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The free energy for hydride transfer reactions of transition metal hydrides is known to be influenced by solvent effects. The first-row transition metal hydride [HNi(dmpe)2][BF4] (dmpe = 1,2-bis(dimethylphosphino)ethane) has starkly different hydride transfer reactivities with CO2 in different solvents. A binary mixture of water and acetonitrile was used to tune the hydride transfer reactivity of HNi(dmpe)2+ with CO2 so that the free energy for this reaction approached zero. Various mole fractions of water were tested and a linear relationship between the hydride transfer free energy and solvent composition was established for 0-0.24 mole fraction of water. A deviation from linearity was found upon moving toward higher mole fractions of water. The tuning of the free energy for hydride transfer allowed HNi(dmpe)2+ to be used as a catalyst for the hydrogenation of CO2. The optimized catalyst conditions produced 58 turnovers at room temperature in 0.082 mole fraction of water using 60 atm of a 1:1 mixture of H2 to CO2 gas.

Understanding the thermodynamics of paramagnetic transition metal hydride complexes, especially of the abundant 3d metals, is important in the design of electrocatalysts and organometallic catalyst...

Studies on N2 activation and transformation by transition metal hydride complexes are of particular interest and importance. The synthesis and diverse transformations of a dinitrogen dititanium hydride complex bearing the rigid acridane-based acri PNP-pincer ligands {[(acri PNP)Ti]2 (μ2 -η1 :η2 -N2 )(μ2 -H)2 } are presented. This complex enabled N2 cleavage and hydrogenation even without additional H2 or other reducing agents. Furthermore, diverse transformations of the N2 unit with a variety of organometallic compounds such as ZnMe2 , MgMe2 , AlMe3 , B(C6 F5 )3 , PinBH, and PhSiH3 have been well established at the rigid acri PNP-ligated dititanium framework, such as reversible bonding-mode change between the end-on and side-on/end-on fashions, diborylative N=N bond cleavage, the formal insertion of two dimethylaluminum species into the N=N bond, and the formal insertion of two silylene units into the N=N bond. This work has revealed many unprecedented aspects of dinitrogen reaction chemistry.

Studying the formation of transition metal hydride complexes via proton-coupled electron transfer is important for developing next-generation molecular catalysts for hydrogen evolution. We report herein the study of stepwise photoinduced reduction and protonation of [Co

The implementation of a sustainable energy economy based on renewable but intermittent energy sources necessitates the efficient electrogeneration of chemical fuels and the efficient utilization of...

Outer-sphere radical hydrogenation of olefins proceeds via stepwise hydrogen atom transfer (HAT) from transition metal hydride species to the substrate. Typical catalysts exhibit M-H bonds that are either too weak to efficiently activate H2 or too strong to reduce unactivated olefins. This contribution evaluates an alternative approach, that starts from a square-planar cobalt(II) hydride complex. Photoactivation results in Co-H bond homolysis. The three-coordinate cobalt(I) photoproduct binds H2 to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H2 via HAT.

The rational development of homogeneous catalytic systems for selective aerobic oxidations of organics has been hampered by the limited available knowledge of how oxygen reacts with important organometallic intermediates. Recently, several mechanisms for oxygen insertion into late transition metal-hydride bonds have been described. Contributing to this nascent understanding of how oxygen reacts with metal-hydrides, a detailed mechanistic study of the reaction of oxygen with the Ir

The factors that influence the selectivity of pincer supported group 10 transition metal hydride catalysts for CO2 hydroboration are investigated. We demonstrate that selective CO2 reduction to eit...

The prevalence of transition metal-mediated hydride transfer reactions in chemical synthesis, catalysis, and biology has inspired the development of methods for characterizing the reactivity of transition metal hydride complexes. Thermodynamic hydricity represents the free energy required for heterolytic cleavage of the metal–hydride bond to release a free hydride ion, H−, as determined through equilibrium measurements and thermochemical cycles. Kinetic hydricity represents the rate of hydride transfer from one species to another, as measured through kinetic analysis. This tutorial review describes the common methods for experimental and computational determination of thermodynamic and kinetic hydricity, including advice on best practices and precautions to help avoid pitfalls. The influence of solvation on hydricity is emphasized, including opportunities and challenges arising from comparisons across several different solvents. Connections between thermodynamic and kinetic hydricity are discussed, and opportunities for utilizing these connections to rationally improve catalytic processes involving hydride transfer are highlighted.

