Metal-hydrogen Interactions Research Articles

One electron is enough: A chemical bonding approach on the hydrogen induced structural transformation in MPd3 (M = Mg, In, Tl) intermetallic compounds

It is crucial to understand the nature of the metal-hydrogen interactions in the intermetallic hydrides in order to develop new hydrogen storage materials with optimum metal-H interactions for desired hydrogen absorption-desorption kinetics. The MPd3 intermetallic compounds (M = Mg, In, Tl) transform to cubic anti-perovskite type (AuCu3-type with hydrogen in the octahedral intersect) structure from ZrAl3-type four-fold ccp-superstructure up on hydrogenation. According to the DFT-based orbital space electronic structure and chemical bonding analysis, this structural transformation is driven by maximizing the coordination of hydrogen surrounded by Pd, forming the Pd (4d) – H (1s), and … Pd–H–Pd … multicenter interactions. These interactions provide sufficient thermodynamic stability for the hydrides to form, with negative binding energies of hydrogen. The binding energies are found to be significantly low, with the possibility of the ease of hydrogen desorption within limited reaction conditions.

A Review on hydrogen embrittlement behavior of steel structures and measurement methods

Hydrogen can be found within metals under a variety of industrial and environmental conditions. Hydrogen-metal interactions can take place through hydrogen embrittlement, hydrogen sulfide corrosion, or hydrogen absorption. Steel and other metals that are exposed to hydrogen may experience a difficulty known as hydrogen embrittlement that affects their mechanical properties. The material's ductility and toughness may be reduced as a result of this phenomena, it also increasing the risk of brittle fracture. In steel, atomic hydrogen mainly diffuses into the microstructure of the steel, causing hydrogen embrittlement. Localized weakening of the bonds between the metal atoms might result from hydrogen atoms occupying interstitial positions in the metal lattice. Especially when under stress, this may lead to a more susceptible to fracture and cracking. Concerns with hydrogen embrittlement arise in sectors like aerospace and oil and gas that use high-strength steels. If not appropriately handled, it may result in catastrophic failures. Use of hydrogen-resistant alloys, appropriate heat treatments, and protection from conditions that promote hydrogen uptake are examples of preventive measures. This literature review paper covers the definition of hydrogen embrittlement (HE), mechanisms causing HE, measurement of hydrogen concentration and preventive measures that restrict hydrogen diffusion to the steel.

A systematic study on the phase diagram and superconductivity of ternary clathrate Ca-Sc-H at high pressures.

This work explores potential high-temperature superconductor materials in hydrogen-rich systems. Here, the crystal structure stabilities of ternary Ca-Sc-H systems under high-pressure (P = 100-250 GPa) and their superconductivities are investigated using the particle swarm optimization methodology combined with first-principles calculations. For the predicted candidate structures of Ca-Sc-H systems, the pressure-dependent phase diagram and thermodynamic convex hull were investigated across a wide range of compositions; the electronic properties of all the predicted phases were analyzed in detail to study the bonding behavior of these stable phases. We identified the crystal structures of four thermodynamically stable compounds: R3̄m-CaScH6, Immm-CaSc2H9,C2/m-Ca2ScH10, and R3̄m-CaScH12. Among them, R3̄m-CaScH12 was predicted to have the highest Tc value (i.e., 173 K) at 200 GPa. The discovery of this previously unreported pressure-induced decomposition of Ca-Sc-H systems will pave the way for investigations on the nature of hydrogen-metal interactions.

