Trigonal Prismatic Structure Research Articles

Structural model of the Ni2B amorphous alloy from the data of molecular dynamics simulation

The structure of the Ni2B amorphous alloy has been investigated by the molecular dynamics method using the Heine-Abarenkov-Animalu and Animalu potentials. The amorphous state is reached by melting a microcrystal of the appropriate composition with subsequent cooling of the melt. The interatomic distances Ni-Ni, B-B, and Ni-B and the boron coordination numbers with respect to boron and nickel are determined from the calculated partial radial atomic distribution functions (RDFs). It is found that, upon transition from the crystalline state to the amorphous state, the antiprismatic-type structure transforms into the trigonalprismatic structure.

Molecular Electrostatic Potential and Electron Density Topography: Structure and Reactivity of (substituted arene)Cr(CO)3Complexes

Structural properties as well as subtle electronic effects occurring in (substituted arene)Cr(CO)3 complexes have been investigated at HF, B3LYP, and MP2 levels using the topographical properties of the molecular electrostatic potential (MESP) as well as electron density. The most stable structures are characterized as syn-eclipsed for an electron-donating arene substituent and anti-eclipsed for an electron-withdrawing arene substituent. The MESP topography unambiguously revealed the electronic nature of the Cr(CO)3 moiety as a very strong electron-withdrawing group to the arene unit of the complex. Interestingly, the substituent effects are strongly felt at the MESP surrounding the carbonyl oxygen rather than the arene ring. The effect of electron correlation significantly changes the values of the MESP at CPs, especially in the case of systems with an electron-withdrawing substituent. The electron density topography revealed that only the three ring carbon atoms that are syn with respect to the carbonyl groups are bonded to the chromium atom via a (3, −1) bond critical point, suggesting a trigonal-prismatic structure for these complexes.

The synthesis and the molecular structure of hexakis(carbonyl)hexafluoroantimonato(V)tungsten(II) undecafluorodiantimonate(V), [W(CO)6(FSbF5)][Sb2F11

The reaction of tungsten hexacarbonyl, W(CO)6, with antimony(V) fluoride, SbF5, in the conjugate Brønsted-Lewis superacid HF-SbF5 at 40 degrees C produces quantitatively the salt [W(CO)6(FSbF5)][Sb2F11] as the main product. The observed 2e- oxidation without any loss of CO is unprecedented. The cation [W(CO)6(FSbF5)]+ is seven coordinated with a distorted C2v capped trigonal prismatic structure. [W(CO)6(FSbF5)][Sb2F11] crystallizes in the monoclinic space group P21 (No. 4). a = 8.2051(12) A, b = 16.511(3) A, c = 8.1432(2) A, beta = 111.5967(6) degrees, V = 1025.8(2) A3, Z = 2. Number of reflections measured = 9112, unique 4410. Residuals on F, I > 3 sigma (I): R (Rw) = 0.023 (0.023). In the [W(CO)6(FSbF5)]+ cation the FSbF5 group is very tightly coordinated to tungsten with the bridging fluorine nearly equidistant from W and Sb. The details of the molecular structure are compared to those to polymeric [[Mo(CO)4]2(cis-mu-F2SbF4)3]x[Sb2F11]x reported by us very recently.

Syntheses and characterization of mixed acetylacetonate–catecholate complexes of zirconium, [Zr3(acac)4(cat)4(MeOH)2], [Zr(acac)2(DBcat)]2 (H2DBcat = 3,5-di-tert-butylcatechol) and [Zr4(μ4-O)(acac)4(DBcat)3(OMe)4(MeOH)

