Nickel deficiency in RENi2-xP2 (RE=La, Ce, Pr). Combined crystallographic and physical property studies

Large single crystals from RENi_2-xP₂ (RE=La, Ce, Pr) were synthesized from the pure elements using Sn as a metal flux, and their structures were established by X-ray crystallography. The title compounds were confirmed to crystallize in the body-centered tetragonal ThCr₂Si₂ structure type (space group I4/mmm (No. 139); Pearson's symbol tI10), but with a significant homogeneity range with respect to the transition metal. Systematic synthetic work, coupled with accurate structure refinements indicated strong correlation between the degree of Ni-deficiency and the reaction conditions. According to the temperature dependent dc magnetization measurements, LaNi_2-xP₂ (x=0.30(1)), as expected, is Pauli-like paramagnetic in the studied temperature regime, while the Ce-analog CeNi_2-xP₂ (x=0.28(1)) shows the characteristics of a mixed valent Ce³⁺/Ce⁴⁺ system with a possible Kondo temperature scale on the order of 1000 K. For three different PrNi_2-xP₂ (x?0.5) samples, the temperature and field dependence of the magnetization indicated typical local moment 4f-magnetism and a stable Pr³⁺ ground state, with subtle variations of T_C as a function of the concentration of Ni defects. Field-dependent heat capacity data for CeNi_2-xP₂ (x=0.28(1)) and PrNi_2-xP₂ (x=0.53(1)) are discussed as well.     

Stereocontrolled palladium(0)-catalyzed preparation of unsaturated azidosugars: an easy access to 2- and 4-aminoglycosides

The palladium-catalyzed substitution of alkyl 4,6-di-O-acetyl-α-d-erythro-hex-2-eno-pyranosides using NaN₃ as the nucleophile gave predominantly the corresponding alkyl 2-azido-2,3,4-trideoxy-α-d-threo-hex-2-enopyranosides in the presence of Pd(PPh₃)₄. However, alkyl 6-O-acetyl-4-azido-2,3,4-trideoxy-α-d-erythro-hex-2-enopyranosides were obtained as the major products using Pd(PPh₃)₄ as the catalyst in the presence of dppb as the added ligand. Conversely, alkyl 6-O-(tert-butyldimethylsilyl)-4-O-methoxycarbonyl-2,3-dideoxy-α-d-hex-2-enopyranosides gave exclusively alkyl 4-azido-6-O-(tert-butyldimethylsilyl)-2,3,4-trideoxy-α-d-erythro-hex-2-enopyranosides in the presence of Pd₂(dba)₃/PPh₃ as the catalyst and Me₃SiN₃ as the nucleophile. The bis-hydroxylation followed by hydrogenation of ethyl 4-azido-2,3,4-trideoxy-α-d-erythro-hex-2-enopyranoside afforded the corresponding 4-amino-α-d-mannopyranoside, when propyl 2-azido-2,3,4-trideoxy-α-d-threo-hex-3-enopyranoside gave the 2-amino-α-d-altropyranoside under the same conditions.     

Ternary rare-earth alumo-silicides—single-crystal growth from Al flux, structural and physical properties

A number of rare-earth alumo-silicides (R-Al-Si) have been synthesized from the corresponding elements by high-temperature reactions, carried out in excess of aluminum to serve as a flux. Under these experimental conditions, large single crystals of all R-Al-Si ternary phases were readily produced. The crystal structures these ternaries adopt were studied by means of powder and single-crystal X-ray diffraction and were classified as follows: (1) the early rare-earths (R=La, Ce, Pr, Nd, Sm, Gd) yield RAl_xSi_2−x, x∼1, non-stoichiometric ternary derivatives of the body-centered α-ThSi₂-type; (2) the late rare-earths (R=Tb, Dy, Ho, Er, Tm) form stoichiometric R₂Al₃Si₂ compounds that crystallize in the C-centered monoclinic Y₂Al₃Si₂-type; (3) the divalent Eu and Yb produce EuAl₂Si₂ and YbAl₂Si₂ with the trigonal CaAl₂Si₂-type, whereas the last lanthanide element, Lu, forms LuAlSi with C-centered orthorhombic YAlGe-type. These structural trends are reviewed, and the evolution of the basic physical properties such as magnetism, heat capacity and electrical resistivity when moving across the series is described in detail.     