Tuning Mo Incorporation in Ni-Fe-Mo Solid Solutions: Towards an Improved Hydrogen Adsorption Energy

The production of hydrogen by water electrolysis has emerged as a promising pathway to achieve a 100% renewable and sustainable hydrogen economy. Up to now, alkaline water electrolysis has been highlighted as a truly alternative strategy for hydrogen production due to the usage of non-expensive electrocatalysts. Ni-based electrocatalysts already have a long century history because of high conductivity, stability, and earth abundancy. However, their strong hydrogen interaction energy slows the Hydrogen Evolution Reaction (HER) activity. Tuning the morphology of the catalyst by increasing the active surface area (e.g., production of nanosheets, nanowires, nanoparticles, etc.) and enhancing the intrinsic activity (e.g., doping, alloying, etc.) can improve the catalytic activity. Binary and ternary alloys based on nickel are a common route to boost their intrinsic activity and tune the morphology. Ternary alloys like CuAlNi, NiMoFe and NiMoW have been previously proposed, although the role of each element is still not fully investigated. Among the alloys, Ni-Fe mixtures have shown better catalytic activity towards HER, and the Mo incorporation has resulted in a reduced onset potential due to favorable hydrogen-metal interaction and development of active sites. The strategy for NiFeMo synthesis is mainly by hydrothermal method or electrodeposition which are limited by low material loading and production volume. Therefore, searching for a technique which is capable of producing trimetallic alloys with a scalable amount is of high interest for the development of hydrogen production. Here, we studied the preparation of FeNi3Mox via a fast and straightforward solution precursor plasma spraying (SPPS) method. The liquid precursors containing metal salts of Ni, Fe, and Mo are used to produce trimetallic solid solutions. The non-stoichiometric nanostructured electrocatalysts are deposited onto stainless-steel mesh (SS-mesh), Ni foam, and carbon paper for the HER in alkaline. We found that tuning Mo content (~9 At%) in the solution precursor conducts to an improved overpotential of 112 mV (iR-corrected) at -10 mA cm-2 and a Tafel slope of 109 mV dec-1 in agreement with a Volmer-Heyrovsky HER route. Energy dispersive X-ray spectroscopy (XPS) has confirmed the reduction in the superficial Ni and Fe oxides with Mo insertion which is directly affecting the electron transport. In addition, to investigate the effect of Mo in more detail, we evaluated the work function using ultraviolet photoelectron spectroscopy (UPS) which shows that Mo incorporation changes linearly the electrocatalyst’s work function and it is highly correlated to the hydrogen binding energy. Consequently, the change in the work function, resulting in a weaker hydrogen adsorption energy which leads to an improved hydrogen production. Theoretical catalytic activity maps from Density Functional Theory (DFT) studies have revealed the enhanced HER activity is the responsibility of the active sites near the superficial Mo atoms. Figure 1

Interplay of hydrogen and point defects in B2-type PdCu: A density functional theory study

To improve the performance of hydrogen-permeable alloys, quantitatively elucidating the metal–hydrogen interactions and identifying the controlling factors that govern the bottleneck step of the hydrogen permeation process are essential. In this study, based on the density functional theory, we characterized the interactions of hydrogen with point defects (such as antisite atoms and vacancies) in B2-type non-stoichiometric PdCu alloys, which are promising candidate materials for low-temperature working membranes for hydrogen purification. We proposed a “double defect” mechanism that produces a new type of defect complex with an atomic configuration similar to the Cu vacancy in B2-type PdCu alloys. We found that both Cu vacancies and double defects can act as strong hydrogen-trapping sites and are thus responsible for the marked decrease in hydrogen diffusivity. In contrast, antisite Cu atoms only repel H atoms and do not significantly hinder hydrogen diffusion. Whereas the binding energy of hydrogen to Cu vacancies and double defects exceeded 0.3 eV when relatively few (one or two) H atoms were trapped, the binding energy of hydrogen to Pd vacancies was substantially smaller (0.02 eV at most). Therefore, in Cu-rich non-stoichiometric PdCu alloys, the double defects may be the major hydrogen trapping sites. Furthermore, the effects of hydrogen on the formation of Pd vacancies, Cu vacancies, and double defects in B2-type PdCu alloys were clarified. Notably, double-defect formation from Pd vacancies in B2-type PdCu was significantly enhanced by the presence of nearby H atoms. This phenomenon, in which vacancies in B2-type intermetallic compounds transform into defect complexes in the presence of hydrogen, is postulated for the first time in this study. The findings of this study contribute to further understanding the driving forces for the formation of vacancy-hydrogen complexes in PdCu alloys at the atomic level and provide theoretical support for identifying the lattice defects that inhibit hydrogen permeability in Pd-based membranes.