Reactions of the acetylacetonate complex [Zr(acac)4] with catechols have been investigated. Two crystalline complexes [Zr3(acac)4(cat)4(MeOH)2] 1 and [Zr(acac)2(DBcat)]22 (H2DBcat = 3,5-di-tert-butylcatechol) are isolated from the respective reactions in toluene, and their solid-state structures established by X-ray diffraction studies. Complex 2 consists of an edge-shared capped trigonal prismatic structure involving two symmetry related [Zr(acac)2(DBcat)] units, while 1 is best visualized as a Zr3 complex, possessing two [Zr(acac)2(cat)(MeOH)] units linked to the central [Zr(cat)2] fragment via four bridging catecholate ligands. Their reactivity was investigated; treatment of 2 with a trace amount of water in a solution of methanol and CH2Cl2 gave a Zr4 oxo complex [Zr4(μ4-O)(acac)4(DBcat)3(OMe)4(MeOH)] 3.

A study on some alkaline earth metal cryptates: Synthesis, characterization, crystal structure and solution structure

Three alkaline earth metal cryptates (Barium(II), Strontium(II), Calcium(II)) were synthesized by (2+3) template condensation of tris(3-aminopropyl)amine with 2,6-diformylpyridine and characterized by some physical methods. The solution structure of [BaL] 2+ (L=cryptand) was studied by electrospray mass spectrometry, 1H- 13C NMR and molecular dynamics method based on analysis of 1H NMR NOE spectrum. The crystal structure presents a tricapped trigonal prism structure in which barium(II) is enclosed within the macrobicycle and is nine-coordinated by six imino nitrogen atoms and three pyridyl nitrogen atoms.

Stable Paramagnetic Half-Sandwich Mo(V) and W(V) Polyhydride Complexes. Structural, Spectroscopic, Electrochemical, Theoretical, and Decomposition Mechanism Studies of [Cp*MH3(dppe)]+ (M = Mo, W)

Compounds Cp*MH3(dppe) (M = Mo, 1; W, 2) are oxidized chemically and electrochemically to the corresponding 17-electron cations 1+ and 2+. Analogous oxidations of 1-d3 and 2-d3 provide 1+-d3 and 2+-d3, respectively. Complex 2+ is stable in CH2Cl2, THF, and MeCN at room temperature. A single-crystal X-ray analysis of the PF6- salt of 2+ shows a geometry for the cation which is intermediate between octahedral and trigonal prismatic, which is reproduced by geometry optimization of the [CpWH3(PH2CH2CH2PH2)]+ model at the B3LYP/LANL2DZ level. Identical calculations on the neutral analogue also reproduce the previously reported trigonal prismatic structure for 1. A blue shift in the M−H stretching vibrations upon oxidation for both Mo and W compounds indicates that a M−H bond strengthening accompanies the oxidation process. The DFT calculations (M−H bond lengths, BDE, and stretching frequencies) are in good agreement with the experimental results. Complex 1+ decomposes in solution at room temperature by one or more of three different mechanisms depending on conditions: H2 reductive elimination, solvent-assisted disproportionation, or deprotonation. In THF or CH2Cl2, a reductive elimination of H2 affords the stable paramagnetic monohydride Cp*MoH(dppe)PF6 (3), which adds a molecule of solvent in CH2Cl2, THF, and MeCN. EPR studies show that the CH2Cl2 molecule coordinates in a bidentate mode to afford a 19-electron configuration. A solvent dependence of the decomposition rate [k(CH2Cl2) ≈ 7.8k(THF) at 0 °C] and an inverse isotope effect [kH/kD = 0.50(3) in CH2Cl2 at 0 °C] indicate the nature of 1+ as a classical trihydride and suggest a decomposition mechanism which involves equilibrium conversion to a nonclassical intermediate followed by a rate-determining associative exchange of H2 with a solvent molecule. In MeCN at 20 °C, a solvent-assisted disproportionation (rate = kdisp[1+]2, kdisp = 3.98(9) × 103 s-1 M-1) and a deprotonation by residual unoxidized 1 (rate = kdeprot[1+][1], kdeprot = 2.8(2) × 102 s-1 M-1) take place competitively, as shown by detailed cyclic voltammetric and thin-layer cyclic voltammetric studies. The stoichiometric chemical oxidation of 1 in MeCN leads to a mixture of [Cp*MoH2(dppe)(MeCN)]+ and [Cp*MoH(dppe)(MeCN)2]2+ by the disproportionation mechanism.

