Evidence for Cluster Orbital Formation in CsSn2X5 Compounds (X=Cl, Br)

The formation of cluster orbitals in CsSn₂Br₅ is discussed and related more generally to tetragonal compounds of the type AB₂X₅ (A=monovalent cation; B=Sn, Pb; X=Cl, Br, I). The crystal structures of CsSn₂Cl₅ and CsSn₂Br₅ have been solved by single-crystal X-ray diffraction. These compounds are isostructural with each other and a range of AB₂X₅ structural analogues. In many AB₂X₅ compounds where B is a subvalent main group metal a tetragonal cell is observed with space group I4/mcm. The structures of CsSn₂Br₅ and CsSn₂Cl₅ are layered with polymeric sheets of [Sn₂X₅]ⁿ⁻_n separated by the Cs⁺ cations. Stereochemical considerations suggest that stabilization of this structural form, rather than the more ionic NH₄Pb₂Cl₅ or NaSn₂Cl₅ structures, is through interaction of the “nonbonding” valence electron pairs on tin with low-lying empty d-orbitals on neighboring X atoms. Electronic structure calculations based on the structural data confirm the likelihood of cluster orbital formation. Crystal data: CsSn₂Cl₅, tetragonal, I4/mcm, a=8.153(1) Å, c=14.882(4) Å, Z=4, R₁=0.0215, wR₂=0.0503 [I>2σ(I)], R₁=0.0393, wR₂=0.0536 (all data); CsSn₂Br₅, tetragonal, I4/mcm, a=8.483(6) Å, c=15.28(2) Å, Z=4, R₁=0.0607, wR₂=0.1411 [(I>2σ(I)], R₁=0.1579, wR₂=0.1677 (all data).     

Synthesis, crystal structures and spectroscopic characterization of two neutral heterobimetallic clusters MS4Cu4(pz)6X2 (where M = Mo (1) or W (2), X = Cl (1) or disordered Cl/Br (2), and pz = 3,5-dimethylpyrazole)

The title complexes were synthesized in acetone by the reaction of [n-Bu₄N]₂[MoS₄Cu₄Cl₄] and pz^Me2 for compound 1, and n-Bu₄NBr, [NH₄]₂[WS₄], CuCl and pz^Me2 for compound 2. X-ray diffraction studies of 1 and 2 demonstrate that four of the six edges of the tetrahedral [MS₄]²⁻ core are bridged by four copper atoms, giving a pentanuclear structure MS₄Cu₄(pz^Me2)₆X₂ (M = Mo, W) with the five metal atoms essentially coplanar. The four Cu atoms exhibit two different coordination modes. Each of one pair of mutually trans Cu atoms is coordinated by two (μ₃-S) atoms and two nitrogen atoms of pz^Me2 rings, giving a distorted tetrahedral CuS₂N₂ arrangement. The other two mutually trans Cu atoms are coordinated by two (μ₃-S) atoms, one nitrogen atom of pz^Me2 and one terminal Cl or Br ligand, giving a distorted tetrahedral CuS₂NX unit. In addition to being structurally studied by X-ray diffraction, the title compounds have been characterized by IR, UV–Vis and ¹H NMR spectroscopy. The IR results, which include low-frequency M–S_b stretching bands, are consistent with the X-ray structural analysis and confirm that the [MS₄]²⁻ cores are coordinated through all four sulfur atoms in the complexes 1 and 2.     

New mixed-halide, boron-centered zirconium cluster phases with different cation distributions within a cluster framework: syntheses and structures of A[(Zr6B)ClxI14−x] (A=Na or Cs, 0x4)