Hydrogen and helium trapping in hcp beryllium

Even though hydrogen-metal surface interactions play an important role in energy technologies and metal corrosion, a thorough understanding of these interactions at the nanoscale remains elusive due to obstructive detection limits in instrumentation and the volatility of pure hydrogen. In the present paper we use analytical spectroscopy in TEM to show that hydrogen adsorbs directly at the (0001) surfaces of hexagonal helium bubbles within neutron irradiated beryllium. In addition to hydrogen, we also found Al, Si and Mg at the beryllium-bubble interfaces. The strong attraction of these elements to (0001) surfaces is underlined with ab-initio calculations. In situ TEM heating experiments reveal that hydrogen can desorb from the bubble walls at T ≥ 400 °C if the helium content is reduced by opening the bubbles. Based on our results we suggest the formation of a complex hydride consisting of up to five elements with a remarkably high decomposition temperature. These results therefore promise novel insights into metal-hydrogen interaction behavior and are invaluable for the safety of future fusion power plants.

Adsorption and dissociation of H2 molecule over first-row transition metal doped C24 nanocage as remarkable SACs: A comparative study

Transition metal doped carbon materials such as carbon nanotubes and fullerenes have been extensively investigated for hydrogen adsorption and storage applications. However, the strength of these hydrogen storage material is mainly dependent on the metal-support and hydrogen-metal interaction energies. In this work, we have designed, and explored transition metal doped C24 (TM@C24) complexes as single atom catalysts for hydrogen dissociation. Adsorption energy results for all studied TM@C24 complexes reveal the high thermodynamic stability of designed TM@C24 catalysts. Among all considered TMs@C24 catalysts, the highest adsorption energy (−6.22 eV) is calculated for Mn@C24 catalyst. Moreover, H2 dissociation mechanism is evaluated for both gas phase and in aqueous media to get insight into the solvent effect. In both gas phase and in aqueous media, the best catalytic activity for hydrogen dissociation is computed for Mn@C24 catalyst with the lowest energy barrier of 0.04 eV and 0.30 eV, respectively. NCI analysis is carried out to confirm the shared shell interactions (covalent interactions) between adsorbed hydrogen and TM@C24 complexes. Furthermore, natural bond orbital and electron density differences analysis are also performed to explore the activation and dissociation of H2 molecule. Overall results reveal that Mn@C24 complex can at as a promising low cost, highly abundant and noble metal free single atom catalyst to effectively catalyze hydrogen dissociation reaction.

Synergistic enhancement of hydrogen interactions in palladium–silicon–gold metallic glass with multilayered graphene

Presence of only a few layers of graphene boosts hydrogen intake of Pd-based metallic glass thin films by 2.6 times with 4.5 times higher electrocatalytic hydrogen evolution reaction activity, a tremendous improvement in metal–hydrogen interactions.

Hydrogen induced structural phase transformation in ScNiSn-based intermetallic hydride characterized by experimental and computational studies

