Rev. A, 135 (1964) 640]. Structural data for compounds (I) and (II) are available from the literature [N. Brničević et al., Inorg. Chem. 31 (1992) 3294]. Compound (I) crystallizes in space group, a = b = 19.127 (4) &Aring ; ; , c = 8.565 (1) &Aring ; ; , and compound (II) in C2/m space group, a = 12.187 (14) &Aring ; ; , b = 15.793 (4) &Aring ; ; , c = 9.182 (6) &Aring ; ; , β = 99.05 (3)o. Magnetic measurements will be discussed in view of the structural data with special attention to the crystallographic origin of anisotropy for compound (II).

Molekule prekursora (Ta_6X_12)X_2&middot ; ; 6ROH (X = Cl, Br ; R = CH_3, C_2H_5) poslužile su za pripravu novih mješovito-ligandnih sustava [R_4N]_2[(Ta_6Cl_12)C_l4(ROH)_2], koji u terminalnim oktaedarskim koordinacijskim položajima, uz molekule alifatskog alkohola, sadrže i atome klora. Spojevi su kristalni, no nisu međusobno izostrukturni. Sličnim postupkom priređeni su i spojevi sastava [R_4N]_2[Ta_6Br_12(C_2H_5OH)_6]Cl_4, odnosno [R_4N]_2[Ta_6Br_12(H_2O)_6]Br_4&middot ; ; 2H_2O, koji u terminalnim koordinacijskim položajima sadrže molekule etanola, odnosno vode. Kristalna struktura riješena je za [(CH_3)_4N]_2[Ta_6Br_12(H_2O)_6]Br_4&middot ; ; 2H_2O [1]. Spoj kristalizira u monoklinskom kristalnom sustavu, prostorna grupa C2/c. Os drugog reda prolazi kroz središte klustera, dok je centar simetrije smješten između klusterskih jedinki. Najbliže susjedne klusterske jedinke poredane su duž kristalografske osi c tvoreći jednodimenzijsku lančastu strukturu. Parametri jedinične ćelije iznose: a = 24, 388 (5) &Aring ; ; ; b = 12, 470 (2) &Aring ; ; ; c = 18, 425 (4) &Aring ; ; ; β = 127, 74 (3)&ordm ; ; ; V = 4431 (2) &Aring ; ; ^3 ; Z = 4. Razmatrat će se strukturna i spektroskopska svojstva novih sustava.

Reakcijom vodenih otopina [M(bpy)_3]Cl_2 i M'_3[NbO(C_2O_4)_3]&middot ; ; nH_2O (M = Fe^2+, Co^2+, Ni^2+ ; bpy = bipiridil, C_10H_8N_2 ; M' = NH^4+, alkalni metali) priređeni su novi kristalni anorganski sustavi [M(bpy)_3]_2[NbO(C_2O_4)_3]Cl&middot ; ; nH_2O. Svi spojevi kristaliziraju u obliku igličastih monokristala (monoklinski kristalni sustav). Ovaj je kristalni oblik metastabilan u otopini i tijekom 2-4 tjedna podliježe procesu postupne, ali potpune pretvorbe u novi kristalni oblik, kristalne poliedre (rompski kristalni sustav). Kristalne strukture riješene su za analoge nikla(II). [Ni(bpy)3]2[NbO(C2O4)3]Cl&middot ; ; 11H2O, kristalne iglice: monoklinski kristalni sustav, prostorna grupa P21/c (Nr. 14) ; a = 23, 875 (3) &Aring ; ; ; b = 13, 265 (1) &Aring ; ; ; c = 23, 115 (2) &Aring ; ; ; β = 101, 69 (1)&ordm ; ; ; V = 7168 (1) &Aring ; ; 3 ; Z = 4. [Ni(bpy)3]2[NbO(C2O4)3]Cl&middot ; ; 12H2O, kristalni poliedri: rompski kristalni sustav, prostorna grupa P212121 (Nr. 19) ; a = 15, 058 (1) &Aring ; ; ; b = 22, 039 (2) &Aring ; ; ; c = 22, 398 (2) &Aring ; ; ; V = 7433 (1) &Aring ; ; 3 ; Z = 4. Razmatraju se strukturna, spektroskopska i magnetska svojstva novih spojeva.

Three new series of mixed-ligand clusters of the [M_6X_12)X_2(RCN)_4] (M = Nb, Ta ; X = Cl, Br ; R = Et, n-Pr, n-Bu) composition have been prepared. It is supposed that four nitrile molecules and two halogene atoms are coordinated to the terminal octahedral coordination sites of the [M_6X_12]^2+ unit.