Two new mixed-halide zirconium cluster phases have been synthesized by solid-state reactions in sealed tantalum containers from the Zr(IV) halides, elemental Zr and B, and NaI or CsCl, respectively. Single-crystal X-ray data were used to determine the crystal structures of Na[(Zr₆B)Cl_3.9I_10.1], and Cs[(Zr₆B)Cl_2.2I_11.8]. Both phases crystallize in a stuffed version of the [Nb₆Cl₁₄] structure type, orthorhombic, space group Cmca (Na[(Zr₆B)Cl_3.87(5)I_10.13]: a=15.787(2) Å, b=14.109(2) Å, c=12.505(2) Å, Z=4, R1(F)=0.0322 and wR2(F²)=0.0842; Cs[(Zr₆B)Cl_2.16(5)I_11.84]: a=15.696(4) Å, b=14.156(4) Å, c=12.811(4) Å, Z=4, R1(F)=0.0404 and wR2(F²)=0.1031). This structure type is constructed of clusters which contain centered (Zr₆Z) octahedra of the type [(Zr₆Z)X₁₂ⁱX₆^a] with Z=B and X=Cl and/or I. In both structures, chlorine and iodine atoms are randomly (to X-rays) distributed on the inner non-cluster-interconnecting ligand positions, whereas those sites which bridge metal octahedra are solely occupied by iodine. The phase widths for both phases have been found to cover 0x4 for A^I[(Zr₆B)Cl_xI_14−x]. Whereas the sodium cations in Na[(Zr₆B)Cl_xI_14−x] occupy 25% of a site which is octahedrally surrounded by halogen atoms, the larger cations in the cesium-containing phase occupy a 12-coordinate site within the cluster network.     

Isolierung und struktur des dreikernigen hexamethylbenzolzirkonium-komplexes [(Me6C6)3Zr3Cl6]2+ [(Al2Cl7)−]2

Crystals of the trinuclear complex [(Me₆C₆)₃Zr₃Cl₆][Al₂Cl₇]₂ have been obtained from the reaction of ZrCl₄, hexamethylbenzene, AlCl₃, and Al in benzene. They are monoclinic, space group C2/2, with Z  4 and lattice parameters a 14.167(3), b 27.779(7), c 15.721(3) Å and β 94.27(4)°. The Zr atoms form a regular triangle. Each pair of Zr atoms is bridged by two Cl atoms. The fifth coordination site of each Zr atom is occupied by a h⁶-Me₆C₆ group. The cation is almost isostructural with the known trinuclear cation [(Me₆C₆)₃Nb₃Cl₆]²⁺. Important distances are: ZrZr 3.35, ZrCl 2.56, and Zrcenter of C₆ ring 2.17 Å. One of the two independent [Al₂Cl₇]^? anions occurs in a staggered conformation and one occurs in an eclipsed conformation.     

Dimensional reductions from 2-D Nb4P2S21 to 1-D ANb2PS10 (A=Na, K, Rb, Cs, Tl) and to 0-D Tl5[Nb2S4Cl8]Cl using halide molten salts

We found new synthetic routes to obtain 1-D quaternary thiophosphate compounds and a 0-D molecular complex containing a Nb₂S₄ core from a 2-D ternary thiophosphate, Nb₄P₂S₂₁. When Nb₄P₂S₂₁ was reacted with alkali metal halides (ACl; A=Na, K, Rb, Cs) or TlCl at 500-700 °C, the -S-S-S- bridges in 2-D Nb₂PS₁₀-S-S₁₀PNb₂ were excised to form a 1-D chain, and cations were inserted between the chains to form ANb₂PS₁₀ (A=Na, K, Rb, Cs, Tl). We also found that thallium chloride (TlCl) is an excellent reagent for further excision, and it substitutes chloride ligands for the sulfur ligands of 2-D Nb₄P₂S₂₁ to form the molecular complex Tl₅[Nb₂S₄Cl₈]Cl. Crystal data for TlNb₂PS₁₀: monoclinic, Pn, a=6.9452(11) Å, b=7.3761(12) Å, 12.873(2) Å, β=104.472(3)°, and Z=2. Crystal data for Tl₅[Nb₂S₄Cl₈]Cl: orthorhombic, Immm, a=7.001(5) Å, b=9.509(7) Å, c=15.546(11) Å, and Z=2.     