Synthesis and characterization of 2-phosphonoethanesulfonic acid and a barium-hydrogenphosphonatoethanesulfonate — BaH(O3P-C2H4-SO3)

Following the strategy of using polyfunctional phosphonic acids for the synthesis of new metal phosphonates, the organic linker molecule 2-phosphonoethanesulfonic acid, H₂O₃P-C₂H₄-SO₃H (1) (H₃L), was synthesized and characterized in detail. The acid was used in a high-throughput (HT) investigation of the system BaCl₂/H₃L/NaOH/H₂O. The HT experiments comprising 48 individual hydrothermal reactions were performed to systematically investigate the influence of pH of the starting mixture as well as the molar ratio Ba²⁺: H₃L. Only two reaction products were observed: small amounts of BaCO₃ under basic conditions and BaH(O₃P-C₂H₄-SO₃) (2). For compounds 1 and 2 the crystal structures were determined from single-crystal X-ray diffraction data (H₂O₃P-C₂H₄-SO₃H: trigonal, P3₂, a=814.58(1), c=861.20(2) pm, Z=3, R1=0.0254, wR₂=0.0758 for I>2σ(I); BaH(O₃P-C₂H₄-SO₃): orthorhombic, Ibam, a=953.39(19), b=855.55(17), c=867.82(17) pm, Z=4, R1=0.0162, wR₂=0.0417 for I>2σ(I)). The structure of H₃L (1) is stabilized exclusively by strong hydrogen bonds. Compound 2 is built up by chains of edge sharing BaO₈ polyhedra. These chains are connected to a three-dimensional network by the -CH₂CH₂- linker of the ligand. Thermogravimetric investigation of compound 2, as well as IR spectra of 1 and 2 are presented.     

Bridge cleavage reactions of cyclopalladated nitrosamines with thioamides and related compounds

The palladacycle [Pd(μ-O₂CMe){κ²C,N-4-MeC₆H₃N(Me)NO}]₂ readily undergoes bridge cleavage reactions with a variety of compounds containing donor functionalities including thioamides, 8-hydroxyquinoline, thioureas, selenoureas, acetylacetone derivatives, dithiocarbamates, xanthates, as well as bidentate N-donors to afford either the monomeric, neutral Pd(II) complexes [Pd{κ²C,N-4-MeC₆H₃N(Me)NO}{L-L}] or the monocationic complexes [Pd{κ²C,N-4-MeC₆H₃N(Me)NO}(N-N)]PF₆ in high yields. A series of 15 different complexes was prepared and fully characterised spectroscopically and, in some cases, by X-ray diffraction. It was also found that the dithiocarbamato complex undergoes a disproportionation reaction in solution to give the bis(cyclometallated) complex [Pd{κ²C,N-4-MeC₆H₃N(Me)NO}₂] as well as the bis(dithiocarbamato) complex [Pd{κ²S-S₂CNEt₂}₂].     

Synthesis and fluorescence properties of covalently linked homo- and hetero-porphyrin dyads containing meso-tolyl porphyrin and meso-furyl porphyrin sub-units

Three porphyrin building blocks with N₄, N₃S and N₂S₂ cores having three meso-furyl groups and one meso-iodophenyl group were synthesized and characterized. The porphyrin building blocks were used to synthesize six porphyrin dyads such as N₄-N₄, N₃S-N₃S, N₂S₂-N₂S₂, N₄-N₃S, N₄-N₂S₂ and N₃S-N₂S₂ containing meso-tolyl and meso-furyl porphyrin sub-units under mild Pd(0) mediated coupling conditions. Steady state fluorescence studies indicated an efficient energy transfer from the meso-tolyl porphyrin sub-unit to the meso-furyl porphyrin sub-unit in all six dyads. This study supported the argument that the meso-furyl porphyrins can be used as good energy acceptors when meso-aryl porphyrins act as energy donors in their metal free form.     

Inorganic-Organic Hybrid Materials: Hydrothermal Synthesis and Characterization of the Metal Diphosphonates M2(O3PCH2C6H4CH2PO3)·2H2O (M=Mn, Ni, Cd)