Catalytic processes to generate (or oxidize) fuels such as hydrogen are underpinned by multiple proton-coupled electron transfer (PCET) steps that are associated with the formation or activation of metal-hydride bonds. Fully understanding the detailed PCET mechanisms of metal hydride transformations holds promise for the rational design of energy-efficient catalysis. Here we investigate the detailed PCET mechanisms for the activation of the transition metal hydride complex CpW(CO)2(PMe3)H (Cp = cyclopentadienyl) using stopped-flow rapid mixing coupled with time-resolved optical spectroscopy. We reveal that all three limiting PCET pathways can be accessed by changing the free energy for elementary proton, electron, and proton-electron transfers through the choice of base and oxidant, with the concerted pathway occurring exclusively as a secondary parallel route. Through detailed kinetics analysis, we define free energy relationships for the kinetics of elementary reaction steps, which provide insight into the factors influencing reaction mechanism. Rate constants for proton transfer processes in the limiting stepwise pathways reveal a large reorganization energy associated with protonation/deprotonation of the metal center (λ = 1.59 eV) and suggest that sluggish proton transfer kinetics hinder access to a concerted route. Rate constants for concerted PCET indicate that the concerted routes are asynchronous. Additionally, through quantification of the relative contributions of parallel stepwise and concerted mechanisms toward net product formation, the influence of various reaction parameters on reactivity are identified. This work underscores the importance of understanding the PCET mechanism for controlling metal hydride reactivity, which could lead to superior catalyst design for fuel production and oxidation.

Two heteroleptic nickel(II) complexes of the type [Ni(bis-NHC)(dRpe)]2+ (1, R = phenyl; 2, R = methyl; bis-NHC = 1,l′:3,3′-bis(1,3-propanediyl)dibenzimidazolin-2,2′-diylidene) have been synthesized and characterized. The complexes exhibit reversible, two-electron reductions at −1.53 and −1.87 V versus Fc+/0, respectively. Through the use of thermodynamic scaling relationships, hydricities for the corresponding nickel(II) hydride complexes 1H and 2H are estimated to be 45.6 and 37.8 kcal mol–1, respectively. Experimental estimation of an upper bound for the hydricities (ΔGH–°(1H) < 50.3 kcal mol–1 and ΔGH–°(2H) < 40.6 kcal mol–1) was determined by selection of an added weak acid such that the hydrogen evolution reaction (HER) becomes exergonic vs endergonic. Further electrochemical studies establish 2 as an efficient hydrogen evolution electrocatalyst, operating at low overpotential (η = 0.4 V) and reasonable rates (TOF ∼ 1000 s–1) using phenol as the Bronsted acid source. This demonstrates the utility of ...

Transition metal hydride (TMH) plays a critical role in the field of chemical catalysis and catalytic transformations. Knowledge of enthalpies for cleaving metal hydride bonds enables the prediction of chemical reactivity, such as for the bond-forming and bond breaking events that occur in a catalytic reaction especially in the hydrogenation of carbon dioxide. Thermochemistry of the first row TMHs, MH+/0/− (M = Sc−Zn) were calculated at both the Density Functional Theory (DFT) level and the CCSD(T) level. Thermochemistry hydricity is the energy required to cleave an M–H bond to generate a hydride ion (H−). The heterolytic bond cleavage with proton dissociation is the acidity of the metal hydride. The homolytic bond-dissociation energy (BDE), involves formation H . Periodic trends of H−, H+, H donor ability are discussed and compared among the different elements and every MH+/0/− systems. Note that, we can find suitable methods and avoid bad methods for different element. The study could give certain theoretical guidance on the catalysis of TMH.

The paper presents a brief overview of the role of metal hydride complexes in understanding the dynamics of hydrogen generation in homogeneous water reduction catalysts. In addition, the kinetics of protonation of [Os(phen)2(CO)(H)]+ and the one-electron reduced form of the complex, [Os(phen)(phen−)(CO)(H)], in acetonitrile solution were examined. The monocationic complex reacted with tosylic acid to form a distinct intermediate species which was found to be a dihydrogen complex; this went on to produce hydrogen and the solvento complex. The one-electron reduced complex was prepared by reduction of the photoexcited monocationic complex with the sacrificial donor BIH. Protonation of the reduced complex by tosylic acid occurred with a rate constant approximately 100 million times larger. The results are used to address possible mechanistic pathways of existing homogeneous catalysts.