The hexanuclear chloro and bromo clusters exhibit a hugh tendency towards the formation of compounds with differently charged cluster units. The cluster [Ta_6Cl_12(C_3H_7CN)_6][(Ta_6Cl_12)Cl_6]&#8226 ; ; 2C_3H_7CN containing [Ta_6Cl_12]^2+ in the cation and [Ta_6Cl_12]^4+ in the anion, prepared recently, is the first example of this tipe.

Low-dimensional antiferromagnetic behaviour is observed for a number of compounds with paramagnetic hexanuclear tantalum halide clusters. Among others it has been observed for [Ta_6Cl_12(CH_3OH)_6]Br_3 (1) and [(Ta_6Cl_12)Cl(H_2O)_5][CdBr_4]&middot ; ; 6H_2O (2). The compound 1 crystallizes in the R-3 space group, a = b = 19.127(4) &Aring ; ; , c = 8.565(1) &Aring ; ; and the compound 2 in the Fd-3m space group, a = 19.974(1) &Aring ; ; . One-dimensional packing motifs in both structures are suggested for the explanation of the observed low-dimensional magnetism.

The hexanuclear halide cluster unit [Ta_6Cl_12(CH_3OH)_6]^3+ is found to exist in two compounds, [Ta_6Cl_12(CH_3OH)_6]Br_3 and [Ta_6Cl_12(CH_3OH)_6]Br_3· 4H_2O that crystallize in rhomoedral (space group R-3) and monoclinic (space group C2/m) cystallographic system, respectively. The unit is an open-shell system with one unpaired electron in HOMO. The magnetic susceptibility measurement shows pure one-dimensional antiferromagnetic behaviour. It is expected that this behaviour is due to the overlap of the HOMO orbitals of two neigbouring cluster units. The overlap is calculated by the GAMESS and GAUSSIAN98 ab.initio programmes.

Istraživane su reakcije ciano-kompleksa K_4[Fe(CN)_6] i K_2[Ni(CN)_4] s heksanuklearnim halogenidnim spojevima niobija i tantala [(M_6X_12)X_2(H_2O)_4]x4H_2O (X = Cl, Br) u vodi, odnosno u smjesi metanol/voda.

In the crystal packing of the title compound each cluster unit has four nearest neighbours tetrahedrally arranged. This implies that possible magnetic interaction should be three-dimensional.

U ovom su radu istraživane reakcije etanolnih prekursora [(M_6X_12)X_2]x6C_2H_5OH (M = Nb, Ta ; X = Cl, Br) s valeronitrilom i benzonitrilom.

Br_16H_36O_18Ta_6Zn, cubic, Fd-3m (No.227), a =20.8276(4) Angstrom, V = 9034.8 Angstrom^3, Z = 8, R_gt(F) = 0.036, wR(F^2) = 0.075, T = 100 K.

Compounds with homonuclear mixed-charge cluster units.

Sinteza i Kristalna struktura spoja s klusterskim jedinkama različitog naboja [Ta6Br12(H2O)6]Cl2.trans-[Ta6Br12(OH)4(H2O)2].18H2O

Opisane su reakcije heksanuklearnih halogenidnih kompleksa s bidentatnim dušikovim bazama.

Reaction of the complexes [M_6X_12(C_2H_5OH)_6]X_2 (M = Nb, Ta ; X = Cl, Br) with N, N-dimethylformamide afforded a new series of clusters of the composition [M_6X_12(C_3H_7NO)_6]X_2. These microcrystalline substances are not isotypic with any other series of niobium and tantalum clusters with the [M_6X_12]^2+ unit. The analytical and spectral data point to the conclusion that all octahedral coordination sites in the hexanuclear cluster units are occupied by the oxygen bonded N, N-dimethylformamide molecules.

Compounds consisting of both cluster cations and cluster anions of the composition [(M_6X_12)(EtOH)_6][(Mo_6Cl_8)Cl_4X_2]xnEtOHxmEt_2O (M = Nb, Ta; X = Cl, Br) have been prepared by the reaction of (M_6X_12)X_2x6EtOH with (Mo_6Cl_8)Cl_4. IR data are given for three compounds. The structures of [(Nb_6Cl_12)(EtOH)_6][(Mo_6Cl_8)Cl_6]x3EtOHx3Et_2O (1) and [(Ta_6Cl_12)(EtOH)_6][(Mo_6Cl_8)Cl_6]x6EtOH (2) have been solved in the triclinic space group P-1 (No. 2). Crystal data: 1, a = 10.641(2) A, b = 13.947(2) A, c = 15.460(3) A, alpha = 65.71(2)^o, beta = 73.61(2)^o, gamma = 85,11(2)^o, V = 2005.1(8) A^3 and Z = 1; 2, a = 11.218(2) A, b = 12.723(3) A, c = 14,134(3) A, alpha = 108,06(2)^o, beta = 101.12(3)^o, gamma = 91,18(2)^o, V = 1874.8(7) A^3 and Z = 1. Both structures are built of octahedral [(M_6Cl_12)(EtOH)_6]^2+ cluster cations and [(Mo6_Cl_8)Cl_6]^2- cluster anions, forming distorted CsCl structure types. The Nb-Nb and Ta-Ta bond lengths of 2.904 A and 2.872 A (mean values), respectively, are rather short, indicating weak M-O bonds. All O atoms of coordinated EtOH molecules are involved in H bridges. The Mo-Mo distances of 2.603 and 2.609 A (on average) are characteristic for the [(Mo_6Cl_8)Cl_6]^2- anion, but there is a clear correlation between the number of hydrogen bridges to the terminal Cl and the corresponding Mo-Cl distances.

