Structural studies of carbene complexes: The crystal structure of the secondary carbene complex RuI2[CHN(CH3)(p-CH3C6H4)](CO)(CN-p-CH3C6H4)(P(C6H5)3)

Crystal and molecular structures of the title compound have been determined from a three-dimensional X-ray analysis using diffractometer data. The crystals are triclinic, space group P$\text{1}$, with Z = 2 in a unit cell of dimensions a = 11.640(1), b = 10.9139(8), c = 16.587(2) Å, α = 87.983(5), β = 99.670(6), λ = 62.250(5)°. Full matrix least squares refinement has given a final R-factor of 0.043 for 2726 reflections for which I > 2σ(I).The crystal structure consists of discrete molecules of neutral complex together with water molecules which are hydrogen bonded into pairs [O ? O separation 2.60 Å]. 0The (H₂0)₂ units do not hydrogen bond to any other atoms. The ruthenium coordination is octahedral with trans carbene and isocyanide, cis iodides, and cis phosphine and carbonyl ligands. The Ru-donor distances are 2.776(2) [I trans to -PPh₃], 2.782(1) [I trans to -CO], 2.342(4) [PPh₃], 1.855(15) [CO], 2.046(15) [C(carbene)], and 1.998(16) Å [C(isocyanide)]. The bond lengths are discussed in terms of the trans effects of the ligands. The C(carbene)-N distance is 1.26(2) Å and the Ru—C(carbene)—N angle is 141.5(5)°.     

X-ray crystal structure of a distorted tetrahedral cluster in the salt [(Ph3P)4Au4N]+ BF4−. Geometrical indication of stable electronic configurations in post-transition metal complexes and the magic number 18-e in centred gold clusters

According to an X-ray investigation (?120°C, λ(Mo-K_α), 6815 reflections, space group P2₁/c, Z = 4, R = 0.068), the cation of the tetraaurated ammonium salt [(Ph₃PAu)₄N]⁺ BF₄^? can be considered as a distorted tetrahedral Au₄(N) cluster built up of four linearly coordinated Au atoms with an interstitial N atom (AuAu 3.012–3.504 Å, AuN 1.93–2.10 Å). On the basis of the structural data on 14-, 16- and 18-electron mononuclear post-transition metal complexes, the magic number of 18 skeletal electrons can be suggested for [Au_nL_{n ? 1}]^m+ clusters with an interstitial Au atom and a quasi-spherical peripheral Au_{n ? 1} polyhedron.     

Synthesis, Characterization, Luminescence, and Electrochemistry of New Tetranuclear Gold(I) Amidinate Clusters: Au4[PhNC(Ph)NPh]4, Au4[PhNC(CH3)NPh]4, and Au4[ArNC(H)NAr]4

The syntheses and the X-ray structures of the tetranuclear gold(I) benzamidinate, Au₄[PhNC(Ph)NPh]₄, and the tetranuclear gold(I) acetamidinate, Au₄[PhNC(CH₃)NPh]₄, clusters are reported. The clusters are produced by the reaction of the sodium salt of an amidine ligand with the gold precursor Au(THT)Cl in a (1:1) stoichiometry. The average Au...Au distance between adjacent Au(I) atoms is ∼2.9 ?, typical of compounds having an aurophilic interaction. The four gold atoms are arranged in a square (Au...Au...Au... = 88–91°) in the acetamidinate and in a distorted square (Au...Au...Au... = 82–97°) in the benzamidinate derivative. Electrochemical oxidation of the tetranuclear complex Au₄[PhNC(Ph)NPh]₄ show three reversible waves at 0.87, 1.19, 1.42 V vs. Ag/AgCl at a scan rate of 100 mV/s in CH₂Cl₂ similar to the three reversible waves seen before from the tetranuclear complexes Au₄[ArNC(H)NAr]₄, Ar = C₆H₄-4-OMe, Ar = C₆H₄-4-Me, and Ar = C₆H₃-3,5-Cl. A summary of the chemistry of the tetranuclear Au(I) amidinate complexes Au₄[ArNC(H)NAr]₄, Ar = C₆H₄-4-OMe, C₆H₃-3,5-Cl, C₆H₄-4-Me, C₆H₄-3-CF₃, C₆F₅, C₁₀H₇ also is presented. The tetranuclear clusters Au₄[ArNC(H)NAr]₄, Ar = C₆H₄-4-OMe, Ar = C₆H₄-3-CF₃, Ar = C₆H₄-4-Me and Ar = C₆H₄-3,5-Cl are the first tetranuclear gold(I) cluster species from group 11 elements to show fluorescence at room temperature. The lifetimes of the naphthyl and trifluoromethylphenyl complexes are in the millisecond range indicating phosphorescent processes. Recently it has been shown that Au₄[ArNC(H)NAr]₄ are very effective catalysts upon calcination for room temperature CO oxidation. Congratulations to Dieter Fenske, a superb synthetic chemist with exceptional talents in cluster chemistry, on the occasion of his 65th birthday.     