Octaaquabromouranium(IV) Tribromide Monohydrate, [UBr(H2O)8]Br3.H2O

The cationic uranium complex, [UBr(H2O)8]3+, presents a tricapped trigonal prismatic structure in the solid state, with U—O distances (at T = 150 K) in the range 2.379 (8)–2.455 (8) Å and a U—Br bond length of 2.885 (2) Å, in addition to an extended network of intermolecular hydrogen-bonding interactions. The title compound is the first octaaqua complex to be structurally characterized for an actinide element.

The Nonoctahedral Structures of d0, d1, and d2 Hexamethyl Complexes

The unusual distorted trigonal-prismatic structure reported recently for hexamethyltungsten is only the tip of the iceberg. None of the d0, d1, and d2 hexamethyl complexes studied here with density functional theory has an octahedral structure. All of them prefer distorted or regular trigonal-prismatic structures (depicted below). The electronic factors which control the structures of these systems are discussed, and spectroscopic predictions are made to facilitate the experimental characterization of the, as yet unknown, d2 systems [Os(CH3)6] and [Ru(CH3)6].

Nanostructure of Metallic Amorphous Alloys —Characterization of Medium-Range Structure and Dynamics by Pulsed Neutron Scattering—

Atoms in metallic amorphous alloys are preferably combined into a configuration of minimizing the local energy in the short-range structure, while the total energy minimum is realized in the periodic long-range structure of crystalline metals. The topological mechanism of crystal-to-amorphous solid state structure transition is demonstrated as a conversion of octahedral structure units into tetrahedral structure units. However, the short-range structure of metallic amorphous alloys often shows similarity to that of their crystalline counterparts. Survival of the local coordination in metallic amorphous alloys is also confirmed by characterizing the dynamic structure. The low-energy excitation often called the Boson peak in inelastic neutron scattering is observed in an energy range of I to 3 meV for metallic amorphous alloys, corresponding to an excess specific heat in addition to the Debye-type harmonic vibration in a temperature range of 10 to 30 K. The medium-range structure of metallic amorphous alloys has a quite unique nature in contrast to the crystalline alloys. In case of metal-metalloid (Pd-Si and Pd-Ge) amorphous alloys it is discussed that the low-energy excitation is correlated to a locally collective atomic motion of hinge-like movement between trigonal prismatic structure units.

Synthesis and characterization of Zr(IV) and Hf(IV) tropocoronand complexes

Zirconium(IV) and hafnium(IV) complexes of the tropocoronands, [MY 2(TC- n, m)] (MZr(IV), Hf(IV); YCl,CH 2Ph,Ch 2SiMe 3) have been prepared and structurally characterized. The compounds exhibit variable stereochemistry depending on the number of methylene units, n and m, in the polymethylene linker chains connecting the two aminotroponiminate rings, although in all cases the cis stereoisomer is formed. With the smallest ligand (TC-3,3), the dialkyl and dichloro complexes display slightly distored trigonal prismatic structures; increasing the size of the macrocyclic ring substantially shifts the geometry at the metal center toward octahedral stereochemistry. Concomitant with the reorientation toward octahedral symmetry, the stability of the complexes decreases and the metal center can attack the CH bonds in the alkyl linker arms of the ligand. Substitution of the latter with xylyl groups eliminates this undesired decomposition pathway while still affording strained complexes. Finally, when the binding cavity of the ligand becomes too large, dinuclear species such as {[Hf(CH 2SiMe 3) 3] 2(TC-5,5)} are obtained.