Mercury(II) halides react with [(M_6Br_(12))Br_2(H_2O)_4]×4H_2O to yield the compounds of the composition [M_6Br_(12)(H_2O)_6][HgBr_2X_2]×12H_2O (M = Nb, Ta; X = Cl, Br; I). The substances crystallize in the cubic crystallographic system and contain complex cations [M_6Br_(12)(H_2O)_6]^(2+) and tetrahedrally coordinated mercury(II) atom in [HgX_4]^(2-) anions. The magnetic susceptibility measurement of [Nb_6Br_(12)(H_2O)_6][HgBr_4]×12H_2O in the temperature range 4.2 to 300 K indicates diamagnetic behaviour; the electrical conductivity measurement on the single crystal gives a value of sigma_(295 K) = 0.01 Omega^(-1) cm^(-1) (E_a = 0.20 eV), characteristic for semiconducting materials. To the same crystallographic system belong the substances with [Ta_6Cl_(12)(H_2O)_6]^(3+) cation. These compounds are paramagnetic with large spin temperature dependent magnetic susceptibility. Data on electronic, magnetic and electrical properties for [M_6Br_(12)(H_2O)_6]^(2+) and [Ta_6Cl_(12)(H_2O)_6]^(3+) are correlated.

Partial substitution of mercury atoms by bismuth and its influence on the structural, superconducting and magnetic properties of the Hg-based oxide superconductors have been investigated. Two series of samples of the nominal compositions Hg_1-xBi_xBa_2Ca_2Cu_3O_8+delta and Hg_1-xBi_xBa_2Ca_3Cu_4O_10+delta, 0 < x < 0.5, have been prepared by the solid state reactions at normal pressure, by using the sealed quartz tube technique. For the bismuth content range of 0.2 < x < 0.25, for the first series and 0.1 < x < 0.20 for the second one, the formation of the Hg-1234 phase in a high volume percentage, with the superconducting transition Tc(onset) ~ 130 K has been found. The X-ray powder diffraction data and the AC magnetic susceptibility measurements have been correlated.

Heksanuklearni etanolatni klusteri [M_6X_(12)(C_2H_5OH)_6]X_2, M = Nb, Ta; X = Cl, Br, sa slabo vezanim molekulama etanola, pokazali su se kao vrlo pogodni polazni spojevi u reakcijama niobijevih i tantalovih M_6-klustera s ligandima koji imaju dušikove atome kao donore. Otapanjem polaznih etanolatnih klustera [M_6X_(12)(C_2H_5OH)_6]X_2 u N,N-dimetilformamidu, uz tehniku rada na visokom vakuumu, priređeni su kristalni spojevi sastava [M_6X_(12)(C_3H_7ON)_6]X_2, M = Nb, Ta; X = Cl, Br, u kojima su svi terminalni oktaedarski položaji u klusterskoj jedinki zauzeti kisikom iz N,N-dimetilformamida. Reakcijom polaznih etanolatnih klustera i 1,5-naftiridina u etanolu dobiveni su spojevi u kojima, u danim uvjetima, nije došlo do potpune izmjene molekula etanola molekulama 1,5-naftiridina, [M_6Cl_(12)(1,5-C_8H_6N_2)_(4/2)(C_2H_5OH)_2]Cl_2. Novi su spojevi opisani na temelju elementarnih analiza, spektroskopskih i magnetskih mjerenja te kristalografskih podataka.

Chemical effects in X-ray spectra (energy shifts, emission probability changes, satellite formation and peak profile changes) may be used for characterization of the chemical circumstances and chemical bonding of the monitored element in the sample. For that purpose an investigation of the chemical effects in ion induced X-ray spectra of the high-Tc based superconductors of series HgBaCaCuO and SrLnCeTaCuO (Ln = Eu, Sm) was carried out using high resolution crystal X-ray spectrometer. The applicability of the chemical effects for the mapping of the crystal phases in the superconducting samples using ion microbeam is discussed.