Metal-hydrogen Distances Research Articles

Neutron Powder Diffraction, x-ray absorption and Mössbauer spectroscopy on Mg2FeH6

We have studied the crystallographic structure, the oxidation states and the electronic structure of the ternary metal hydride Mg2FeH6 prepared by high-pressure and high-temperature reaction from the simple hydrides. This procedure provides a well-crystallized sample essential to perform Neutron Powder Diffraction (NPD) experiments. By NPD we accessed to subtle crystallographic details of Mg2FeH6, such as the hydrogen occupancy and the metal-hydrogen distances and angles; a full hydrogen composition with six hydrogen atoms per unit formula is determined. No magnetic structure was observed by NPD at 2 K. The oxidation states of the atoms inside the material and the electronic structure of Mg2FeH6 have been evaluated by x-ray absorption near-edge spectroscopy (XANES). Unexpectedly, the Fe K-edge spectrum shows a zero oxidation state of Fe atoms in agreement with Mössbauer data. Our results suggest a metallic nature of the hydride in contrast with the ionic character previously reported for this compound.

Hydrogen Vibrational Excitation Spectra of CaF<SUB>2</SUB>-Type Metal Hydrides Synthesized from Ti-Based BCC Solid Solution Alloys

Hydrogen vibrational excitation was studied for CaF2-type metal hydrides synthesized from Ti-based BCC solid solution alloys using inelastic incoherent neutron scattering (IINS). Ti1:0V1:1Mn0:9H4:5 and Ti0:7V1:2Cr1:1H4:8 showed IINS spectra similar to that reported for TiH2. The first three peaks were isolated but the higher excitation peaks were not clear. Analysis of the spectra using curve-fitting with Gauss functions revealed that the hydrogen vibration of Ti1:0V1:1Mn0:9H4:5 is harmonic but that of the Ti0:7V1:2Cr1:1H4:8 is deviated from harmonic, which reflects a trumpet-type potential. The relation between metal-hydrogen distance and vibrational excitation energy for the above two hydrides and Ti1:1Cr1:4Mo0:3H 5 was compared with a series of CaF2-type binary metal hydrides. All the hydrides of the Ti-based alloys had lower vibrational excitation energies than the binary metal hydrides for the corresponding metal-hydrogen distances. [doi:10.2320/matertrans.MA201014]

Iron–nickel mixed metal nitrido clusters: Synthesis and solid state structure of the anions [HFe5NiN(CO)14]2− and [HFe4Ni2N(CO)13]2−

The two clusters [HFe5NiN(CO)14]2− (1) and [HFe4Ni2N(CO)13]2− (2) were obtained by reaction of [Fe4N(CO)12]− and [Ni6(CO)12]2− in refluxing MeCN and EtCN, respectively, along with other Fe–Ni mixed metal clusters. Their solid state structures were determined on the [PPh4]+ salts, and both have an octahedral metal cage, containing an interstitial nitrogen atom. The two Ni atoms in 2 are cis, with a Ni–Ni separation of 2.724(1)Å. The two anions have different stereochemistry of the carbonyl ligands: in 1, five CO’s are semi-bridging, and the remaining nine are terminal; in 2 there are three asymmetric bridging and ten terminal ligands (two for each iron and one for each nickel). The hydride ligands were located in the final difference maps, both bridging a Ni–Fe edge of the clusters but, thanks to the better quality of the diffraction data, the metal-hydrogen distances were refined only in 2. In this cluster, the Fe–H and Ni–H bond lengths are 1.77(2) and 1.79(2)Å, respectively.

Hydrogen bonding in the LaNi3BH3 hydride

The electronic structure of the intermetallic LaNi3B as well as the novel hydride LaNi3BH3 have been theoretically investigated by means of quantum chemistry methods. We employed a mixed approach to investigate the electronic structure of these compounds: state-of-the-art energy band calculations and molecular cluster computations. We computed the energy bands and the total and partial density of states using both the linear-augmented plane waves and projector-augmented wave methods. In addition the electronic structure of three representative clusters of the local environment of Ni atoms was investigated by quantum chemistry ab initio molecular calculations. In this report, we discuss the chemical bonding and we investigated the H site occupancy energies and correlate this estimate with the occupancy fraction and metal-hydrogen distances experimentally observed.