Structural Trends in Group 4 Metal Tetraaza Macrocycle Complexes. Molecular Structures of (Me4taen)Zr(OtBu)2 and (Me4taen)Hf(NMe2)2

The tetraaza macrocycle complexes (Me4taen)Zr(OtBu)2 (4) and (Me4taen)Hf(NMe2)2 (5) have been prepared and characterized by X-ray crystallography. Both species adopt cis structures with distorted octahedral metal geometries. These structures are similar to that observed earlier for (Me4taen)Zr(NMe2)2 (3) but quite different from the trigonal prismatic structures observed for (Me4taen)ZrCl2 (1) and (Me4taen)Zr(CH2Ph)2 (2). The structures of these d0 (Me4taen)MX2 species are rationalized on electronic grounds. The conformational properties of the Me4taen2- macrocycle and the covalent character of and absence of a strong π-component in the M−X bonds favor trigonal prismatic structures for 1 and 2, whereas M−X π-bonding in 3−5 promotes distortion toward octahedral structures. Compound 4, C22H40N4O2Zr, crystallizes in the triclinic space group P1 with a = 9.6080(8) A, b = 9.7407(8) A, c = 14.5916(12) A, α = 92.714(1)°, β = 107.808(1)° γ = 99.810(1)°, and Z = 2. Compound 5, C18H34N6Hf·0.5(C7H8), crystallizes i...

Enantiomerization of pendant-arm triaza macrocyclic lithium(I) and sodium(I) complex ions.

The first reported enantiomerization of six-coordinate alkali metal complex ions, represented by 1,4,7-tris(2-hydroxyethyl)-1,4,7-triazacyclononanelithium(I) and its sodium(I) analogue in methanol, has been characterized by variable-temperature 13C{1H} n.m.r. spectroscopy. The respective kinetic parameters are: k = (1·11 ± 0·05) × 106 and (2·27 ± 0·09) × 105 s-1 at 298·2 K, ΔH = 27·2 ± 0·3 and 21·7±0·2 kJ mol-1, and ΔS = –36·3 ± 1·3 and –69·6±1·2 J K-1 mol-1. Molecular orbital calculations show that these enantiomers have distorted trigonal prismatic structures consistent with the interpretation of the n.m.r. spectra. For the Li+, Na+, K+, Rb+ and Cs+ complex ions log(K/dm3 mol−1) = 3·13±0·09, 3·52±0·05, 3·23±0·05, 2·78±0·10 and 2·47±0·08, respectively, at 298·2 K and I = 0·05 mol dm-3 (NEt4 ClO4) in methanol, where K is the complex ion stability constant.

Enantiomerization of Pendant-Arm Triaza Macrocyclic Lithium(I) and Sodium(I) Complex Ions

The first reported enantiomerization of six-coordinate alkali metal complex ions, represented by 1,4,7-tris(2-hydroxyethyl)-1,4,7-triazacyclononanelithium(I) and its sodium(I) analogue in methanol, has been characterized by variable-temperature 13C{1H} n.m.r. spectroscopy. The respective kinetic parameters are: k = (1·11 ± 0·05) × 106 and (2·27 ± 0·09) × 105 s-1 at 298·2 K, ΔH = 27·2 ± 0·3 and 21·7±0·2 kJ mol-1, and ΔS = –36·3 ± 1·3 and –69·6±1·2 J K-1 mol-1. Molecular orbital calculations show that these enantiomers have distorted trigonal prismatic structures consistent with the interpretation of the n.m.r. spectra. For the Li+, Na+, K+, Rb+ and Cs+ complex ions log(K/dm3 mol−1) = 3·13±0·09, 3·52±0·05, 3·23±0·05, 2·78±0·10 and 2·47±0·08, respectively, at 298·2 K and I = 0·05 mol dm-3 (NEt4 ClO4) in methanol, where K is the complex ion stability constant.