Hydride transfer between transition metal hydride complexes and carbon dioxide is a known reaction, where the thermodynamically favored direction of hydride transfer determines whether CO2 reduction or formate oxidation occurs. Analysis of a growing database of thermodynamic parameters for transition metal hydride complexes now provides clear demarcation between metal hydrides which will function as oxidases and as reductases. The turning point is set at the hydricity of formate (44 kcal/mol in acetonitrile). Here, we utilize hydricity as a framework to reevaluate the catalytic activity and proposed mechanisms for formate oxidation and CO2 reduction with several Ni and Rh P2N2 (P2N2 = 1,5-diaza-3,7-diphosphacyclooctane) complexes, respectively. The series of Ni P2N2 complexes have hydricities between 55–64 kcal/mol and are active catalysts for the electrochemical oxidation of formate. A surprising correlation of increased rate of electrochemical oxidation with decreased overpotential, η, is observed. The ...

The hydricity ΔG°H– of a metal hydride is an important parameter for describing the reactivity of such complexes. Here, we compile a comprehensive data set consisting of 51 transition-metal hydride complexes [M-H](n−1)+ with known ΔG°H– values in acetonitrile for which the one-electron reduction of the parent complex [M]n+ is reversible. Plotting the hydricity as a function of respective E1/2(Mn+/(n–1)+) yields a robust linear correlation. While this correlation has been previously noted for limited data sets, our analysis demonstrates that this trend extends over a wide range of metal identities, ligand architectures, structural geometries, and overall charges of the metal hydride. This correlation is modeled using established thermochemical cycles relating the hydricity and homolytic bond free energy of the metal–hydride bond. The linear trend of the model enables the estimation of hydricity simply on the basis of the reduction potential of the parent complex and thus provides a guide for the rational d...

Density functional theory (DFT) is used to calculate the relative free energies of deprotonation of the isomers of iron-group hydride complexes MHXL4 where M = Fe, Ru, Os, L4 = (CO)4 or (PMe2CH2CH2PMe2)2 for a wide range of anionic ligands X. The free energies of the most stable isomers are used to calculate relative pKa values where Ka refers to the acid dissociation constant for the equilibrium MHXL4 → [MXL4]- + H+. These are used to test the proposal that the pKa for a given metal complex in THF can be simply calculated by adding the contributions to the total pKa value from each ligand L; these are called ligand acidity constants (LAC) AL used in the LAC equation [R. H. Morris, J. Am. Chem. Soc., 2014, 136, 1948-1959]. The AL are calculated using AL = 0.2 as a reference for the hydride ligand. The AL of certain less polarizable X ligands are found to be fairly constant (±1 to ±2 units) and consistent with the proposed LAC method for a range of the complexes considered: 2 for Me-, 1 for OH-, 1 for NH2-, 0 for B(OCH2CH2O-)-, -1 for OMe-, -2 for SH- and -2 for SMe-. Other ligands have more variable AL values (±3 to ±5 units) because of high polarizability or other reasons: 1 for OtBu-, -1 for F-, -2 for BMe2-, -2 for NMe2-, -3 for Cl-, -3 for PMe2-, -3 for Br-, -5 for I-, -6 for CN- and -12 for SiCl3-. Iodide stabilizes the anion [MIL4]- more than does fluoride in [MFL4]- making iodide the more acidifying ligand despite its lower electronegativity. DFT is also used to validate the charge correction in the LAC equation.

International audience; While the spectrum of NiH has been extensively studied, only the Ω = 3/2 and 5/2 components of the strongly interacting 2Δ, 2Π and 2Σ+ molecular states arising from the 3d9 ground-state configuration of Ni+ have been reported for the deuterated form, NiD. We report on laser excitation and resolved fluorescence experiments that definitively locate rotational levels of v = 0–2 in the 2Σ+ state, and of v = 0 and v = 1 in the low-lying 2Π1/2 state of 58NiD.