Understanding an interrelation between the structure, chemical composition and hydrogenation properties of intermetallic hydrides is crucial for the improvement of their hydrogen storage performance. Ability to form the hydrides and to tune the thermodynamics and kinetics of their interaction with hydrogen is related to their chemical composition. Some features of the metal–hydrogen interactions remain however poorly studied, including chemistry of Sc-containing hydrides. ZrNiAl-type ScNiSn-based intermetallic hydride has been probed in the present work using a broad range of experimental techniques including Synchrotron and Neutron Powder Diffraction, 119Sn Möessbauer Spectroscopy, hydrogenation at pressures reaching several kbar H2 and hydrogen Thermal Desorption Spectroscopy studies. Computational DFT calculations have been furthermore performed. This allowed to establish the mechanism of the phase-structural transformation and electronic structure changes causing a unique contraction of the metal lattice of intermetallic alloy and the formation of the ...H-Ni-H-Ni… chains in the structure with H atoms carrying a partial negative charge. Such hydrogen absorption accompanied by a formation of a covalent Ni-H bonding and causing an unusual behavior contracts to the conventionally observed bonding mechanism of hydrogen in metals as based on the metallic bonding frequently accompanied by a jumping diffusion movement of the inserted H atoms – in contrast to the directional Metal-Hydrogen bonding observed in the present work. At high applied pressures ScNiSnH0.83 orthorhombic TiNiSi type hydride is formed with H atoms filling Sc3Ni tetrahedra. Finally, this study shows that scandium closely resembles the behavior of the heavy rare earth metal holmium.

The power of multifunctional metal hydrides: A key enabler beyond hydrogen storage

The metal-hydrogen interaction and its equilibrium conditions allow for distinct properties in metal hydrides (MHs). Based on these properties, MHs have been found to enable a range of novel technologies from thermal compression to sensors, and catalysis. Ni-MH batteries are currently the main application of hydrides in the market. However, new battery concepts such as Li-MgH 2 , Mn-MH, and hydride-based solid electrolytes have emerged. Fuel cells based on hydrides have also been proposed to convert the chemical “hydride energy” into electrical energy. Based on their unique thermodynamic properties, MHs are also the basis of new concepts in hydrogen compression, heat pumps, cooling systems, and thermal energy storage. Other important applications, including catalysis and chemical speciation have also been considered owing the chemical properties of hydrides. Sensors and smart mirrors based on the dynamic optical, structural, and electrical properties of MHs have been developed. This review summarizes current state-of-the-art along the multiple applications of MHs and provides recommendations on the future progress required to enable a more widespread adoption of MHs beyond their use as hydrogen storage materials. • Hydrides have a broad range of applications. • Enabling alternative applications beyond hydrogen storage could help in finally enabling the commercialization of hydrides. • The continuous development of new hydrides materials remains a critical enabler.

Hydrogen permeation in metals and alloys: A Brief Review

This is an integrative review deals with the hydrogen-metal interaction mechanisms, reversible and irreversible traps and highlights the use of electrochemical permeation as an important tool of evaluation the Hydrogen embrittlement (HE) mechanism. Due to its simplicity, flexibility, low cost and low risk, this technique is widely used in the study of hydrogen transport and diffusion parameters, contributing to the selection of more resistant steels and alloys. On the other hand, it´s important toothose new efforts should be made to develop and implement new techniques that can be applied to different temperature and pressure conditions.

New Insights into Phase Separation of Cerium Hydrides under Pressure.

We report theoretical calculations of the static ground-state structures and pressure-induced phase transformations of four cerium hydrides: CeH, CeH2, CeH2.5, and CeH3. Under pressure, the experimental CaF2-type structure of CeH2 is likely to disproportionate to face-centered cubic (fcc) Ce and a cubic Pm3̅n (β-UH3 type) structure of CeH3 above 6 GPa. At further increasing pressures, fcc Ce will transform to a tetragonal I4/mmm structure above 12 GPa, while CeH3 moves through the following sequence of phases: Pm3®n (β-UH3 type) → Pm3̅n (A15 type) → R3®m; the corresponding transition pressures are calculated to be 10 and 70 GPa, respectively. The tetragonal I41/amd structure of CeH2.5 has the similar decomposition as that of CeH2. Finding this previously unreported pressure-induced decomposition of CeH2 will pave the way for investigations on the nature of hydrogen–metal interactions.