Crystal and Molecular Structures of Hexamethyltungsten and Hexamethylrhenium

The structure of hexamethyltungsten, W(CH 3 ) 6 , was determined by x-ray single-crystal diffraction at −163°C. The molecule has a strongly distorted trigonal prismatic structure with C 3 ν symmetry. This irregular structure is not a result of intermolecular forces, but rather represents its true molecular structure. A similar structure, which deviates less from the ideal trigonal prismatic structure, was determined for hexamethylrhenium, Re(CH 3 ) 6 . Although these structures violate the simplistic models used to predict the geometry of molecules, they are at least in part explainable by the molecular orbital model.

Surface Structure of Single-Crystal MoS2(0002) and Cs/MoS2(0002) by X-ray Photoelectron Diffraction

The surface structure of a clean MoS2(0002) and an in situ Cs surface-doped MoS2(0002) has been studied using high-resolution X-ray photoemission spectroscopy. The X-ray photoelectron diffraction (XPD) patterns of the Mo 3d5/2 and S 2p core levels from a clean, well-ordered MoS2(0002) show forward focusing intensity maxima along the directions of nearest neighbors in MoS2 in both the polar and azimuthal angle scans. The XPD patterns in the azimuthal angle scan exhibit a pronounced photoelectron intensity maximum at every 60°, reflecting the 6-fold rotational symmetry of the basal plane. In addition, because of the finite electron escape depth and the short-range order of the scattered photoelectrons, the azimuthal scans of both the Mo 3d5/2 and the S 2p core levels further display the 3-fold rotational symmetry of the trigonal prismatic local structure, which MoS2(0002) possesses. The deposition of Cs onto the MoS2(0002) surface at room temperature did not introduce any significant changes either in the l...

Ordering and manipulation of MoS2 platelets on differently charged micas by atomic force microscopy

Atomic force microscopy (AFM) has been used to investigate how dispersed MoS2 platelets (from colloidal suspensions) deposit onto three natural (Muscovite) mica (001) surfaces. Different platelet arrangements are observed, which are attributed to defects and charging effects of the mica (as well as the concentration of starting colloid). The provenance of the mica influences the self-organisation of the platelets into long tape-like assemblies (South Dakota mica) or individual flakes (Alps mica). Atomic scale imaging of the tapes reveals a distorted octahedral (Oh)-based local structure, different from the trigonal prismatic structure found in the 2H-polytype of annealed MoS2, in agreement with previous structural results on water-dispersed MoS2 platelets. The buckling and susceptibility to stripping of the tapes is ascribed to the presence of a water layer between the substrate mica and MoS2, and after stripping by the tip, the platelets ultimately form small clusters. The ordering of these clusters depends not only on the defect and charge structure of the mica, but also on complex hydration reactions between the H2O layer associated with the MoS2 and K + ions of the mica. Relatively symmetrical squares may be lifted out of the tapes, supporting the presence of weak bonding between tape and mica. Conversely, Alps and Bihar micas give rise to separate platelets, which are resistent to tip manipulation, which is attributed to hydrophobic interaction between MoS2 platelets and the mica surface.

The Structure of Hexamethyltungsten, W(CH3)6: Distorted Trigonal Prismatic with C3 Symmetry

Ab initio and density functional calculations show that the equilibrium structure of hexamethyltungsten is a distorted trigonal prism of C3 symmetry (with local C3v symmetry for the WC6 skeleton). A regular prismatic D3 structure (with D3h skeleton) is found to be ca. 20 kJ mol-1 higher in energy at correlated levels of theory. It is a transition state connecting two C3 minima. These results extend a recent gas-phase electron diffraction study which favored a regular prismatic structure but could not rule out a distortion to C3v. The failure of a previous theoretical study to locate the distorted minimum is due to the neglect of electron correlation and to some other restrictions during the structure optimizations. Correlation is important, e.g. for the description of hyperconjugative “agostic” C−H → W interactions which are found to be pronounced in W(CH3)6. Structures optimized with gradient-corrected or hybrid density functionals, or at the MP2 level, describe these interactions well. The observed single-line 13C and 1H NMR spectra are explained by dynamic motions due to the low D3 inversion and methyl rotation barriers. 13C chemical shifts calculated using density functional theory differ by ca. 18 ppm between the two nonequivalent sets of methyl groups in the distorted trigonal prismatic structure. Low-temperature NMR experiments could be useful to confirm this value and thus the distortion. Harmonic vibrational frequency analyses are consistent with experimental results and have been used to characterize stationary points on the potential energy surface. Differences between the structural preferences of W(CH3)6 and WH6 are investigated via detailed bonding analyses.