The complex transition metal hydride Mg3CrH8 has been previously synthesized using high pressure conditions. It contains the first group 6 homoleptic hydrido complex, [Cr(II)H7]5-. Here, we investigated the formation of Mg3CrH8 by in situ studies of reaction mixtures of 3MgH2-Cr-H2 at 5 GPa. The formation of the known orthorhombic form (o-Mg3CrH8) was noticed at temperatures above 635 °C, albeit at a relatively slow rate. At temperatures around 750 °C a high temperature phase formed rapidly, which upon slow cooling converted into o-Mg3CrH8. The phase transition at high pressures occurred reversibly at ∼735 °C upon heating and at ∼675 °C upon slow cooling. Upon rapid cooling, a monoclinic polymorph (m-Mg3CrH8) was afforded which could be subsequently recovered and analyzed at ambient pressure. m-Mg3CrH8 was found to crystallize in P21/n space group (a = 5.128 A, b = 16.482 A, c = 4.805 A, β = 90.27°). Its structure elucidation from high resolution synchrotron powder diffraction data was aided by first-principles DFT calculations. Like the orthorhombic polymorph, m-Mg3CrH8 contains pentagonal bipyramidal complexes [CrH7]5- and interstitial H-. The arrangement of metal atoms and interstitial H- resembles closely that of the high pressure orthorhombic form of Mg3MnH7. This suggests similar principles of formation and stabilization of hydrido complexes at high pressure and temperature conditions in the Mg-Cr-H and Mg-Mn-H systems. Calculated enthalpy versus pressure relations predict o-Mg3CrH8 being more stable than m-Mg3CrH8 by 6.5 kJ/mol at ambient pressure and by 13 kJ/mol at 5 GPa. The electronic structure of m-Mg3CrH8 is very similar to that of o-Mg3CrH8. The stable 18-electron complex [CrH7]5- is mirrored in the occupied states, and calculated band gaps are around 1.5 eV.

A large number of industrially relevant enzymes depend upon dihydronicotinamide adenine dinucleotide (NADH) and dihydronicotinamide adenine dinucleotide phosphate (NADPH) cofactors, which are too expensive to be added in stoichiometric amounts. Existing NAD(P)H-recycling systems suffer from low activity, or the generation of side products. This review focuses on NAD(P)H cofactor regeneration catalyzed by transition metal complexes such as rhodium, ruthenium and iridium complexes using cheap reducing agents such as hydrogen (H2) and ethanol, which have attracted increasing attention as sustainable energy carriers. The catalytic mechanisms for the regioselective reduction of NAD(P)+ are discussed with emphasis on identification of catalytically active intermediates such as transition metal hydride complexes. Applications of NAD(P)H-recycling systems to develop artificial photosynthesis are also discussed.

After remaining elusive for many years, terminal hydride states have now been identified in several native and mutant [FeFe]-hydrogenases. In this Perspective, hydride states of [FeFe]-hydrogenases are considered on the basis of hydricity, a thermodynamic parameter that has been instrumental for the rational design of synthetic molecular catalysts. Importantly, it is shown how the hydricity of [FeFe]-hydrogenases can inspire future research efforts in both the study of hydrogenases and the design of molecular catalysts. By using hydricity, quantitative linear free energy relationships can be developed to relate the driving force and rate of different [FeFe]-hydrogenase variants. Further development of these correlations across a wide range of hydrogenase variants can potentially lead to new insights into the structure-activity relationships of [FeFe]-hydrogenases. The hydricity of [FeFe]-hydrogenase is also compared to select transition metal hydride complexes, which emphasizes the strong electronic communication between the diiron active site and the extended protein scaffold of the enzyme. A mechanism is proposed for how key hydrogen bonding interactions might affect the hydricity of [FeFe]-hydrogenases, providing a basis for the emulation of these structural features in synthetic molecular complexes.

The hydrogen-release reaction of a complex transition metal hydride, LaMg2 NiH7 , composed of La3+ , 2×Mg2+ , [NiH4 ]4- and 3×H- , was studied by thermal analyses, powder X-ray, and neutron diffraction and inelastic neutron scattering. Upon heating, LaMg2 NiH7 released hydrogen at approximately 567 K and decomposed into LaH2-3 and Mg2 Ni. Before the reaction, covalently bound hydrogen (Hc °v. ) in [NiH4 ]4- exhibited a larger atomic displacement than H- , although a weakening of the chemical bonds around [NiH4 ]4- and H- was observed. These results indicate the precursor phenomenon of a hydrogen-release reaction, wherein there is a large atomic displacement of Hc °v. that induces the hydrogen-release reaction rather than H- . As an isothermal reaction, LaMg2 NiH7 formed LaMg2 NiH2.4 at 503 K in vacuum for 48 h, and LaMg2 NiH2.4 reacted with hydrogen to reform LaMg2 NiH7 at 473 K under 1 MPa of H2 gas pressure for 10 h. These results revealed that LaMg2 NiH7 exhibited partially reversible hydrogen-release and uptake reactions.