Preparation,crystal structure,and properties of the mixed-valence compound Nb3Se5Cl7

The new compound Nb₃Se₅Cl₇ was prepared by heating 2NbSe₂Cl₂ + 1NbCl₄ at 530°C for 2–3 weeks. The compound is monoclinic with a = 7.599, b = 12.675, c = 8.051Å; β = 106.27°; space group $\text{P2}{}_1\text{m}$. The corresponding bromide, Nb₃Se₅Br₇ (obtained by decomposition of NbSe₂Br₂ under NbSeBr₃), is isotypic with a = 7.621, b = 12.833, c = 8.069Å; β = 106.21°. from the crystal structure and XPS spectra it follows that Nb₃Se₅Cl₇ can be formulated as: [Nb⁴⁺₂Nb⁵⁺₁(Se₂)^2?₂Se^2?₁Cl^?₇]. The structure consists of chains of composition [Nb⁴⁺₂(Se₂)^2?₂Cl^?₅], to which side chains [Nb⁵⁺Se^2?Cl^?₂] are attached. The Nb⁴⁺ atoms form pairs (NbNb = 2.94 Å) which explains that Nb₃Se₅Cl₇ is a diamagnetic semiconductor with a band gap (1.59 eV at 5°K, 1.49 eV at 300°K) very similar to that of NbSe₂Cl₂.     

Zinc selenium oxochloride, β‐Zn2(SeO3)Cl2, a synthetic polymorph of the mineral sophiite

Dizinc selenium dichloride trioxide, β‐Zn₂(SeO₃)Cl₂, a monoclinic polymorph of the orthorhombic mineral sophiite, has a structure built of distorted ZnO₄Cl₂ octahedra, ZnO₂Cl₂ tetrahedra and SeO₃E tetrahedra (E being the 4s² lone pair of the Se^IV ion), joined through shared edges and corners to form charge‐neutral layers. The Cl atoms and the Se lone pairs protrude from each layer towards adjacent layers. The main structural difference between the mineral and synthetic polymorphs lies in the packing of the layers.     

A2VNb6Cl18 (A=Rb, In, Tl): synthesis, crystal structure and magnetic properties of a new series of quaternary niobium chloride cluster compounds

A new series of quaternary niobium chloride cluster compounds corresponding to the general formula, A₂VNb₆Cl₁₈ (A=Rb, Tl or In), has been prepared in sealed quartz tubes from a mixture containing NbCl₅, Nb, VCl₃ and RbCl, or In or Tl metal by solid state reactions at 750°C. The structure of Tl₂VNb₆Cl₁₈ was determined using single-crystal X-ray diffraction: Crystal data: rhombohedral, (No. 148), a=9.1122(17), c=25.178(7) Å, V=1810.5(7) ų and Z=3. The full-matrix least-squares refinement against F² converged to R₁=0.0515, wR₂=0.1104 (all data). The structure consists of discrete octahedral cluster units, [Nb₆Clⁱ₁₂Cl^a₆]⁴⁻ linked by V²⁺ and A⁺ cations, located in a 6-coordinated octahedral and 12-coordinated anticubeoctahedral chloride environment, respectively. The intra-cluster bond lengths indicate 16 valence electrons per cluster. Magnetic susceptibility studies show paramagnetic behavior with a magnetic moment of 3.37 μ_B per formula unit. Electrical resistivity measurements indicate a semiconducting behavior.     

Beiträge zur Chemie der Elemente Niob und Tantal. 84 Die Niob- und Tantalkomplexe [Me6X] X · n H2O mit Me = Nb,Ta; Xi = Cl,Br; Xa = Cl,Br, J

On the chemistry of the elements niobium and tantalum. 84. The niobium and tantalum complexes [Me₆X]X · n H₂O with Me = Nb, Ta; X¹ = Cl, Br; X^a = Cl, Br, J The known and unknown compounds mentioned in the title were prepared. In this group of compounds four different crystal structures (A, B, C, D) occur. Lattice constants are given of the six compounds with structure C which crystallize in the hexagonal system and are isotypic with Ba₂[Nb₆Cl₁₂]Cl₆. Regarding the IR-spectra and the thermal behaviour, possible principles of structure are discussed.     