Molecular structure of η5-[Pd(CH2CHCMeCH2CH2CHCMe2)-(MeCN)](BF4), A cationic π-allylpalladium complex with an almost in-plane intramolecular olefin coordination

The complex η⁵-[Pd(CH₂CHCMeCH₂CH₂CHCMe₂)-(MeCN)](BF₄), a model for a key intermediate in diene polymerization, crystallizes in the monoclinic space group P2₁/c, a 11.362(2), b 13.655(4), c 10.046(2) Å, β 134.80(1)°. The structure was solved by conventional Patterson and Fourier syntheses and refined by full matrix least squares techniques to a final discrepancy index R = 0.058 for 1710 independent reflections. The palladium and nitrogen atoms, the center of gravity of the allyl triangle, and the middle point of the coordinated double bond are coplanar. The side chain of the organic moiety is located in the syn position with respect to the η³-allyl group. The orientation of the coordinated double bond, which forms an angle of 26° with the coordination plane, is novel for palladium(II) complexes.     

Barreleneiridium(I) complexes. Crystal structures of [Ir(Me3TFB)(η6-C6H4Me2)]ClO4 and [Ir(TFB)(η5-PhNPh2)]BF4·CH2Cl2 (TFB = tetrafluorobenzobarrelene)

A new series of cationic areneiridium(I) complexes of formula [Ir(barrelene)(arene)]⁺ or [Ir(barrelene)(PhNRPh)]⁺ (R= Ph or H) have been synthesized from neutral iridium complexes of the type [IrY(barrelene)]_x (barrelene = Me₃TFB, Y = Cl or OMe (x = 2), Y = acac (x = 1); barrelene = TFB, Y = OMe (x = 2), Y = acac (x = 1)). The crystal structures of [Ir(Me₃TFB)(1,4-C₆H₄Me₂)]ClO₄ and [Ir(TFB)(PhNPh₂)]BF₄·CH₂Cl₂ have been determined by X-ray diffraction. They crystallize in the space groups Pbca and Pna2₁ respectively with lattice constants of 17.6947(11), 15.8072(10), 16.0019(11) Å and 9.8059(2), 20.8097(9), 14.3367(4) Å. Final R factors were 0.063 and 0.042 for the observed data. Both complexes show a staggered arrangement between the arene and the TFB moieties and deviation from planarity of the coordinated arene ligands. In the second complex the IrC and NC distances, the CNC angle, the type of arene puckering, and the spectroscopic data indicate a distortion of the coordinated arene towards a η⁵-coordinated iminocyclohexadienyl form.     

X-ray crystal structure of (η5-pentamethylcyclopentadienyl) (η3-1,3-dimethylallyl)chlororhodium: localized “Di-π-olefin-σ” bonding of cyclopentadienyl

The crystal structure of (η⁵-C₅Me₅)(η³-MeHCCHCHMe)RhCl at ?120°C was determined (R = 0.041 for 1790 reflections). The molecule has approximate mirror symmetry. The cyclopentadienyl ring is bent by 6.8°, acquiring an envelope-like conformation, and its bonding with Rh is of a partially-localized η⁴:η¹ type with RhC(“π-diolefin”) of 2.206–2.235 Å and RhC(“σ-bonded”) of 2.151 Å. The syn-arrangement of the Me groups in the π-allyl ligand, assigned by NMR spectra, is confirmed.     