Lattice dynamics of high-pressure hydrides studied by inelastic neutron scattering

Due to the small mass and anomalously large neutron scattering cross-section of proton (about 80 barns compared to a few barns for other nuclei), inelastic neutron scattering is considered as one of the most effective tools in studying optical vibrations of hydrogen atoms in metal hydrides. The current review is focused on the binary hydrides of 3d- and 4d-metals of groups VI–VIII, which were produced at high hydrogen pressures of several gigapascals in relatively large quantities of hundreds of mg, quenched to low temperature and studied by INS ex situ at ambient pressure with high statistical accuracy. One of the unusual effects revealed by INS is a strong increase in the strength of the metal-hydrogen interactions with decreasing atomic number of the d-metal accompanied by an increase in the Me-H distance. Based on the available experimental results, the spectra g(E) of the phonon density of states and temperature dependencies CV(T) of the heat capacity at constant volume at T up to 1000 K have been derived in this paper and presented both in the figures and in digital form. This provides the reference data for the theoretical investigations of the crystal structures and compositions of new practically important hydrides giving the opportunity to validate calculation methods by comparing the calculated g(E) and CV(T) with the accurate experimental dependencies for the binary hydrides. Recent INS studies showed [R.A. Klein et al., J. Alloy. Compd. 894 (2022) 162381] that the fingerprints of anomalously short H-H separations of 1.6 Å violating the “2 Å rule” can be easily and unambiguously identified in the complex INS spectra of quaternary hydrides (La,Ce)NiInH1+x. This makes neutron spectroscopy an attractive means for obtaining valuable data in the search for novel hydrides with a record high hydrogen capacity.

Multilayer crystal-amorphous Pd-based nanosheets on Si/SiO2 with interface-controlled ion transport for efficient hydrogen storage

This contribution shows an unusually high hydrogen storage of multilayer amorphous (A)-crystalline (C) Pd–Si based nanosheets when stacked in the right order. Samples with A/C/A/C/A stacking sequence exhibit 40 and 12 times larger hydrogen sorption than monolithic crystalline and amorphous samples, respectively. The maximum capacitance calculated from the fitting of electrochemical impedance measurements of the same sample is twice larger than that of the conventional polycrystalline Pd films of similar thickness. Five times higher diffusion coefficient calculated from modified Cottrell equation is obtained compared to specimens with C/A/C/A/C stacking. For the A/C/A/C/A multilayers, nanobubbles with diameters of 1–2 nm are homogeneously distributed at Si/SiO2 interface, and PdHx crystal formation in these regions confirms hydrogen-metal interactions. Furthermore, corrosion-resistant amorphous top layer permits larger amounts of hydrogen ion transfer to inner layers. Thus, hydrogen storage and production can be enhanced by smart design of multilayers targeted for proton exchange membrane electrolysis or fuel cells.

Investigating the non-classical M-H2 bonding in OsClH3(PPh3)3

Dihydrogen ligation to metal centers results in a wide range of metal-hydrogen interactions. Compounds containing non-classical dihydrogen ligands in which the H–H contacts are significantly longer than the 0.74 Å distance found in solid H2 are classified as Kubas-like compounds, stretched dihydrogen ligands, or compressed dihydrides, depending on the H–H contact length. Compounds containing stretched dihydrogen and compressed dihydride ligands are somewhat rare. Developing a more complete understanding of the metal-hydrogen and hydrogen-hydrogen interactions in these species may provide insights into the mechanisms for the oxidative addition reaction and hydrogen storage. Here, we use diffraction and inelastic neutron scattering (INS) measurements, paired with both molecular and solid-state density functional theory (DFT), to fully characterize the nature of the metal-hydrogen and hydrogen-hydrogen interactions in a previously reported compound, OsClH3(PPh3)3. In that report, the H–H distance was measured as 1.48(2) Å by single-crystal neutron diffraction and was described as a “stretched” dihydrogen. The INS-generated phonon density of states is well described by the DFT calculations, including those normal modes dominated by the vibrational motions of the three Os-ligated hydrogen atoms. The resulting calculated electron charge density landscape indicates that there is no remnant bonding character between the Os-ligated hydrogen atoms in the compound. Based on these findings, we update the understanding of the metal-hydrogen bonding in this complex and reclassify it as a compressed dihydride. As such, OsClH3(PPh3)3 represents a rare example of a non-classical compressed dihydride species.