Mixed Valence Tungsten (IV,V) Compounds with Layered Structures (Part IV): Syntheses,Crystal Structures,and Conductivity of BiX2[W2O2X6] (X = Cl,Br)

The reaction of metallic bismuth with either tungsten tetrachlorideoxide WOCl₄ at 650 K or tungsten tetrabromideoxide WOBr₄ at 670 K, respectively, leads to BiX₂[W₂O₂X₆] (X = Cl, Br) as black, lustrous crystal needles. The crystal structure determinations (triclinic, P$\bar{1}$ ) show the two isotypic structures to be closely related to Hg_0.55[W₂O₂Cl₆] with the presence of 1D‐polymeric W₂O₂X₆ double strands. Dinuclear [Bi₂X₄]²⁺ cations are embedded in the host structure via secondary W–X ··· Bi bonds. Unlike the other members of theM_y[W₂O₂X₆] structure family, which crystallize monoclinic and show crystallographic equivalent tungsten atoms, BiX₂[W₂O₂X₆] has independent tungsten sites. Nevertheless, an assignment of an individual oxidation state to the tungsten atoms within the W₂ group (W–W 2.8321(4) Å for X = Cl, 2.8985(4) Å for X = Br) is not possible and a dynamic intervalent state W(IV, V) is assumed. Electrical conductivity measurements for BiCl₂[W₂O₂Cl₆] show semi‐conductive behavior with a very small band gap of 70 meV and a high conductivity of around 0.5 Ω^–1cm^–1 at temperatures above 220 K. A temperature dependence of the activation energy of charge transport is present and interpreted by the Varshni model.     

Syntheses and Structures of the Quaternary Copper Tellurides K3Ln4Cu5Te10 (Ln=Sm, Gd, Er), Rb3Ln4Cu5Te10 (Ln=Nd, Gd), and Cs3Gd4Cu5Te10

Six quaternary alkali-metal rare-earth copper tellurides K₃Ln₄Cu₅Te₁₀ (Ln=Sm, Gd, Er), Rb₃Ln₄Cu₅Te₁₀ (Ln=Nd, Gd), and Cs₃Gd₄Cu₅Te₁₀ have been synthesized at 1123 K with the use of reactive fluxes of alkali-metal halides ACl (A=K, Rb, Cs). All crystallographic data were collected at 153 K. These compounds crystallize in space group Pnnm of the orthorhombic system with two formula units in cells of dimensions (A₃Ln₄, a, b, c (Å)): K₃Sm₄, 16.590(2), 17.877(2), 4.3516(5); K₃Gd₄, 16.552(4), 17.767(4), 4.3294(9); K₃Er₄, 16.460(4), 17.550(4), 4.2926(9); Rb₃Nd₄, 17.356(1), 17.820(1), 4.3811(3); Rb₃Gd₄, 17.201(2), 17.586(2), 4.3429(6); Cs₃Gd₄, 17.512(1), 17.764(1), 4.3697(3). The corresponding R₁ indices for the refined structures are 0.0346, 0.0315, 0.0212, 0.0268, 0.0289, and 0.0411. The three K₃Ln₄Cu₅Te₁₀ structures belong to one structure type and the Rb₃Ln₄Cu₅Te₁₀ (Ln=Nd, Gd) and Cs₃Gd₄Cu₅Te₁₀ structures belong to another one, the difference being the location of one of the three unique Cu atoms. Both structure types are three-dimensional tunnel structures that contain similar Ln/Te fragments built from LnTe₆ octahedra and CuTe₄ tetrahedra. The CuTe₄ tetrahedra form ¹_∞[CuTe⁵⁻₃] and ¹_∞[CuTe³⁻₂] chains. The alkali-metal atoms, which are in the tunnels, are coordinated to seven or eight Te atoms.     