The three isostructural transition metal diphosphonates M₂(O₃PCH₂C₆H₄CH₂PO₃)·2H₂O (M=Mn, Ni, Cd) were hydrothermally synthesized using p-xylenediphosphonic acid and the corresponding metal salts. The structures were refined in the orthorhombic space group Pca2₁ from X-ray powder diffraction data: Mn₂(O₃PCH₂C₆H₄CH₂PO₃)·2H₂O (1): a=983.71(7), b=582.72(4), c=2173.5(2) pm, V=1245.8(1) 10⁶ pm, Z=4, wR_p=0.079, R_p=0.062, R_F=0.081; Ni₂(O₃PCH₂C₆H₄CH₂PO₃)·2H₂O (2): a=951.18(3), b=562.31(2), c=2178.47(6) pm, V=1165.2(1) 10⁶ pm, Z=4, wR_p=0.072, R_p=0.054, R_F= 0.095; Cd₂(O₃PCH₂C₆H₄CH₂PO₃)·2H₂O (3): a=1005.19(3), b=594.37(2), c=2186.08(8), V=1304.3(1) 10⁶ pm, Z=4, wR_p=0.067, R_p=0.052, R_F=0.059. The structures are built up from corner-linked [MO₆] polyhedra (M=Mn, Ni, Cd) forming inorganic metal oxide layers. These layers are linked by the organic diphosphonic acids acting as pillars. Magnetization measurements of 1 confirm the presence of divalent ions and indicate antiferromagnetic ordering at low temperatures. Thermogravimetric as well as IR spectroscopic studies are also presented.     

Inorganic-organic hybrid materials: synthesis and crystal structure determination from powder diffraction data of Pb2(O3PCH2C6H4CH2PO3)

A new lead diphosphonate, Pb₂(O₃PCH₂C₆H₄CH₂PO₃) was hydrothermally synthesized from tetraethyl α,α′-p-xylenediphos-phonate and Pb(NO₃)₂. The structure was solved and refined using X-ray powder diffraction data. It crystallizes in the monoclinic space group P2₁/c, with a = 467.84(2), b = 2007.98(9), c = 639.10(2) pm, β = 101.020(3)°, V = 589.31(4) 10⁶ pm, Z=2, wR_p=0.034, R_p=0.027, R_F2=0.061, R_F=0.036. The structure is built from corner-linked [PbO₄] polyhedra, containing a lone pair of electrons. These polyhedra are connected to layers by phosphonate groups, RPO₃²⁻ and through the organic diphosphonic acid to a three-dimensional structure. Thermogravimetric as well as IR spectroscopic studies are also presented.     

The electronic structure of FeO 4 2− , RuO4, RuO 4 − , RuO 4 2− and OsO4 by the HFS-DVM method

The electronic structures of FeO ₄ ^2? , RuO₄, RuO ₄ ^? , RuO ₄ ^2? and OsO₄ have been investigated using the Hartree-Fock-Slater Discrete Variational Method. The calculated ordering of the valence orbitals is 2t ₂, 1e, 2a ₁, 3t ₂ andt ₁ with thet ₁ orbital as the highest occupied. The first five charge transfer bands are assigned as:t ₁→2e(v ₁), 3t ₂→2e(v ₂),t ₁→4t ₂(v ₃), 3t ₂→4t ₂(v ₄) and 2a ₁→4t ₂(v ₅). It is suggested that ad-d transition should be observed at 1.5 eV in RuO ₄ ^? and RuO ₄ ^2? .     

Ni nanoparticles as electron-transfer mediators and NiSx as interfacial active sites for coordinative enhancement of H2-evolution performance of TiO2

The development of efficient photocatalytic H₂-evolution materials requires both rapid electron transfer and an effective interfacial catalysis reaction for H₂ production. In addition to the well-known noble metals, low-cost and earth-abundant non-noble metals can also act as electron-transfer mediators to modify photocatalysts. However, as almost all non-noble metals lack the interfacial catalytic active sites required for the H₂-evolution reaction, the enhancement of the photocatalytic performance is limited. Therefore, the development of new interfacial active sites on metal-modified photocatalysts is of considerable importance. In this study, to enhance the photocatalytic evolution of H₂ by Ni-modified TiO₂, the formation of NiS_x as interfacial active sites was promoted on the surface of Ni nanoparticles. Specifically, the co-modified TiO₂/Ni-NiS_x photocatalysts were prepared via a two-step process involving the photoinduced deposition of Ni on the TiO₂ surface and the subsequent formation of NiS_x on the Ni surface by a hydrothermal reaction method. It was found that the TiO₂/Ni-NiS_x photocatalysts exhibited enhanced photocatalytic H₂-evolution activity. In particular, TiO₂/Ni-NiS_x(30%) showed the highest photocatalytic rate (223.74 μmol h^?1), which was greater than those of TiO₂, TiO₂/Ni, and TiO₂/NiS_x by factors of 22.2, 8.0, and 2.2, respectively. The improved H₂-evolution performance of TiO₂/Ni-NiS_x could be attributed to the excellent synergistic effect of Ni and NiS_x, where Ni nanoparticles function as effective mediators to transfer electrons from the TiO₂ surface and NiS_x serves as interfacial active sites to capture H⁺ ions from solution and promote the interfacial H₂-evolution reaction. The synergistic effect of the non-noble metal cocatalyst and the interfacial active sites may provide new insights for the design of highly efficient photocatalytic materials.     

