A single-crystal neutron diffraction study at 20 K has revealed accurate hydride ligand positions in the first stable hydride derivative of a divalent group-14 metal, [2,6-Trip(2)C(6)H(3)Sn(mu-H)](2)-4C(6)H(6), Trip = 2,4,6-tri-isopropylphenyl [B.E. Eichler, P.P. Power, J. Am. Chem. Soc. 122 (2000) 8785]. In the solid state this dimeric complex assumes a trans C-2h geometry with two bridging hydrides (Sn-H 1.943(7) angstrom, angle C-Sn-H 92.4(2)degrees, angle Sn-H-Sn' 106.9(3)degrees, angle H-Sn-H' 73.1(3)degrees). The bulky Trip ligand serves to stabilize the Sn-H bonds. The tin atoms carry lone pairs, and, as determined previously by X-ray diffraction and reported by Eichler and Power, the tin coordination accordingly is pyramidal as evidenced by the sum of the three bond angles around tin of 257 degrees. To our knowledge this is the first neutron diffraction study of a tin hydride complex to be reported. The neutron diffraction measurements were carried out using the time-of-flight Laue SCD instrument at the Argonne Intense Pulsed Neutron Source.

Transition-metal _σ-complexes [2] are coordination compounds in which two electrons in an X-H _σ-bond form a dative bond with a transition metal as illustrated in Figure 1. This bond can be further stabilized by π-backbonding from the metal to the X-H σ* antibonding orbital. Transition-metal σ-complexes are typically reactive intermediates that precede oxidative addition of substrates having an X-H bond. σ-complexes are, therefore, intermediates in catalytic hydrogenation (X = H), activation and functionalization of hydrocarbons (X = C), hydro-silylation (X = Si), and hydroboration (X = B) reactions.

Crystal structures of all five crystalline methyl D-pentofuranosides, methyl alpha-D-arabinofuranoside (1), methyl beta-D-arabinofuranoside (2), methyl alpha-D-lyxofuranoside (3), methyl beta-D-ribofuranoside (4) and methyl alpha-D-xylofuranoside (5) have been determined by means of cryogenic X-ray and neutron crystallography. The neutron diffraction experiments provide accurate, unbiased H-atom positions which are especially important because of the critical role of hydrogen bonding in these systems. This paper summarizes the geometrical and conformational parameters of the structures of all five crystalline methyl pentofuranosides, several of them reported here for the first time. The methyl pentofuranoside structures are compared with the structures of the five crystalline methyl hexopyranosides for which accurate X-ray and neutron structures have been determined. Unlike the methyl hexopyranosides, which crystallize exclusively in the C(1) chair conformation, the five crystalline methyl pentofuranosides represent a very wide range of ring conformations.