Elevated Catalytic Activity of Ruthenium(II)–Porphyrin‐Catalyzed Carbene/Nitrene Transfer and Insertion Reactions with N‐Heterocyclic Carbene Ligands

Bis(NHC)ruthenium(II)–porphyrin complexes were designed, synthesized, and characterized. Owing to the strong donor strength of axial NHC ligands in stabilizing the trans M?CRR′/M?NR moiety, these complexes showed unprecedently high catalytic activity towards alkene cyclopropanation, carbene C? H, N? H, S? H, and O? H insertion, alkene aziridination, and nitrene C? H insertion with turnover frequencies up to 1950 min^?1. The use of chiral [Ru(D₄‐Por)(BIMe)₂] ( 1 g ) as a catalyst led to highly enantioselective carbene/nitrene transfer and insertion reactions with up to 98 % ee. Carbene modification of the N terminus of peptides at 37 °C was possible. DFT calculations revealed that the trans axial NHC ligand facilitates the decomposition of diazo compounds by stabilizing the metal–carbene reaction intermediate.     

Arylmercury(II) compounds involving intramolecular coordination via 2-Me2NCH2- and chiral (S)-2-Me2NCHMe-ring substituents

Arylmercury compounds of the type Ar₂Hg and ArHgX (X = Cl, OAc) have been synthesized and characterized by ¹H and ¹³C NMR spectroscopy; the Ar group was either 2-Me₂NCH₂C₆H₄ or (S)-2-Me₂NCH(Me)C₆H₄, both of which contain N-donor ligands. The observation of anisochronous NMe resonances in (S)-2-Me₂NCH(Me)C₆H₄HgX (X = Cl, OAc) at low temperature indicates that in solution the mercury centre is three-coordinate as a result of stable intramolecular HgN coordination     

Synthesis and Rearrangement of Stable NHC→Silylene Adducts and Their Unique Reactivity towards Cyclohexylisocyanide

The syntheses and reactivity of the two N‐heterocyclic carbene (NHC)→ silylene complexes 2 and 4 have been investigated. The latter are easily accessible by reaction of the zwitterionic, N‐heterocyclic silylene LSi: 1 [L=Ar‐N‐C(=CH₂)CH?C(Me)‐N‐Ar, Ar=2,6‐iPr₂C₆H₃] with 1,3,4,5‐tetramethylimidazol‐2‐ylidene and 1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene, respectively. While compound 2 undergoes facile rearrangement above ?20 °C to give the unsymmetrical N‐heterocyclic silylcarbene 3 , the derivative 4 remains unchanged even after boiling in benzene. The remarkable reactivity of 3 and 4 towards cyclohexylisocyanide has been examined which leads in a unique series of C? H, Si? H, and C? N bond activations to the new triaminosilanes 5 and 6 , respectively. The novel compounds 3 , 4 , 5 , and 6 were fully characterized by ¹H, ¹³C, and ²⁹Si NMR spectroscopy, EI‐MS, elemental analysis, and single‐crystal X‐ray diffraction.     

Cleavage of the carbon-silicon bonds in trimethylsilylacetylenes by trans[(PR3)2PtX(R′OH)]PF6 cations and formation of cationic alkoxycarbene complexes of platinum(II)

Cationic alkoxycarbene complexes of platinum(II) have been isolated in the reactions of trans-[(PR₃)₂PtX(R′OH)]PF₆ (X  H or Me; R′  Me or Et) with Me₃SiCCR′′ (R′′  H, Me or SiMe₃). In these reactions cleavage of the carbon-silicon bond by the nucleophilic attack of alcohol has been observed. These carbene complexes have been characterized by elemental analyses and by IR, ¹H and ¹³C NMR spectral data. ¹³C NMR chemical shift data for carbene carbon atoms suggest that the carbene carbon may be very positively charged.     

Synthesis,Characterization, Luminescence,and Electrochemistry of the Tetranuclear Gold(I) Amidinate Clusters,Precursors to CO Oxidation Catalysts: Au4[(ArNC(H)NAr)]4, Au4[(PhNC(Ph)NPh)]4 and Au4[PhNC(CH3)NPh)4]