The antiferromagnetic structures of KFeS2, RbFeS2, KFeSe2, and RbFeSe2 and the correlation between magnetic moments and crystal field calculations

The ternary compounds KFeS₂, RbFeS₂, KFeSe₂, and RbFeSe₂ were prepared and their crystal structures were determined from single-crystal diffractometer data. The atomic arrangement in the isotypic compounds (space group C2/c) is characterized by tetrahedra of chalcogen atoms. Those tetrahedra are centered by iron ions and linked by edges, thus forming chains of ¹_∞[FeX⁻_4/2] frameworks (X S or Se). Susceptibility measurements are reported. Neutron difraction experiments on powdered samples revealed magnetic structures in the antiferromagnetically ordered state. The magnetic moments of the iron ions shows a significant dependence of the atomic arrangement. Therefore calculations based on a simple point charge model are discussed to correlate the magnetic moments and crystal field splittings.     

Heat capacity, enthalpy and entropy of strontium niobate Sr2Nb2O7 and calcium niobate Ca2Nb2O7

The heat capacity and the enthalpy increments of strontium niobate Sr₂Nb₂O₇ and calcium niobate Ca₂Nb₂O₇ were measured by the relaxation time method (2–300 K), DSC (260–360 K) and drop calorimetry (720–1370 K). Temperature dependencies of the molar heat capacity in the form C_pm = 248.0 + 0.04350T − 3.948 × 10⁶/T² J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and C_pm = 257.2 + 0.03621T − 4.434 × 10⁶/T² J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇ were derived by the least-square method from the experimental data. The molar entropies at 298.15 K, S_m°(298.15 K) = 238.5 ± 1.3 J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and S_m°(298.15 K) = 212.4 ± 1.2 J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇, were evaluated from the low-temperature heat capacity measurements.     

The dichloromethane induced fragmentation of ferrocenylmethyldimethylamine. Mechanistic aspects and crystallographic and electrochemical investigation of the (FcCH2)2NMe2 and FcCH2NMe2H ions

Ferrocenylmethyldimethylamine, FcCH₂NMe₂, reacts with CH₂Cl₂ in either the presence or absence of non-coordinating counterions to give equimolar amounts of the bis(ferrocenylmethyl)dimethyl ammonium salts (FcCH₂)₂NMe₂⁺X⁻ (X⁻=PF₆⁻, SbF₆⁻, BPh₄⁻ or Cl⁻, 1a–d) and the corresponding protonated ammonium salts FcCH₂NMe₂H⁺ which have been isolated as the SbF₆⁻ and Cl⁻ salts 2b,d. The reaction proceeds via fragmentation of an intermediate quaternary chloromethylated ammonium ion to chloromethylferrocene, FcCH₂Cl, and dimethyliminium chloride NMe₂CH₂⁺Cl⁻. The parent amine acts as a nucleophile toward FcCH₂Cl to give 1a–d and as a base toward NMe₂CH₂⁺ to give FcCH₂NMe₂H⁺, NMe₂H and (Me₂N)₂CH₂. The FcCH₂Cl intermediate is intercepted by NEt₃ while KCN or LiH do not successfully compete with FcCH₂NMe₂. A new, non-toxic, selective, high-yield route to 1d is also presented. Electrochemistry and UV–vis spectroelectrochemistry reveal, that the two identical redox centers in 1a–d are essentially non-interacting. Individual E_1/2 values have been determined for different solvents by digital simulation. The corresponding ferrocenium salts were prepared by either chemical or electrochemical means and accordingly characterized. Our studies are augmented by X-ray structure analyses of 1b, 1d and 2d. 1d contains three different cation conformers and four molecules of water per unit cell. The latter are hydrogen bonded to the chloride counterions to form one-dimensional infinite chains parallel to the a axis.     