A novel family of layered bismuth compounds II: the crystal structures of Pb0.6Bi1.4Rb0.6O2Z2, Z=Cl, Br, and I

Using various synthetic approaches, we have prepared over 50 new multinary bismuth oxyhalides which crystallize in four layered structure types. Most of the compounds belong to the three previously reported structure types involving fluorite- and CsCl-like metal-oxygen vs. metal-halogen layers as well as single or double halide ion sheets. The majority of Bi_2−xA_xQ_0.6O₂Z₂ (A=Li, Na, K, Ca, Sr, Ba, Pb; Q=Rb, Cs; Z=Cl, Br, I) compounds crystallize in the tetragonal structure of Pb_0.6Bi_1.4Cs_0.6O₂Cl₂ (Y2) while both Bi_1.4Ba_0.6Q_0.6O₂I₂ (Q=Rb, Cs) oxyiodides adopt its orthorhombically distorted, partially ordered version. Due to the lower degree of substitution, the fluorite-like layers in the Y2 structure accommodate more A cations than previously known for related Bi compounds. However, very large Tl⁺ or Rb⁺ give compounds with another, as yet unknown, structure. We discuss the influence of size and charge of A cations and stoichiometry of [Bi_2−xA_xO₂] fluorite layers on structure and stability of layered oxyhalides of bismuth. Also, we predict formation of isostructural compounds with smaller Q cations like Tl⁺ and K⁺.     

Distribution de la densite des electrons de valence dans Cu2As

The distribution of valency electron density in Cu₂As (C38) has been determined by Fourier synthesis using as coefficients the values ΔF = F_obs ? F_core (F_core corresponds to the structure factors of the inner orbitals). The bonding between the pyramidal-site copper atoms and the arsenic atoms is exposed, as well as the bonding between tetrahedral-site and pyramidal-site copper atoms.The structural evolution of the unit cell from the Cu₂Sb-type (C38) to the Fe₂P-type (C22) and Co₂P-type (C23) can be related to the metal-metalloid interaction. This interaction mainly involves the pyramidal-site metal atoms in the Cu₂Sb-type, and the tetrahedral-site metal atoms in the Fe₂P- and Co₂P-types.     

Laserspectroscopic investigations of the lithium-D-lines in magnetic fields

Laser-atomic-beam investigations of the lithiumD ₁- andD ₂-line in magnetic fields were performed using cw-laser excitation and fluorescence detection. For both isotopes⁶Li and⁷Li, the hyperfine splittings of the ground level 2² S _1/2 and the upper level of theD ₁-line, 2² P _1/2, as well as the isotopic shifts of theD ₁- and theD ₂-line were determined from the registrations without field. In magnetic fields, Zeeman- and Paschen-Back-effects of the lines were studied. Using the Zeeman pattern of theD ₁-line for a calibration of the field strength, values for the hyperfine constantsA andB of the 2² P _3/2-level of⁷Li could be derived from the Zeeman pattern of theD ₂-line.     

From a Well-Defined Organozinc Precursor to Diverse Luminescent Coordination Polymers Based on Zn(II)-Quinolinate Building Units Interconnected by Mixed Ligand Systems

Introduction of photoactive building blocks into mixed-ligand coordination polymers appears to be a promising way to produce new advanced luminescent materials. However, rational design and self-assembly of the multi-component supramolecular systems is challenging from both a conceptual and synthetic perspective. Here, we report exploratory studies that investigate the potential of [Zn(q)₂]₂[tBuZn(OH)]₂ complex (q = deprotonated 8-hydroxyquinoline) as an organozinc precursor as well as a mixed-ligand synthetic strategy for the preparation of new luminescent coordination polymers (CPs). As a result we present three new 2D mixed-ligand Zn(II)-quinolinate coordination polymers which are based on various zinc quinolinate secondary building units interconnected by two different organic linker types, i.e., deprotonated 4,4′-oxybisbenzoic acid (H₂obc) as a flexible dicarboxylate linker and/or selected bipyridines (bipy). Remarkably, using the title organozinc precursors in a combination with H₂obc and 4,4′-bipyridine, a novel molecular zinc quinolinate building unit, [Zn₄(q)₆(bipy)₂(obc)₂], was obtained which self-assembled into a chain-type hydrogen-bonded network. The application of the organometallic precursor allowed for its direct reaction with the selected ligands at ambient temperature, avoiding the use of both solvothermal conditions and additional base reagents. In turn, the reaction involving Zn(NO₃)₂, as a classical inorganic precursor, in a combination with H₂obc and bipy led to a novel 1D coordination polymer [Zn₂(q)₂(NO₃)₂(bipy)]. While the presence of H₂obc was essential for the formation of this coordination polymer, this ditopic linker was not incorporated into the isolated product, which indicates its templating behavior. The reported compounds were characterized by single-crystal and powder X-ray diffraction, elemental analysis as well as UV-Vis and photoluminescence spectroscopy.     