A series of over 30 iron, ruthenium, and osmium hydride phosphine complexes are reported, along with their MIII/II redox potentials. The complexes are of the type MH(PP)n(X) or [MH(PP)n(L)]+, where PP is one of the following bidentate phosphine ligands: dppe, dtpe, depe, and dtfpe, with n = 2; or the tetradentate phosphine ligand meso-tet-1, with n = 1. The electrochemical data of these complexes and those from the literature are used to determine the Lever EL parameter of −0.65 V for the hydride ligand for iron and ruthenium. For osmium, however, the EL value for the hydride ligand is found to be more positive at only −0.37 V, an increase which is attributed to Os-H σ bond strengthening due to relativistic effects. The correlation holds for irreversible oxidations as well as reversible ones. These EL values can now be used along with Lever’s equations to predict redox potentials of other iron-group hydride complexes.

Aqueous hydride transfer is a fundamental step in emerging alternative energy transformations such as H2 evolution and CO2 reduction. "Hydricity," the hydride donor ability of a species, is a key metric for understanding transition metal hydride reactivity, but comprehensive studies of aqueous hydricity are scarce. An extensive and self-consistent aqueous hydricity scale is constructed for a family of Ru and Ir hydrides that are key intermediates in aqueous catalysis. A reference hydricity is determined using redox potentiometry and spectrophotometric titration for a particularly water-soluble species. Then, relative hydricity values for a range of species are measured using hydride transfer equilibria, taking advantage of expedient new synthetic procedures for Ru and Ir hydrides. This large collection of hydricity values provides the most comprehensive picture so far of how ligands impact hydricity in water. Strikingly, we also find that hydricity can be viewed as a continuum in water: the free energy of hydride transfer changes with pH, buffer composition, and salts present in solution.

Solid-state materials containing rotatable polyanions, such as B12 H122−, constitute a peculiar class of ionic conductors due to their unique transport behavior, where rotating polyanions promote phase transitions to disordered phases with several orders of magnitude enhancement in cation conductivities. A major drawback is the high temperature required to activate rotation and thereby low conductivities at room temperature. Here, we elucidate a mechanism to drastically reduce the temperature based on the use of pseudorotation in high-H coordination hydride complexes. We demonstrate this mechanism for an existing complex transition metal hydride Li5MoH11 containing MoH93−, and we present a strong potential of this material to unprecedentedly exhibit a high lithium ion conductivity of 7.9 × 10–2 S cm−1 at room temperature.

The synthesis and reactivity of an NPN-chelating ligand containing a nontrigonal phosphorous triamide center (L1 = P(N(o-N(2-pyridyl)C6H4)2) is reported. Metalation of L1 with RuCl2(PPh3)3 gives RuCl2(PPh3)(L1) (2). By contrast, metalation of L1 with RuHCl(CO)(PPh3)3 yields RuCl(CO)(PPh3)(L1H) (3), a chelated 10-P-5 ruthenahydridophosphorane, via net insertion into the Ru–H bond. Hydride abstraction from 3 with Ph3CPF6 gives [RuCl(CO)(PPh3)(L1)]PF6 (4); reaction of 4 with NaBH4 returns 3.

The problem of activating N2 and its subsequent hydrogenation to form NH3 has been approached from many directions. One of these approaches involves the use of transition metal hydride complexes. Recently, transition metal hydride complexes of Ti and Ta have been shown to activate N2, but without catalytic formation of NH3. Here, we show that at elevated temperatures (400 °C, 5 MPa), solid-state hydride-containing Ti compounds (TiH2 and BaTiO2.5H0.5) form a nitride-hydride surface similar to those observed with titanium clusters, but continuously (∼7 days) form NH3 under H2/N2 flow conditions to achieve a catalytic cycle, with activity (up to 2.8 mmol·g·–1·h–1) almost comparable to conventional supported Ru catalysts such as Cs–Ru/MgO or Ru/BaTiO3 that we have tested. As with the homogeneous analogues, the initial presence of hydride within the catalyst is critical. A rare hydrogen-based Mars van Krevelen mechanism may be at play here. Conventional scaling rules of pure metals predict essentially no activ...