Risk-based inspection planning for hydrogen technologies: review of currents standards and suggestions for modification

The research addressing loss of integrity (LOI) for hydrogen containment technologies requires a multidisciplinary perspective. However, lack of collaboration between materials and process safety engineers is reflected in the theory addressing the LOI phenomena. Even though the potential degrading mechanisms related to hydrogen-metals interaction are being extensively studied, they are not explicitly considered by standards and recommended practices in relation to plant inspection planning. This inevitably introduces additional uncertainties while planning for inspections and predictive maintenance and leads to the necessity to address and consider generic mechanisms of material degradation within the framework of LOI development prediction. A review of the available standards and recommended practices for inspection planning is here conducted to identify how metal-hydrogen interactions are currently considered in terms of degradation mechanisms. Focus is given to risk-based approaches, which include the assessment of potential accident scenarios as consequence of LOI. In particular, the association of metal-hydrogen mechanisms with damage factors influencing the predicted LOI frequency is studied to understand their impact both on the risk and the inspection typology. Appropriate knowledge of material degradation mechanisms for hydrogen containment is of paramount importance as it allows not only for a correct design of the equipment of interest, but also for a suitable inspection and maintenance planning so to guarantee its integrity through an effective and informed risk-based approach. A few modifications to the current standards are suggested as a result of this work.

Enhancement of Interfacial Hydrogen Interactions with Nanoporous Gold-Containing Metallic Glass.

Contrary to the electrochemical energy storage in Pd nanofilms challenged by diffusion limitations, extensive metal-hydrogen interactions in Pd-based metallic glasses result from their grain-free structure and presence of free volume. This contribution investigates the kinetics of hydrogen-metal interactions in gold-containing Pd-based metallic glass (MG) and crystalline Pd nanofilms for two different pore architectures and nonporous substrates. Fully amorphous MGs obtained by physical vapor deposition (PVD) co-sputtering are electrochemically hydrogenated by chronoamperometry. High-resolution (scanning) transmission electron microscopy and corresponding energy-dispersive X-ray analysis after hydrogenation corroborate the existence of several nanometer-sized crystals homogeneously dispersed throughout the matrix. These nanocrystals are induced by PdHx formation, which was confirmed by depth-resolved X-ray photoelectron spectroscopy, indicating an oxide-free inner layer of the nanofilm. With a larger pore diameter and spacing in the substrate (Pore40), the MG attains a frequency-independent impedance at low frequencies (∼500 Hz) with very high Bode magnitude stability accounting for enhanced ionic diffusion. On the contrary, on a substrate with a smaller pore diameter and spacing (Pore25), the MG shows a larger low-frequency (0.1 Hz) capacitance, linked to enhanced ionic transfer in the near-DC region. Hence, the nanoporosity of amorphous and crystalline metallic materials can be systematically adjusted depending on AC- and DC-type applications.