Lattice distortions in layered type arsenides LnTAs2 (Ln=La-Nd, Sm, Gd, Tb; T=Ag, Au): Crystal structures, electronic and magnetic properties

The lanthanide coinage-metal diarsenides LnTAs₂ (Ln=La, Ce-Nd, Sm; T=Ag, Au) have been reinvestigated and their structures have been refined from single crystal X-ray data. Two different distortion variants of the HfCuSi₂ type are found: PrAgAs₂, NdAgAs₂, SmAgAs₂, GdAgAs₂, TbAgAs₂, NdAuAs₂ and SmAuAs₂ crystallize as twofold superstructures in space group Pmcn with the As atoms of their planar layers forming zigzag chains, whereas LaAgAs₂, CeAgAs₂ and PrAuAs₂ adopt a fourfold superstructure (space group Pmca) with cis-trans chains of As atoms. The respective atomic positions can be derived from the HfCuSi₂ type by group-subgroup relations. The compounds with zigzag chains of As atoms exhibit metallic behaviour while those with cis-trans chains are semiconducting as measured on powder pellets. The majority of the compounds including 4f elements show antiferromagnetic ordering at T_N<20 K.     

Mononuclear, trinuclear, and hetero-trinuclear supramolecular complexes containing a new tri-sulfonate ligand and cobalt(II)/copper(II)-(1,10-phenanthroline)2 building blocks

Novel mononuclear, trinuclear, and hetero-trinuclear supermolecular complexes, [Co(phen)₂(H₂O)(HTST)]·2H₂O (1), [Co₃(phen)₆(H₂O)₂(TST)₂]·7H₂O (2), and [Co₂Cu(phen)₆(H₂O)₂(TST)₂]·10H₂O (3), have been synthesized by the reactions of a new tri-sulfonate ligand (2,4,6-tris(4-sulfophenylamino)-1,3,5-triazine, H₃TST) with the M²⁺ (M=Co, Cu) and the second ligand 1,10-phenanthroline (phen). Complex 1 contains a cis-Co(II)(phen)₂ building block and an HTST as monodentate ligand; complex 2 consists of two TST as bidentate ligands connecting one trans- and two cis-Co(II)(phen)₂ building blocks; complex 3 is formed by replacing the trans-Co(II)(phen)₂ in 2 with a trans-Cu(II)(phen)₂, which is the first reported hetero-trinuclear supramolecular complex containing both the Co(II)(phen)₂ and Cu(II)(phen)₂ as building blocks. The study shows the flexible multifunctional self-assembly capability of the H₃TST ligands presenting in these supramolecular complexes through coordinative, H-bonding and even π-π stacking interactions. The photoluminescent optical properties of these complexes are also investigated and discussed as well as the second-order nonlinear optical properties of 1.     

A novel germanate, Cu2Fe2Ge4O13, with a four tetrahedra oligomer

The structure of Cu₂Fe₂Ge₄O₁₃, previously thought to be CuFeGe₂O₆, has been determined from single-crystal X-ray diffraction data to be monoclinic, P2₁/m, a=12.1050(6), b=8.5073(4), c=4.8736(2) Å, β=96.145(1)°, Z=2, with R1=0.0231 and wR2=0.0605. The unique structure has an oligomer of four germanate tetrahedra, cross-linked laterally by square-planar copper ions, joined end-to-end by a zigzag chain of edge-sharing iron oxide octahedra. Running along the a-direction the metal oxide chain consists of alternating Cu-Cu and Fe-Fe dimers. A hypothetical series of homologous structures (Cu_n−2Fe₂Ge_nO_3n+1 with n=3,4,…,∞) with different length germanate oligomers is proposed, where as n increases, the infinite chain of the CuGeO₃ is approached. In this context, Cu₂Fe₂Ge₄O₁₃ is viewed as being built from blocks of CuGeO₃ and the Fe oxide chains. This material has significance to the study of low-dimensional mixed-spin systems.     