Heterodinuclear hydridoplatinum and -palladium complexes, (dppe)HPt–MLn (MLn = MoCp(CO)3 (3a), WCp(CO)3 (3b)), (dppe)Pt(μ-H)(μ-CO)Mn(CO)4 (3c), (dppe)Pt(μ-H)(μ-CO)FeCp(CO) (3d), cis- L ′ 2 HPt–MLn (5aa: Lʹ = PPh3; MLn = MoCp(CO)3, 5ba: Lʹ = PPh3; MLn = WCp(CO)3, 5ab: Lʹ = PMePh2; MLn = MoCp(CO)3, 5ac: Lʹ = PMe2Ph; MLn = MoCp(CO)3), (dppe)Pd(μ-H)(μ-CO)MLn (MLn = MoCp(CO)2 (7a), WCp(CO)2 (7b)), (dppe)Pd(μ-H)(μ-CO)Mn(CO)4 (7c) are prepared by the oxidative addition of mononuclear transition-metal hydride complexes to zero-valent platinum or palladium complexes. The reactions of the heterodinuclear hydride complexes 3a, 3d and 7a with electron deficient alkenes and alkynes such as dimethyl fumarate or DMAD cause reductive elimination at the Pt or Pd center to give the Pt(alkene or alkyne)(dppe) and MHLn,, suggesting reversibility of this process. The DFT calculations suggest that these reactions are controlled by the thermodynamic stability and the electron rich alkene complex of Pt(0) (or Pd(0)) are preferable to prepare these heterodinuclear hydride complexes by the oxidative addition.

Despite many exploratory studies over the past several decades, the presently known transition metals that form homoleptic transition-metal hydride complexes are limited to the Groups 7-12. Here we present evidence for the formation of Mg3 CrH8 , containing the first Group 6 hydride complex [CrH7 ](5-) . Our theoretical calculations reveal that pentagonal-bipyramidal H coordination allows the formation of σ-bonds between H and Cr. The results are strongly supported by neutron diffraction and IR spectroscopic measurements. Given that the Group 3-5 elements favor ionic/metallic bonding with H, along with the current results, the true boundary for the formation of homoleptic transition-metal hydride complexes should be between Group 5 and 6. As the H coordination number generally tends to increase with decreasing atomic number of transition metals, the revised boundary suggests high potential for further discovery of hydrogen-rich materials that are of both technological and fundamental interest.

Features of the electronic structure of adducts of transition metal hydride complexes (Cp*M(dppe)H, dppe is the 1,2-(diphenylphosphino)ethane, M = Fe, Ru, Os; CpM(CO)3H, M = Mo, W) with acids and bases were analyzed with the ADF2014 program using energy decomposition analysis (EDA) by the Ziegler-Rauk method combined with the natural orbitals for chemical valence theory (ETS-NOCV). The nature of orbital interactions in the complex determines the reaction pathway: σMH → σ*OH interaction leads to the proton transfer to hydride ligand, nM → σ*OH leads to the metal atom protonation, nN → σ*MH implies the metal hydride deprotonation, and σMH → n*B corresponds to the hydride transfer to Lewis acid. It was shown that M-H bond polarization change has the similar character upon the formation of complexes with Bronsted and Lewis acids. The ease of polarization of M-H bonds in complexes CpM(CO)3H determines their reactivity as proton and hydride ion donors.

Ninefold coordination of hydrogen is very rare, and has been observed in two different hydride complexes comprising rhenium and technetium. Herein, based on a theoretical/experimental approach, we present evidence for the formation of ninefold H- coordination hydride complexes of molybdenum ([MoH9]3−), tungsten ([WH9]3−), niobium ([NbH9]4−) and tantalum ([TaH9]4−) in novel complex transition-metal hydrides, Li5MoH11, Li5WH11, Li6NbH11 and Li6TaH11, respectively. All of the synthesized materials are insulated with band gaps of approximately 4 eV, but contain a sufficient amount of hydrogen to cause the H 1s-derived states to reach the Fermi level. Such hydrogen-rich materials might be of interest for high-critical-temperature superconductivity if the gaps close under compression. Furthermore, the hydride complexes exhibit significant rotational motions associated with anharmonic librations at room temperature, which are often discussed in relation to the translational diffusion of cations in alkali-metal dodecahydro-closo-dodecaborates and strongly point to the emergence of a fast lithium conduction even at room temperature.