A summary of the chemistry of the tetranuclear Au(I) amidinate complexes is presented. Tetranuclear Au(I) amidinate clusters are produced by the reaction of the sodium salt of a amidine ligand with the gold precursor Au(THT)Cl in a (1:1) stoichiometry. The structures of the tetranuclear Au₄[ArNC(H)NAr]₄, Ar = C₆H₄‐4‐OMe, C₆H₃‐3,5‐Cl, C₆H₄‐4‐Me, C₆H₄‐3‐CF₃, C₆F₅, C₁₀H₇ and the tetranuclear Au₄[(PhNC(Ph)NPh]₄ and Au₄[PhNC(CH₃)NPh]₄ have been characterized by X‐ray crystallography. The average Au···Au distance between adjacent Au(I) atoms is ?3.0 Å, typical of compounds having an aurophilic interaction. The four gold atoms are located at the corner of a rhomboid with the amidinate ligands bridged above and below the near plane of the four Au(I) atoms. The angles at Au···Au···Au in the cyclic units are between 70° and 116°. The tetranuclear gold(I) amidinate clusters each show different luminescence behavior. The tetranuclear clusters Au₄[(ArNC(H)NAr]₄, Ar = C₆H₄‐4‐OMe, Ar = C₆H₄‐3‐CF₃, Ar = C₆H₄‐4‐Me and Ar = C₆H₄‐3,5‐Cl are the first tetranuclear gold(I) cluster species from group 11 elements that show fluorescence at room temperature. The tetranuclear naphthyl derivative Ar = C₁₀H₇ is luminescent only at 77 K. The pentafluorophenyl derivative Ar = C₆F₅ does not show any photoluminescence in the solid state nor in the solution. The lifetimes of the naphthyl and trifluoromethylphenyl complexes are in the millisecond range indicating phosphorescent processes. Electrochemical and chemical oxidation studies of the tetranuclear Au(I) amidinate clusters are presented. The tetranuclear complexes Au₄[ArNC(H)NAr]₄, Ar = C₆H₄‐4‐OMe, Ar = C₆H₄‐4‐Me, and Ar = C₆H₃‐3,5‐Cl, show three reversible waves at 0.75, 0.95, 1.09 V vs. Ag/AgCl at a scan rate of 500 mV/s in 0.1 M Bu₄NPF₆/CH₂Cl₂ at a Pt working electrode in CH₂Cl₂. Three reversible waves at 0.87, 1.19, 1.42 V vs. Ag/AgCl at a scan rate of 100 mV/s are also observed for the tetranuclear complex Au₄[PhNC(Ph)NPh]₄ in CH₂Cl₂. The pentafluorophenyl amidinate derivative, Au₄[ArNC(H)NAr]₄, Ar = C₆F₅ shows no oxidation wave below 1.8 V. Recently it has been shown that Au₄[ArNC(H)NAr]₄ is a very effective catalyst precursor for room temperature CO oxidation.     

Crystal and molecular structure of (η5-trimethylsilylcyclopentadienyl)(η4-tetraphenylcyclobutadiene)cobalt

The structure of (η⁵-trimethylsilylcyclopentadienyl)(η⁴-tetraphenylcyclobutadiene)cobalt, ((CH₃)₃SiC₅H₄)Co((C₆H₅)₄C₄), has been determined by single crystal X-ray diffraction techniques using three-dimensional data collected on an automatic diffractometer. The crystals are monoclinic, space group P2₁/c, with lattice parameters a 11.551(1), b 16.352(6), c 18.324(2) », β 122.85(1)° with four molecules in the unit cell. The structure consists of discrete molecules in which a cobalt atom is sandwiched between the η⁵-cyclopentadienyl (Cp) and the η⁴-cyclobutadience (Cb) ligands bonded to the metal in the hapto mode. The perpendicular distances Co?(Cp) and Co?(Cb) of 1.688 and 1.699 », respectively, as well as the dihedral angle of 6.9° between the two rings and the distortions of their side groups, indicate steric interactions within the molecule. The Cb ring is planar within 0.015 » and has a rectangular shape with edges of 1.480(5) and 1.463(3) ». The Cp ring, which is planar to within 0.005 », appears to be highly distorted by the trimethylsilyl group, which induces a lengthening of the C—C distances involving the substituted carbon atom (1.440(7) ») and a narrowing of the corresponding bond angle (105.3(2)°).     