Synthesis and properties of BF2- & PO2-complexes of mono meso-heterocycle substituted 25-oxasmaragdyrins and derivatives

BF₂- and PO₂-smaragdyrins containing one five membered heterocycle such as pyrrole, thiophene and furan at one of the meso-position of corresponding 25-oxasmaragdyrins were synthesized by treating the appropriate mono meso-heterocycle substituted 25-oxasmaragdyrin with BF₃.OEt₂ and POCl₃ respectively in CH₂Cl₂ under mild reaction conditions. All macrocycles were thoroughly characterized by HR-MS and 1D and 2D NMR spectroscopy. The presence of a five membered heterocycle in place of a six membered aryl group significantly alters the electronic properties of the smaragdyrin macrocycle as reflected in their spectral and electrochemical properties. The meso-pyrrole BF₂-smaragdyrin was subjected to a Vilsmeier-Haack reaction to prepare meso-(α-formyl pyrrolyl) BF₂-smaragdyrin which was subsequently used to prepare meso-(α-dipyrromethanyl pyrrolyl) BF₂-smaragdyrin. The further use of meso-heterocycle substituted BF₂- and PO₂-oxasmaragdyrins was demonstrated by treating meso-pyrrolyl BF₂-smaragdyrin with pentafluorobenzaldehyde in CHCl₃ under mild acid catalysed conditions to afford an unusual dipyrromethanyl bridged BF₂-smaragdyrin dyad which exhibits excellent photophysical properties.     

Some binuclear complexes of iron and cobalt with isocyanide ligands

Whereas Co₂(CO)₈ and RNC (R= Me, Et, and Cy) react to give mixtures of [(RNC)₅Co] [Co(CO)₄] and the covalent, carbonyl-bridged [(RNC)_mCo₂(CO)_8?m] derivatives (m = 1–3), [(π-dienyl)Fe(CO)₂]₂ give only [(π-dienyl)₂Fe₂(CO)_4?n(CNR)_n] complexes (dienyl = C₅H₅, MeC₅H₄ and C₉H₇; n = 1–2) that exist in solution as mixtures of cis- and trans-CO- and RNC-bridged tautomers with the μ-RNC species decreasing in importance as the bulk of R increases.     

Synthesis, structure and reactivity of binuclear metal-metal bonded molybdenum(V) and tungsten(V) thioselenohalides: Molecular structure of Mo2(μ-S2)2Cl6(SeCl2)2 and W2(μ-S2)2Cl6(SeCl2)2

Thioselenohalide complexes Mo₂(μ-S₂)₂Cl₆(SeCl₂)₂ (I), Mo₂(μ-S₂)₂Br₆(SeBr₂)₂ (II), and W₂(μ-S₂)₂Br₆(SeBr₂)₂ (III) were synthesized by the reactions of corresponding metal halides or carbonyls or molybdenum metal with excesses of S₂ X ₂+Se₂ X ₂ mixtures. The complex W₂(μ-S₂)₂Cl₆(SeCl₂)₂ (IV) was obtained by an exchange reaction between (III) and excess of Se₂Cl₂. Coordination of the neutral SeX ₂ ligands to thiohalidesM ₂(μ-S₂)₂ X ₆ results in higher thermal stability, and suggests the possibility to synthesize SeX ₂ complexes of the unstable parent tungsten thiohalides. An unusual oxidative addition reaction of (I) was detected: {fx27-1} Both (I) and (IV) were characterized by X-ray crystal structure analysis. They are isostructural and form discrete molecules. Bridging S ₂ ^2? ligands are coordinated perpendicularly to the metal-metal bond;d(M?M)=2.8066 Å and 2.793 Å for I and IV, respectively. Nonequivalence of chlorine atoms which are bound to the metal atom, relate to nonequivalence of halogen atoms in the complexesM ₂(μ?S₂)₂ X ₈ ^2? . Chlorine atomstrans to SeCl₂ ligands form short bonds with the metal; the corresponding³⁵Cl NQR frequency is increased. The selenium dichloride ligand is ambidentate. The selenium atom binds as a donor to the metal and as an acceptor to two chlorine atoms which are also bound covalently to the same metal atom.