This chapter describes the activation of dinitrogen by various transition metal hydride complexes. A number of mononuclear transition metal hydride complexes can incorporate dinitrogen, but they are usually difficult to induce N–N bond cleavage. In contrast, multimetallic hydride complexes can split and hydrogenate dinitrogen through cooperation of the multiple metal hydrides. In this transformation, the hydride ligands serve as the source of both electron and proton, thus enabling the cleavage and hydrogenation of dinitrogen without extra reducing agents and proton sources. Generally, the reactivity of the metal hydride complexes is significantly influenced by their composition (nuclearity) and metal/ligand combination.

A simple equation (pKa(THF) = ∑AL + Ccharge + Cnd + Cd6) can be used to obtain an estimate of the pKa of diamagnetic transition metal hydride and dihydrogen complexes in tetrahydrofuran, and, by use of conversion equations, in other solvents. It involves adding acidity constants AL for each of the ligands in the 5-, 6-, 7-, or 8-coordinate conjugate base complex of the hydride or dihydrogen complex along with a correction for the charge (Ccharge = -15, 0 or 30 for x = +1, 0 or -1 charge, respectively) and the periodic row of the transition metal (Cnd = 0 for 3d or 4d metal, 2 for 5d metal) as well as a correction for d(6) octahedral acids (Cd6 = 6 for d(6) metal ion in the acid, 0 for others) that are not dihydrogen complexes. Constants AL are provided for 13 commonly occurring ligand types; of these, nine neutral ligands are correlated with Lever's electrochemical ligand parameters EL. This method gives good estimates of the over 170 literature pKa values that range from less than zero to 50 with a standard deviation of 3 pKa units for complexes of the metals chromium to nickel, molybdenum, ruthenium to palladium, and tungsten to platinum in the periodic table. This approach allows a quick assessment of the acidity of hydride complexes found in nature (e.g., hydrogenases) and in industry (e.g., catalysis and hydrogen energy applications). The pKa values calculated for acids that have bulky or large bite angle chelating ligands deviate the most from this correlation. The method also provides an estimate of the base strength of the deprotonated form of the complex.

Transition metal hydride nanoparticles are promising hydrogen storage materials as they store and release hydrogen at near-ambient conditions. Their storage capacities can be modulated by adjusting their size, shape, morphology, and composition. It is however unclear why these factors influence hydrogen storage properties. Here, we present a first-principles study on six 55-atom PtPd nanoclusters to elucidate the effects of nanoparticle morphology and composition on the hydrogen storage characteristics of small nanoclusters. We employ the cluster expansion methodology to analyze how the spatial distribution and energetics of hydrogen adsorption and absorption evolve as a function of hydrogen loading. Our results reveal that at near-ambient pressures and above, hydrogen is actively stored in both the surface—top sites on Pt, and bridge sites on Pd—and the subsurface. The hydrogen storage capacity is controlled by the binding strength of hydrogen, itself influenced by two factors: i) the interaction energy between hydrogen atoms, which dictates how binding strength varies with increasing loading; and ii) the intrinsic binding affinity of a metal cluster for hydrogen, which dictates the magnitude of the nearly-uniform offsets in binding strength across all hydrogen loadings. Bimetallic clusters with alloyed shells and core-shell morphologies modulate these factors by tuning the distribution of hydrogen binding strengths on the surface and subsurface respectively. We shed light on the factors that affect hydrogen storage in clusters and provide insights for the rational design of metal clusters with maximal hydrogen storage capacities.

Metal hydrides are suitable for the compact, efficient and safe storage of hydrogen. Considering hydride-based hydrogen storage tanks, the enhancement of the heat and gas transport properties of the hydride bed is crucial for increased (un-)loading dynamics of the tank. In this contribution, pelletized composites of different hydrogen storage materials (lithium amide, sodium alanate, magnesium hydride and transition metal hydride Hydralloy C5) with expanded natural graphite (ENG) are discussed. The materials were admixed with up to 25 wt.% ENG and compacted at compaction pressures up to 600 MPa. The resulting hydride-ENG pellets exhibit an increased effective thermal conductivity which can be tuned in a wide range. The pellets have an increased volumetric H 2 storage capacity compared to loose hydride powders. High gas permeability in radial direction and sufficient thermal conductivity (>10 W m −1 K −1 ) in combination with a stable pellet structure indicate a high potential to use suchlike prepared hydride-ENG composites for hydrogen storage applications with high loading dynamics.