The reaction of a mixture of 1 equiv of PhPH(2) and 2 equiv of PhNHSiMe(2)CH(2)Cl with 4 equiv of Bu(n)Li followed by the addition of THF generates the lithiated ligand precursor [NPN]Li(2).(THF)(2) (where [NPN] = PhP(CH(2)SiMe(2)NPh)(2)). The reaction of [NPN]Li(2).(THF)(2) with TaMe(3)Cl(2) produces [NPN]TaMe(3), which reacts under H(2) to yield the diamagnetic dinuclear Ta(IV) tetrahydride ([NPN]Ta)(2)(mu-H)(4). This hydride reacts with N(2) with the loss of H(2) to produce ([NPN]Ta(mu-H))(2)(mu-eta(1):eta(2)-N(2)), which was characterized both in solution and in the solid state, and contains strongly activated N(2) bound in the unprecedented side-on end-on dinuclear bonding mode. A density functional theory calculation on the model complex [(H(3)P)(H(2)N)(2)Ta(mu-H)](2)(mu-eta(1):eta(2)-N(2)) provides insight into the molecular orbital interactions involved in the side-on end-on bonding mode of dinitrogen. The reaction of ([NPN]Ta(mu-H))(2)(mu-eta(1):eta(2)-N(2)) with propene generates the end-on bound dinitrogen complex ([NPN]Ta(CH(2)CH(2)CH(3)))(2)(mu-eta(1):eta(1)-N(2)), and the reaction of [NPN]Li(2).(THF)(2) with NbCl(3)(DME) generates the end-on bound dinitrogen complex ([NPN]NbCl)(2)(mu-eta(1):eta(1)-N(2)). These two end-on bound dinitrogen complexes provide evidence that the bridging hydride ligands are responsible for the unusual bonding mode of dinitrogen in ([NPN]Ta(mu-H))(2)(mu-eta(1):eta(2)-N(2)). The dinitrogen moiety in the side-on end-on mode is amenable to functionalization; the reaction of ([NPN]Ta(mu-H))(2)(mu-eta(1):eta(2)-N(2)) with PhCH(2)Br results in C-N bond formation to yield [NPN]Ta(mu-eta(1):eta(2)-N(2)CH(2)Ph)(mu-H)(2)TaBr[NPN]. Nitrogen-15 NMR spectral data are provided for all the tantalum-dinitrogen complexes and derivatives described.

The crystal structure of N-methylacetamide (C3H7NO), M-r = 73.095, has been determined from single-crystal neutron diffraction data at two temperatures, 250 and 276 K, above and below the previously reported phase transition located at 274 K in this work. Crystal data: 250 K [276 K]: space group Pn2la [Pn2(1)m], a = 9.671(2) [4.878(1)] Angstrom, b = 6.613(6) [6.567(1)] Angstrom, c = 7.218(1) [7.332(2)] Angstrom; V = 465.7(6) [234.9(5)] Angstrom (3); D-n = 1.043 [1.034] g.cm(-3), R(F-2) = 0.168 [0.134], wR(F-2) = 0.062 [0.051], S = 1.18 [1.11]. This new investigation of the structure of N-methylacetamide was undertaken in order to assess a recent suggestion based on inelastic neutron scattering spectroscopy that transfer of the amide proton along the peptide hydrogen bond may be responsible for the vibrational anomalies. While we found no evidence for proton transfer along the N-H...O hydrogen bond (d(NH) = 1.025(17) Angstrom, and d(H...O) 1.856(14) Angstrom for at T = 250 K) at either temperature evidence for some molecular disorder is present in accord with our previous C-13 NMR studies. In addition, we find short intramolecular contacts between the amide hydrogen atom and those on both neighboring methyls, which may well affect the vibrational properties of the respective molecular groups.

A new type of hydrogen-bonding interaction has recently been recognized that utilizes the σ-bonding electron pair E-H as the acceptor where E stands for a main-group element such as boron, or for a transition metal.Neutron diffraction studies and quantummechanical calculations have indicated that these interactions are significantly attractive, with energies comparable to those of conventional hydrogen bonds that utilize lone-pair electrons for acceptors.The H ≡ H contacts involved may be as much as 0.5 Å shorter than the conventional van der Waals distance of 2.4 Å, so that the term 'dihydrogen bond' has been used to denote this type of interaction.