The electronic properties of intermetallic hydrides with the K2PtCl6 structure

The authors present results from electronic structure calculations of the intermetallic hydrides M2M'H6 (M=Mg, Ca, Sr, M'= Fe, Ru, Os). Most of these hydrides have been recently synthesized and characterized, all showing the K2PtCl6 cubic structure. They have determined their energy bands and densities of electronic states by means of the non self-consistent augmented-plane-wave method. The results show that the nine hydrides are semiconductors with relatively large energy gaps. These isostructural and isoelectronic families of hydrides have been studied in order to establish correlations between the ordering of the electronic states and the energy gaps in terms of differences in metal-hydrogen distances and atomic energy level. They observe that the splitting between the bonding and antibonding M' d(eg) orbitals is responsible for the increase of the energy gap as the atomic number of M'increases. Nevertheless, the presence of d states from the divalent metals Ca and Sr modifies the nature of the unoccupied conduction states above the energy gaps. In general, a reduction of the energy gap is obtained in the Ca- and Sr-derived hydrides.

The first examples of intermolecular weak (agostic) γ-methyl-metal interactions in organolanthanide complexes: The synthesis and X-ray structures of [{Yb(η-CP″) 2}∞.] and [{Eu(η-CP″) 2}∞] [CP″ C 5H 3(SiMe 3) 2-1,3

The lanthanocene(II) complexes [{Yb(η-CP ) ́ 2}∞] ( 1) and [{Eu)η-Cp) 2}∞] ( 2) [Cp″ C 5H 3(SiMe 3) 2-1,31 were prepared by the desolvation via sublimation of [Yb(η-Cp″) 2(OEt 2)] ( 3) and [Eu(η-CP″) 2(THF)] ( 4), respectively. The single crystal X-ray structures of 1 and 2 reveal that each complex has unique intermolecular agostic-like interactions. 1 adopts a bent metallocene conformation, Cent(Cp1)YbCent(Cp2) 138.0° and the mean YbC bond length 2.662 Å, and a close contact between the metal of one YbCp″ unit and a methyl group of a neighbouring such unit, Yb ⋯ C(9)″ 2.872(7) Å. For 2, the low temperature X-ray study shows a similar short intermolecular contact Eu(2′) ⋯ C(62) 3.091(6) Å [ cf. the mean Eu(2)C bond length of 2.804 Å], with the closest intermolecular metal-hydrogen distance Eu(2′) ⋯ H(62A) being 2.70 Å. Complex 2 also exhibits an unprecedented conformation with a cyclopentadienyl ring bridging η 5, η 3 between two non-equivalent europium atoms.

Embedded cluster calculations for hydrogen chemisorption on the (111) surfaces of fcc Co, Ni, Cu, Rh, Pd and Ag

Abstract We present a muffin tin based calculation on (TM) 3 H, (TM) 7 H and (TM) 19 H clusters embedded at the surface of an effective jellium-like medium whose potential is treated in scattering length approximation. We consider the changes occurring when the d-like perturbation of the TM muffin tins is switched on. The broad chemisorption-induced resonance seen for H on the effective jellium surface is narrowed and shifted down in energy. Furthermore the occupation of this resonance is increased from about 1.1 electrons to about 1.4 (on 3d metals) or 1.8 (on 4d metals), due to d-like states dropping down from the d band to form a relatively welldefined “bonding state”. An antibonding state containing about 0.4 electrons is formed at the top of the d band. The results are compared with other calculations and with photoemission data. Implications for the metal-hydrogen distance and (for Ni) the demagnetizing effect of hydrogen chemisorption are discussed. We use the change in total single particle energy when the d-like perturbation is switched on to estimate trends in chemisorption energy along the 3d and 4d series. In the 3d case experimental data is available on the difference in chemisorption energy between Ni and Cu which is in reasonable agreement with our estimate.