Hydrogen-Driven Surface Segregation in Pd Alloys from Atomic-Scale Simulations

The safe widespread application of hydrogen-based fuels requires sensors that are long-term stable, inexpensive, hydrogen-specific, and have a short response time. In this regard, optical sensing based on nanostructured Pd alloys has shown great potential, but challenges remain, including understanding and controlling the surface composition under operation conditions. While the latter is crucial for long-term functionality and stability, it is experimentally very challenging to obtain accurate atomic-scale information. Here, we therefore scrutinize the behavior of two particularly relevant surface alloys, {111} AuPd and {111} CuPd, in H environments ranging from vacuum to fully covered surfaces. To this end, we employ a combination of alloy cluster expansions trained using density functional theory calculations, Monte Carlo simulations, and thermodynamic analysis to obtain the H coverage as well as the layer-by-layer composition of the near-surface region as a function of H partial pressure, temperature, and bulk composition. To overcome the symmetry reduction implicit to surface systems, we exploit local symmetries, which enables us to achieve accurate and reliable models at a low computational cost. In the case of AuPd, Au segregates to the surface in vacuum while Pd segregates to the surface at 100% H coverage, and the transition between these regimes occurs over a narrow H pressure interval. In the case of CuPd, on the other hand, the H coverage varies much more gradually with H pressure and is coupled to a nonmonotonic variation of the Cu concentration in the topmost surface layer. While there is a pronounced Cu depletion both at 0 and 100% H coverage, Cu enrichment is observed at 50% coverage at Cu bulk concentrations up to at least 10%, providing a nontrivial explanation for an apparent discrepancy between experiment and calculations that was observed previously. At the same time, layer 2 continuously shifts from Cu enrichment to Cu depletion with increasing H coverage. The results demonstrate the rich behavior that can result from the coupling of metal–metal and metal–hydrogen interactions at surfaces, even in apparently simple but concentrated systems. Moreover, they underline the advantages of simulations that account for temperature and pressure effects as well as models that can accurately capture the interactions over a wide composition range.

Hydrogen Evolution Activity of Mesoporous Pd-Based Metallic Glass Assemblies with Tunable Pore Architecture and Film Thickness

The necessity for efficient green energy production compels scientists to develop advanced nanostructured metallic alloys with tunable properties. This contribution places emphasis on tuning pore architecture and film thickness of mesoporous Pd – Cu – Si thin films sputtered on Si substrates for enhanced electrocatalytic and hydrogen sorption/desorption activity and their comparison with the state-of-the-art electrocatalysts. The small Tafel slope of 44 mV dec–1 with a rapid rise in the cathodic current rendering low hydrogen overpotentials of 24 mV at 10 mA cm–2, and the largest sorption/desorption peaks for the T4H2 electrode suggest its use for a fuel cell electrode. Moreover, the kinetic parameters described by the equivalent circuit model have a very good correlation with Tafel slopes and sample mass. The 10 nm T1H1 sample has one of the highest capacitive responses where the hydrogen-metal interaction takes place and is promising as a nano-charger and sensing of hydrogen.

Electronic Communication in Binuclear Osmium- and Iridium-Polyhydrides.

Reactions of polyhydrides OsH6(PiPr3)2 (1) and IrH5(PiPr3)2 (2) with rollover cyclometalated hydride complexes have been investigated in order to explore the influence of a metal center on the MHn unit of the other in mixed valence binuclear polyhydrides. Hexahydride 1 activates an ortho-CH bond of the heterocyclic moiety of the trihydride metal–ligand compounds OsH3{κ2-C,N-[C5RH2N-py]}(PiPr3)2 (R = H (3), Me (4), Ph (5)). Reactions of 3 and 4 lead to the hexahydrides (PiPr3)2H3Os{μ-[κ2-C,N-[C5RH2N-C5H3N]-N,C-κ2]}OsH3(PiPr3)2 (R = H (6), Me (7)), whereas 5 gives the pentahydride (PiPr3)2H3Os{μ-[κ2-C,N-[C5H3N-C5(C6H4)H2N]-C,N,C-κ3]}OsH2(PiPr3)2 (8). Pentahydride 2 promotes C—H bond activation of 3 and the iridium-dihydride IrH2{κ2-C,N-[C5H3N-py]}(PiPr3)2 (9) to afford the heterobinuclear pentahydride (PiPr3)2H3Os{μ-[κ2-C,N-[C5H3N-C5H3N]-N,C-κ2]}IrH2(PiPr3)2 (10) and the homobinuclear tetrahydride (PiPr3)2H2Ir{μ-[κ2-C,N-[C5H3N-C5H3N]-N,C-κ2]}IrH2(PiPr3)2 (11), respectively. Complexes 6–8 and 11 display HOMO delocalization throughout the metal–heterocycle-metal skeleton. Their sequential oxidation generates mono- and diradicals, which exhibit intervalence charge transfer transitions. This notable ability allows the tuning of the strength of the hydrogen–hydrogen and metal–hydrogen interactions within the MHn units.