The X-ray crystal structure of [{(triphos)H(3-x)Ir}(mu-H)(x) {Au(PR3)}][PF6] (triphos = CH3C(CH2PPh2)(3), x = 2) shows that the gold atom builds two almost equal Ir-H-Au bridges with the he'IrH3(triphos)' building block. The Ir-H-Au bridging parameters are typical of three-center-two-electron interactions. The X-ray crystal structure of [{(triphos)H(3-y)Ir}(mu-H)(y) {Au(PR3)}(2)][PF6](2) shows that each gold atom builds two Ir(mu(2)-H)Au bridges with the three hydrides of the 'IrH3(triphos)' building block; one Ir(mu(3)-H)Au-2 bridge is also present (y = 3). The relative positions of the Ir, H, Au and P atoms show that typical three-center-two-electron interactions predominate in this compound, in which there is no direct Au-Au bonding. The neutron diffraction structure of [{(triphos)Ir}(mu-H)(2){Au(PPh3)}(3)][PF6](2) confirms the earlier hypothesis that only two of the three Ir-Au edges are associated with a hydride with formation of Ir(mu(2)-H)Au bridges. The presence or absence of the latter ligand changes the Ir-Au distance only marginally, in contrast to the general trend in hydride clusters. It is shown that the formation of a 'classical' cluster in this set of compounds requires a quadrimetallic unit and the two additional electrons generated by loss of a proton from an Ir-fl bond in the trication [{(triphos)Ir (mu(2)-H)(3){Au(PR3)}(3)}(3+). (C) 2000 Elsevier Science S.A. All rights reserved.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTA Cyclometalated Resting State for a Reactive Molybdenum Amide: Favorable Consequences of β-Hydrogen Elimination Including Reductive Cleavage, Coupling, and ComplexationYi-Chou Tsai, Marc J. A. Johnson, Daniel J. Mindiola, Christopher C. Cummins, Wim T. Klooster, and Thomas F. KoetzleView Author Information Department of Chemistry, Room 2-227, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307 Chemistry Department, Brookhaven National Laboratory Upton, New York 11973-5000 Cite this: J. Am. Chem. Soc. 1999, 121, 44, 10426–10427Publication Date (Web):October 21, 1999Publication History Received26 May 1999Published online21 October 1999Published inissue 1 November 1999https://pubs.acs.org/doi/10.1021/ja9917464https://doi.org/10.1021/ja9917464rapid-communicationACS PublicationsCopyright © 1999 American Chemical SocietyRequest reuse permissionsArticle Views1610Altmetric-Citations124LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (2)»Supporting Information Supporting Information SUBJECTS:Adducts,Anions,Chemical structure,Ligands,Nitrogen compounds Get e-Alerts

Boraneamines tend to have close N-Hdelta+...delta-H-B contacts as a result of the intermolecular interaction of the NH proton with the BH bond by a novel type of hydrogen bond (the dihydrogen bond). A CSD structural search provides characteristic metric data for the interaction: the H ... H distance is in the range 1.7-2.2 Angstrom, and the N-H ... H group tends to be linear while B-H ... H tends to be bent. The reported X-ray structure of BH3NH3 seemed to provide a singular exception in having bent N-H ... H and linear B-H ... H. Our neutron diffraction structure of BH3NH3 now shows that the B and N atoms must be reversed from the assignment previously published. With the correct assignment we find the expected bent B-H ... H and linear N-H ... H arrangement in the closest intermolecular N-H ... H-B interaction (d(HH) = 2.02 Angstrom).

A single-crystal neutron diffraction analysis along with density functional calculations and incoherent inelastic neutron scattering studies has conclusively shown that the dihydrogen adduct of [P2N2]Zr(μ-η2-N2)Zr[P2N2] (1) (where P2N2 = PhP(CH2SiMe2NSiMe2CH2)2PPh) is [P2N2]Zr(μ-η2-N2H)(μ-H)Zr[P2N2] (2), the complex with a bridging hydride and a N−N−H moiety, and not the dihydrogen complex [P2N2]Zr(μ-η2-N2)(μ-η2-H2)Zr[P2N2] (3), as was proposed on the basis of X-ray crystallographic data. In addition, DFT calculations show that the reaction of 1 with both H2 and SiH4 is exothermic while an endothermic reaction is found for the reaction of 1 with CH4.