Embedded cluster calculations for hydrogen chemisorption on the (111) surfaces of fcc Co, Ni, Cu, Rh, Pd and Ag

We present a muffin tin based calculation on (TM) 3H, (TM) 7H and (TM) 19H clusters embedded at the surface of an effective jellium-like medium whose potential is treated in scattering length approximation. We consider the changes occurring when the d-like perturbation of the TM muffin tins is switched on. The broad chemisorption-induced resonance seen for H on the effective jellium surface is narrowed and shifted down in energy. Furthermore the occupation of this resonance is increased from about 1.1 electrons to about 1.4 (on 3d metals) or 1.8 (on 4d metals), due to d-like states dropping down from the d band to form a relatively welldefined “bonding state”. An antibonding state containing about 0.4 electrons is formed at the top of the d band. The results are compared with other calculations and with photoemission data. Implications for the metal-hydrogen distance and (for Ni) the demagnetizing effect of hydrogen chemisorption are discussed. We use the change in total single particle energy when the d-like perturbation is switched on to estimate trends in chemisorption energy along the 3d and 4d series. In the 3d case experimental data is available on the difference in chemisorption energy between Ni and Cu which is in reasonable agreement with our estimate.

Structures and stabilities of the group IIIa dihydrides

The CaF 2-type dihydrides of the group IIIa elements represent a relatively large family of similar metallic hydrides for which relationships between interatomic distances and stabilities can be derived. Available data for the thermodynamic properties of formation of these dihydrides were assembled and the enthalpies and free energies of formation are compared with the metal-hydrogen distances in the hydrides. The data show that for these hydrides there is a maximum in the stability-distance curve; the most stable dihydrides are HoH 2 and DyH 2 with metal-hydrogen distances of 2.237Å and 2.252 Å respectively. A modified Born-Mayer ionic model is applied to this family of dihydrides. In addition to the usual Coulomb (attraction) and overlap (repulsion) terms, an additional attraction term is introduced to take into account residual metallic cohesion in the hydride lattice. The results obtained from this model are compared with the thermochemical data using the BornHaber cycle. In making this comparison a parameter β (which has units of length) is introduced which accounts for the influence of the 4f electronic structure on the repulsive potential for the lanthanide dihydrides. It is shown that the relation between stability and the sum of the metallic radius and the tetrahedral hole radius (in the metal) is similar to that between stability and the actual metal-hydrogen distance in these dihydrides. This similarity is used to rationalize observed correlations between stability and tetrahedral hole sizes in intermetallic compound hydrides.

Localized Modes in Tantalum Hydrides Studied by Neutron Inelastic Scattering

The local vibration spectra of hydrogen atoms dissolved in tantalum have been studied by the energy-loss scattering of pulsed neutrons. The observed time-of-flight spectra show split peaks of the local modes near 120 and 175 meV, which are almost invariant with changing concentration (H/Ta=0.1, 0.5 and 0.7) and configuration of hydrogen at 20° and 85°C. From comparison with previous neutron results on niobium and vanadium hydrides, it is shown that the energies of the split local modes are nearly independent of the metal-hydrogen distance, and the relative intensity and the FWHM of the high energy mode increase in the disordered phase at 85°C in comparison with the ordered phase at 20°C.

Molecular structure determination by n.m.r. spectroscopy

Abstract

Studies of Hydrogen Vibrations in Transition Metal Hydrides by Thermal Neutron Transmissions

The total scattering cross sections of hydrogen atoms for thermal neutrons were measured as a function of the neutron energy for TiH 1.3 , TiH 1.6 , TiH 1.9 , TiH 2.0 , VH 0.25 , NbH 0.85 and TaH 0.7 at room temperature. The results clearly show the effect of the inelastic scattering of neutrons, due to excitation of the hydrogen vibrations, and are utilized to determine the energy levels of the optical vibrations in the hydride lattices. They are 0.135 eV for TiH 1.3-2.0 , 0.165 eV for VH 0.25 , 0.150 eV for NbH 0.85 , and 0.145 eV for TaH 0.7 . It was found that there is an approximately linear relation between the energies of the optical phonons and the metal-hydrogen distances reported on various hydrides, except for palladium hydride.

The crystal structure of thorium and zirconium dihydrides by X-ray and neutron diffraction

Thorium forms a tetragonal lower hydride of composition ThH{sub 2}. The hydrides ThH{sub 2}, ThD{sub 2} and ZrD{sub 2} have been studied by neutron diffraction in order that hydrogen positions could be determined. The hydrides are isomorphous, and have a deformed fluorite structure. Metal-hydrogen distances in thorium hydride are unusually large, as in UH{sub 3}. Thorium and zirconium scattering amplitudes and a revised scattering amplitude for deuterium are reported.