Symmetry constraints in energy transfer between state-selected Ar*(3P2, 3P0) metastable atoms and ground state H atoms

The excitation-transfer reaction in thermal energy collisions of state-selected metastable Ar*(³P₂) and Ar*(³P₀) atoms with ground state H atoms, giving excited H*(n = 2) atoms, has been studied with the stationary afterglow technique. The rate constant for the excitation of H atoms by Ar*(³P₂) has been found to be more than one order of magnitude larger than in excitation by Ar*(³P₀). This difference in the reactivity of two metastable species is explained to be a consequence of the attractive nature of the D(²II) and E(²Σ⁺) potentials that develop from the Ar*(³P₂)+H entrance channel and which give curve crossing with the B(²II) and C(²Σ⁺ potentials, respectively, leading to the Ar+H*(n=2) exit channel, whereas only a repulsive ⁴II (Ω=$\text{1}\text{2}$) potential develops from the Ar*(³P₀+H entrance channel.     

Mechanism of formation of carbene complexes by reaction of cis-[PdCl2(PPh3)(CNR)] with secondary amines

A kinetic study is reported for the reactions of secondary aromatic amines p-YC₆H₄NHR (Y = MeO, Me, H; R = Me, Et) with the isocyanide complexes cis-[PdCl₂(p-XC₆H₄NC)(PPh₃)] (X = Me, H, Cl) leading to the carbene derivatives cis-[PdCl₂ {C(NH-p-C₆H₄X)NR-p-C₆H₄Y} (PPh₃)] in 1,2-dichloroethane at 25°C. A stepwise mechanism is proposed which involves a direct nucleophilic attack of the entering amine on the isocyanide carbon followed by proton transfers to the final carbene complexes. These take place both intramolecularly in a four-membered cyclic transition state and by the agency of one further amine molecule serving as a proton acceptor-donor in a six-membered transition state. Competition experiments with primary amines and trends in rate parameters are discussed to support the mechanism.     

Reactions of tetracyanoethylene with transition-metal acetylides: synthesis and structure of Rh(CCPh) [η2-C2(CN)4] (NCMe)(PPh3)2

The title complex was obtained from the adduct of C₂(CN)₄ and Rh(CCPh)-(CO)(PPh₃)₂ by simple substitution of CO in refluxing acetonitrile. Crystals of the complex are orthorhombic, with a 10.058(2), b 20.008(4), c 21.594(5) Å, space group P2₁2₁2₁, Z  4. The rhodium has approximate trigonal bipyramidal coordination, with apical NCMe and C₂Ph ligands: RhC₂Ph, 1.939(18); RhC(olefinic), 2.151, 2.157(19); RhN, 2.051(16); RhP, 2.377, 2.397(6) Å.     

Gold(III) adducts of 2-vinyl- and 2-ethylpyridine and cyclometallated derivatives of 2-vinylpyridine: Crystal structure of the cyclometallated derivative [Au(k-C,N-CH2CH(Cl)-C5H4N)(PPh3)Cl][PF6]

Reaction of Na[AuCl₄] with 2-vinylpyridine (vinpy) and 2-ethylpyridine (etpy) affords the N-bonded adducts Au(Rpy)Cl₃ (R = CH₂CH, vinpy; CH₃CH₂, etpy). Cationic adducts, [Au(vinpy)₂Cl][X]₂ (X = BF₄, PF₆) and [Au(etpy)₂Cl₂][BF₄], were also obtained by reaction of Au(Rpy)Cl₃ with Rpy (1:1) and excess NaBF₄ or KPF₆. Thermal activation of Au(vinpy)Cl₃ in water gives the five-membered cycloaurated derivative [Au(k²-C,N-CH₂CH(Cl)-C₅H₄N)Cl₂] formally resulting through a trans nucleophilic addition of a chloride onto the CC bond. No cyclometallated derivatives are obtained by reactions of Au(etpy)Cl₃. An X-ray crystal structure determination on the PPh₃ derivative [Au(k²-C,N-CH₂CH(Cl)-C₅H₄N)(PPh₃)Cl][PF₆] was carried out.     