Electronic absorption spectra, optical line strengths, and crystal-field energy-level structure of Nd3+ in hexagonal [Nd(H2O)9](CF3SO3)3

Abstract Optical absorption measurements are used to locate and assign 79 crystal-field (Stark) levels split out of the 4f 3 electronic configuration of Nd 3+ in single crystals of neodymium nonahydrate tris(trifluoromethanesulfonate), [Nd(H 2 O) 9 ](CF 3 SO 3 ) 3 , denoted hereafter as NdTRF. Line strengths are determined for 56 transitions between Stark levels. Single crystals of NdTRF have hexagonal, P6 3 /m(C 6h 2 ) space-group symmetry, and each Nd 3+ ion is coordinated to nine water molecules in a slightly distorted tri-capped trigonal prism structure of C 3h symmetry. The energy-level structure of 4f 3 (Nd 3+ ) in NdTRF is analyzed in terms of a model Hamiltonian that includes consideration of both one-electron crystal-field and two-electron correlation-crystal-field interactions, the latter interactions making important contributions to the energy-level structures of several 4f 3 [ SL ] J multiplet manifolds. Line strength data obtaibned for NdTRF is analyzed in terms of a general 4f → 4f transition intensity model in which the details of the 4f-electron/crystal-field/electric-dipolar radiation-field interactions are represented in parametric form. The derived parameters provide information about structural properties and interaction mechanisms that contribute to 4f → 4f transition intensities. The general crystal-field and intensity parameters are further analyzed in terms of a set of ‘intrinsic’ parameters that are related to contributions from individual Nd 3+ OH 2 pairwise interactions.

A new simple criterion for distinguishing the types of structures in eight-coordinate complexes: The pattern of bond angles

Square antiprismatic, triangular dodecahedral and (rectangularly) bicapped trigonal prismatic (or hendecahedral) eight-coordinate structures are shown to manifest distinct patterns of LML′ bond angles, which are validated by many straightforward examples from the literature. Our criterion was used to re-classify seven published structures: in one of these, a completely new type of eight-coordination, christened the C s decahedron, has been discovered. The same type of analysis has been applied to the rarer types of eight-coordination: the cube, the hexagonal bipyramid, the axially bicapped trigonal antiprism and the trigonally bicapped octahedron, and the axially bicapped trigonal prism: useful insights have thus been obtained concerning several published structures.

Crystal structures of transition metal carbides

The factors contributing to the formation of carbides of complex crystal structure by transition metals such as Cr are discussed in terms of the cells of metal (M) atoms around central carbon (C) atoms. The octahedral cell is favoured by M–M repulsion, which occurs particularly in high C compounds of the group IVA and VA metals. The main alternative, the trigonal prismatic cell, isfavoured by a better angular separation of M–M and C–M bonds, which is important in WC; and also favoured by stronger M–M attraction, as occurs in low C compounds of group VIA and later metals, and by smaller M atomic sizes, as in the first long period metals. Low C content provides an opportunity, in trigonal prismatic structures, for the formation of complex structures which have the advantage of providing a C atom with more than six M neighbours. This advantage also appears strongly in the M23C6 structure where the cell is a square antiprism with eight M neighbours around its central C atom.MST/3142