Tris(triisopropylsilyl)silane (iPr(3)Si)(3)SiH has been synthesized and studied by X-ray and neutron diffraction. It possesses an unusual structure in which the four silicon atoms are nearly coplanar, angle Si-Si-Si = 118.41(5)degrees. The Si-H distance is found to have a normal value of 1.506(2) Angstrom. Thermal and room-temperature photochemical decomposition of (iPr(3)-Si)(3)SiH leads to the elimination of iPr(3)SiH and the generation of bis(triisopropylsilyl)silylene, [(iPr(3)Si)(2)Si:]. Reactions of(iPr(3)Si)(2)Si: include precedented insertions into H-Si bonds and addition to the pi-bonds of olefins, alkynes, and dienes. Despite theoretical predictions of a triplet ground state for [(iPr(3)Si)(2)Si:], stereospecific addition to cis- and trans-2-butene was observed.

The molecular structures of the eight-coordinate tungsten hydride complexes W(PMe3)(4)H2X2 (X = F, Cl, Br, I) and W(PMe3)(4)H2F(FHF) have been determined by single-crystal X-ray diffraction; W(PMe3)(4)H2Cl2 and W(PMe3)(4)H2F(FHF) have also been analyzed by single-crystal neutron diffraction, thereby accurately locating the positions of the hydride ligands. The structures of all of these complexes are similar and are based on a trigonal dodecahedron, with a distorted tetrahedral array of PMe3 ligands in which two of the PMe3 ligands are displaced over the halide substituents. However, the initial structures derived for both W(PMe3)(4)H2Cl2 and W(PMe3)(4)H2F(FHF) did nos exhibit the aforementioned geometry, but were based on an arrangement in which the two transoid-PMe3 ligands are displaced toward the two cis-PMe3 groups, rather than tilted toward the chloride ligands. Interestingly, the unexpected structures for W(PMe3)(4)H2Cl2 and W(PMe3)(4)H2F(FHF) were discovered to be the result of an artifact due to the presence of a heavy atom in a polar space group, which allowed the X-ray structure solutions to refine into most deceptive false minima. Specifically, for the structures corresponding to the false minima, the transoid-PMe3 ligands were incorrectly located in positions that are related to their true locations by reflection perpendicular to the polar axis. In effect, the incorrect molecular structures are a composite of the two possible true polar configurations which are related by a reflection perpendicular to the polar axis, i.e. a "partial polar ambiguity". Of most importance, the solutions corresponding to the false minima are characterized by low R values and well-behaved displacement parameters, so that it is not apparent that the derived structures are incorrect. Thus, for space groups with a polar axis, it is necessary to establish that all of the atoms in the asymmetric unit belong to a single true polar configuration.

The complexes [(PEt3)2(Ar)Pt(μ-H)PtH(PEt3)2][BPh4] (Ar=Ph,2,4-Me2C6H3, 2,4,6-Me3C6H22) were prepared and characterized. Multinuclear, multidimensional NMR studies of these complexes show that, in solution, (i) they exist in rapidly intercoverting conformers which retainn the bent Pt(μ-H)PtH moieties found in the solid state, (ii) the coordination at each platinum atom is square planar, (iii) the two coordination planes are perpendicular to each other and (iv) the aryl group is perpendicular to the coordination plane of the platinum atom to which it is bonded. The complex [PEt3)2Pt(μ-H)PtH(PEt3)2][BPh4] does not react with C2H4 and CH2:CH·CO2Me. At −60°C or above, [(PEt3)2(Ph)Pt(μ-H)PtH(PEt3)2)][BPh4] reacts with CO giving the platinum(II) cations trans-[PtX(CO)(PEt3)2](Ph)∗ (X=H and Ph), and [PtH(PEt3)3]+ and the platinum(0) carbonyl cluster [Pt4(μ-CO)3)4]. The cluster structure of I(PEt3)2(Ph)·Pt(μ-H)PtH(PEt3)2][BPh4, obtained by neutron diffraction, shows that the Pt-H-Pt bond angle in this cation is 125(1)° indicating that the interaction between these three atoms is weak.