Hexanuclear Niobium Cluster Compounds with Protonated N-Base Cations

Octahedral clusters of the [M₆X₁₂] type offer numerous possibilities to form structural arrangements through different choices of bonding situations. In this paper a series of new cluster compounds of the transition metal niobium is described, which consist of the [Nb₆Cl₁₈]^2–, and in one case [Nb₆Cl₁₈]^3–, anion and protonated N-base cations ([MIm-H]⁺, [nPr₃N-H]⁺, [TMGu-H]⁺, and [Tzn-H]⁺). They all are prepared using water scavenger compounds [SOCl₂ or (Ac)₂O] under oxidising conditions, resulting in two-electron (or one-electron, respectively) oxidized cluster units with respect to the starting material [Nb₆Cl₁₄(H₂O)₄] · 4H₂O. Of five members of this group single-crystal X-ray structures were determined. The cluster anions exist in all structures as discrete units. The acidic H atoms of all N-bases are hydrogen bonded to H acceptors, in 4 cases to outer, exo bonded Cl atoms of the cluster unit and in one case to the O atom of a co-crystallized THF molecule. In [TMGu-H]₂[Nb₆Cl₁₈] chains of cluster anions exist hydrogen-bonded through bridging [TMGu-H]⁺ cations. ESI mass spectra of [MIm-H]₂[Nb₆Cl₁₈] · 2SOCl₂ and [TMGu-H]₂[Nb₆Cl₁₈] show the expected isotopic distribution patterns for the anions together with other peaks associated to chloride mass losses and/or reduction processes.     

Low-Temperature Triclinic Distortion in NASICON-Type LiSn2(PO4)3

The triclinic nature of the low-temperature modification of LiSn₂(PO₄)₃, heretofore considered as monoclinic, has been proved by detailed indexing of its X-ray diffraction powder pattern. The triclinic character of most low-temperature polymorphs of NASICON-type LiMe^IV₂(PO₄)₃is tentatively hypothesized, from this evidence and additional indications.     

Compounds with Cluster Cations together with Cluster Anions [(M6X12)(EtOH)6]2+ besides [(Mo6Cl8)Cl4X2]2– (M = Nb,Ta; X = Cl,Br) – Synthesis and Crystal Structures

Compounds consisting of both cluster cations and cluster anions of the composition [(M₆X₁₂)(EtOH)₆][(Mo₆Cl₈)Cl₄X₂] · n EtOH · m Et₂O (M = Nb, Ta; X = Cl, Br) have been prepared by the reaction of (M₆X₁₂)X₂ · 6 EtOH with (Mo₆Cl₈)Cl₄. IR data are given for three compounds. The structures of [(Nb₆Cl₁₂)(EtOH)₆][(Mo₆Cl₈)Cl₆] · 3 EtOH · 3 Et₂O 1 and [(Ta₆Cl₁₂)(EtOH)₆][(Mo₆Cl₈)Cl₆] · 6 EtOH 2 have been solved in the triclinic space group P1 (No. 2). Crystal data: 1 , a = 10.641(2) Å, b = 13.947(2) Å, c = 15.460(3) Å, α = 65.71(2)°, β = 73.61(2)°, γ = 85.11(2)°, V = 2005.1(8) ų and Z = 1; 2 , a = 11.218(2) Å, b = 12.723(3) Å, c = 14.134(3) Å, α = 108.06(2)°, β = 101.13(2)°, γ = 91.18(2)°, V = 1874.8(7) ų and Z = 1. Both structures are built of octahedral [(M₆Cl₁₂)(EtOH)₆]²⁺ cluster cations and [(Mo₆Cl₈)Cl₆]^2– cluster anions, forming distorted CsCl structure types. The Nb–Nb and Ta–Ta bond lengths of 2.904 Å and 2.872 Å (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 Å and 2.609 Å (on average) are characteristic for the [(Mo₆Cl₈)Cl₆]^2– anion, but there is a clear correlation between the number of hydrogen bridges to the terminal Cl and the corresponding Mo–Cl distances.     

Ta3SBr7—A New Structure Type in theM3QX7Family (M=Nb, Ta;Q=S, Se, Te;X=Cl, Br, I)