SYNTHESIS OF NOVEL BIDENTATE (Te,N) LIGANDS-2-ARYLTELLUROETHYLAMINES AND THEIR COMPLEXATION WITH MERCURY (II)

Abstract

Sodium aryltellurolate (ArTe^?Na⁺, where Ar = 4-MeOC₆H₄ or 4-EtOC₆H₄) reacts with 2- bromoethylamine resulting in the (Te, N) ligands 2-aryltelluroethylamine (ArTeCH₂CH₂NH₂, 1) which have been characterized by elemental analyses, molecular weight, IR, ¹H and ¹³C NMR spectra. With HgCI₂, they form HgC1₂·1 type of complexes. IR, ¹H and ¹³C NMR spectra of the complexes suggest that 1 ligates as a bidentate ligand with respect to Hg(II). Osmometric molecular weight measurements suggest that on heating the mercury complex HgCl₂·lb (Ar = 4-EtOC₆H₄) in solution, relatively less soluble species result. It seems to have two Hg atoms bridged by two (Te, N) ligands. The HgC₂·la (Ar = 4-MeOC₆H₄) has very low solubility in organic solvents and. therefore, seems to be dimerized or polymerized during the synthesis. Analysis of CH₂ rocking bands in IR spectra suggests that two CH₂ groups of the ligands are most probably in a gauche conformation in the mercury complexes.     

Crystal structure of a methyl(methoxy)carbene complex of nickel(II), trans-[Ni(C6Cl5)(PMe2Ph)2{C(OMe)Me}]BF4

The structure of a nickel(II) complex, trans-[Ni(C₆Cl₅)(PMe₂Ph)₂{C(OMe)Me}]BF₄, containing the simplest alkyl(alkoxy)carbene ligand has been determined by X-ray crystallography (R = 0.091). The geometry around the nickel atom is square-planar. The comparatively short NiC(1) bond length of 1.843(10) Å showed the presence of π-bonding in the nickel-carbene bond.     

Group IB organometallic chemistry: XXIX. Synthetic and structural aspects of polynuclear arylcopperlithium compounds Ar4Cu2Li2 (“arylcuprates”) and interaggregate exchange phenomena in Ar4Cu4/Ar4Li4/Ar4Cu2Li2 systems

The thermally stable arylmetal-IB-lithium compounds (2-Me₂NCHZC₆H₄)₄M₂Li₂ (M = Cu, Ag or Au; Z = H or Me) and (2-Me₂NC₆H₄)₄M₂Li₂ have been prepared by a $\text{2}\text{1}$ molar reaction of the aryllithium compounds with the corresponding metal-IB halide (Cu or Ag) or metal-lB halide phosphine complex (BrAuPPh₃). These tetranuclear complexes were also made by an interaggregate exchange reaction of the pure arylmetal-IB clusters with the aryllithium compound.The structure of these compounds in solution consists of aryl groups bridging one metal-lB and one lithium atom of a trans M₂Li₂ core. The four built-in ligands coordinate to lithium resulting in two-coordination at M and four-coordination at Li. These conclusions were based on ¹H and ¹³C NMR spectroscopic data (J(AgC(1)), J(LiC(1)) of solutions of these tetranuclear compounds as well as on the ¹⁹⁷Au Mössbauer data of solid (2-Me₂NC₆H₄)₄Au₂Li₂ (IS 5.65 mm/s and QS 12.01 mm/s).The interaggregate exchange between the tetranuclear species is discussed in terms of an associative mechanism involving formation of an octanuclear intermediate in which the aryl groups can migrate via (3c-2e)edge-(2c-2e)corner(3c-2e)edge movements without M₂Ar bond cleavage.Some aspects of the organic reactions in which organocuprates are involved as intermediates are discussed in terms of the novel structural information.     

Reactions of silicon-, germanium- and tin-containing carbene complexes of tungsten Ph3E-CW(OBu)3 (E=Si, Ge, Sn) with hydrogen chloride: crystal structures of carbene complexes Ph3E-CHWCl2(OBu)2 (E=Si, Ge)

The novel organosilicon, -germanium and -tin-containing carbene complexes of tungsten of the type Ph₃E-CHWCl₂(OBu^t)₂ (E=Si, Ge, Sn) have been prepared by the reaction of heteroelement-containing carbene complexes of tungsten Ph₃E-CW(OBu^t)₃ (E=Si, Ge, Sn) with hydrogen chloride. The tin-containing carbene complex was identified in solution by ¹H NMR spectroscopy. Silicon- and germanium-containing carbene complexes were isolated in high yields as crystalline solids and characterized by elemental analysis, IR, ¹H NMR, ¹³C NMR and ²⁹Si NMR spectroscopy and X-ray diffraction studies. The geometry of the W atoms in the compounds can be described as a distorted square pyramid.