It has finally been possible to test the strategy presented in 1991 for the design of a ground-state triplet silylene: attaching to the divalent silicon atom groups sufficiently bulky to open the R-Si-R angle to a value such that the triplet silylene is lower in energy than the singlet, and lowering the electronegativity of the substituents to minimize the {open_quotes}crossover angle{close_quotes} above which the ground state is a triplet. Tris(triisopropylsilyl)silane has been synthesized as a precursor to (I-Pr{sub 3}Si){sub 2}Si:, and both thermal and photochemical dissociation to the silylene has been successful, and the multiplicity of the ground state will be reported. X-ray and neutron diffraction studies revealed novel features of (I-Pr{sub 3}Si){sub 3}SiH such as the near coplanarity of the four silicon atoms and the length of its Si-H bond.

The octahedral hexazirconium cluster compound [PPh4](3)[Zr6Cl18H5] has been structurally characterized by both neutron and X-ray single-crystal diffraction studies. The compound [PPh4](3)[Zr6Cl18H5]. 3CH(2)Cl(2) crystallizes in the triclinic space group PT with unit cell parameters of a = 15.993(3), b = 22.237(3), and c = 14.670(4) Angstrom, alpha = 95.31(1), beta = 112.07(2), and gamma = 82.06(2)degrees, V = 4784(2) Angstrom(3), and Z = 2 at ambient temperature and a = 15.780(6), b = 21.96(3), and c 14.521(7) Angstrom, alpha = 94.96(8), beta = 111.59(4), and gamma = 81.72(5)degrees, V = 4627(11) Angstrom(3), and Z = 2 at T = 15 K. The hydrogen atoms in the cluster anion, [Zr6Cl18H5](3-), were found to be distributed at the centers of the eight triangular faces of the Zr-6 octahedron from neutron diffraction data. The occupancy parameters of the sites range from 0.32 to 0.92 with a total of 5.3(1) hydrogen atoms per cluster, close to the value from H-1 NMR measurement (5.0). The average Zr-H distance is 1.96(4) Angstrom. A variable temperature H-1 NMR study indicated that the cluster hydrogen atoms undergo rapid movement at room temperature. One of the five hydrogen atoms in the cluster [Zr6Cl18H5](3-) was readily removed as a proton with primary linear amines with formation of the corresponding ammonium cations, while the cluster anion, [Zr6Cl18H5](3-), was thus converted into a new cluster anion, [Zr6Cl18H4](4-). The feasibility of such a deprotonation reactions is controlled by the size of both the Lewis base and the cavity available on the Zr-3 triangular faces of the Zr-6 clusters, and also by the basicity of the deprotonating reagents. Two products, [PPh4](4)[Zr6Cl18H4]. 4CH(2)Cl(2) and [H3NEt](4)[Zr6Cl18H4]. 4MeCN from the deprotonation reactions were characterized by X-ray crystallography.

The ability of organisms to exercise control over crystal growth is wonderfully exemplified by skeleton formation in echinoderms. A sea urchin spine is a unique composite of a single crystal of calcite and glycoproteins intercalated inside the crystal during its growth. Here we performed a detailed morphological and high-resolution synchrotron X-ray diffraction study of the textures of synthetic and biogenic calcite crystals. We show that the intracrystalline macromolecules from sea urchin spines, when allowed to interact with growing calcite crystals in vitro, selectively reduce the coherence lengths and degrees of alignment of the perfect domains in specific crystallographic directions. These directions also correspond to the newly-developed stable faces. In contrast, the defect distribution of young sea urchin spines composed entirely of spongy stereomic structure is much more isotropic. In mature spines containing secondarily filled-in wedges of calcite, the degree of anisotropy is intermediate between that of the synthetic crystals and the young spines. The macromolecules extracted from young and mature spines are, however, very similar. These observations demonstrate the inherent capability of occluded matrix macromolecules to finely differentiate between crystal planes by stereochemical recognition processes. They also show that in biologically-produced calcite crystals this process can be overridden to produce a more isotropic material.