The layered compound Ta₃SBr₇has been prepared by direct reaction of the elements at 550°C for 2 weeks in evacuated Pyrex ampules, and its structure determined by single-crystal X-ray diffraction. Ta₃SBr₇crystallizes with monoclinic symmetry in space groupCm;a=12.249(2) Å,b=7.071(2) Å,c=8.829(2) Å,β=134.421(8)°,V=546.16(23) ų,Z=2,R=0.027,R=0.032. The structure, related to the Cd(OH)₂type, consists of triangular tantalum clusters located between every other layer of a closed-packed mixed S/SBr ordered anion framework. The slightly distorted, quasi-infinite²_∞[Ta₃SBr₇] slabs stack parallel to the crystallographicabplane in a new variant, with each successive²_∞[Ta₃SBr₇] slab related by a 1/2b+c stacking vector. This stacking mode is the sixth layered structure type observed in theM₃QX₇system. Ta₃SBr₇represents the first Ta₃QX₇compound discovered that is not isostructural with its niobium counterpart and is the second of the two one-slab per unit cell stacking types possible in theM₃QX₇system Nb₃SBr₇being the other. Synthesis, structural characterization, and lattice energy calculations of Ta₃SBr₇and related compounds are reported.     

Schwingungsspektren der Clusterverbindungen (M6X)X · 8 H2O,M = Nb,Ta; Xi = CI,Br; Xa = CI,Br, I

Vibrational Spectra of the Cluster Compounds (M₆X₁₂ⁱ) · 8H₂O, M = Nb, Ta; Xⁱ = Cl, Br; X^a = Cl, Br, I IR and, for the first time, Raman spectra at 80 K of the cluster compounds (M₆X)X · 8H₂O; M = Nb, Ta; Xⁱ = Cl, Br; X^a = Cl, Br, I, have been recorded, characterized by typical frequencies of the (M₆X) unit, which are only slightly influenced by the terminal X^a ligands. The most intense line with the depolarisation ≈? 0.2 in all Raman spectra is caused by inphase movement of all atoms and assigned to the symmetric metal-metal vibration v₁, observed for the clusters (Nb₆Cl) at 233–234, for (Nb₆Br) at 186–187, for (Ta₆Cl) at 199–203, and for (Ta₆Br) at 176–179 cm^?1. The IR spectra exhibit in the same series intense bands at 233, 204, 207, and 179 cm^?1, assigned to the antisymmetric metal-metal vibration. The metal-metal frequencies are significantly higher than discussed before. The tantalum clusters show on excitation with the krypton line 647.1 nm in the region of a d–d transition at 645 nm a resonance Raman effect with series of overtones and combination bands. In case of (Ta₆Br) another polarisized band is observed at 229 cm^?1 and assigned to the Ta? Brⁱ vibration v₂. From the progressions of v₁ and v₂ anharmonicity constants of about ?3 cm^?1 are calculated indicating a strong distortion of the potential curves.     

Synthesis and structures of new niobium cluster compounds with pyridinium cations: (PyrH)2[Nb6Cl18]·EtOH (Pyr: pyridine, Et: ethyl) and the cubic modification of (PyrH)2[Nb6Cl18]

Slow crystallization of (PyrH)₂[Nb₆Cl₁₈] from hot ethanol solution affords triclinic (PyrH)₂[Nb₆Cl₁₈]·EtOH. Treatment of [Nb₆Cl₁₄(H₂O)₄]·4H₂O with pyridine in a methanol solution gives the second title compound, the cubic modification of (PyrH)₂[Nb₆Cl₁₈]. Both structures were determined by single crystal X-ray diffraction, (PyrH)₂[Nb₆Cl₁₈]·EtOH: P1¯, a=9.3475(3), b=9.3957(3), c=10.8600(3) Å, α=82.582(1)°, β=78.608(1)°, and γ=78.085(1)°, Z=1, R₁(F)/wR₂(F²)=0.0254/0.0573, cub.-(PyrH)₂[Nb₆Cl₁₈]: Fd3¯m, a=19.935(2) Å, Z=8, R₁(F)/wR₂(F²)=0.0557/0.1796. The cluster compounds contain isolated, molecular [Nb₆Clⁱ₁₂Cl^a₆]²⁻ cluster anions with an octahedron of metal atoms edge bridged by chlorido ligands with additional ones on all the six exo positions. These cluster anions are separated by the pyridinium cations and ethanol solvent molecules, respectively. For the cubic modification of (PyrH)₂[Nb₆Cl₁₈], a structural comparison is given to the known rhombohedral modification using the group-subgroup relations as expressed by a Bärnighausen tree.