Metal-hydride Bond Research Articles

Plasma-Assisted One-Step Direct Methanol Conversion to Ethylene Glycol and Hydrogen: Process Intensification

This perspective reports a process intensification strategy that converts methanol into ethylene glycol (MeOH-2-EG) in a single step to circumvent multi-step naphtha cracking into ethylene followed by ethylene epoxidation to ethylene oxide (EO) and the subsequent hydrolysis of EO to ethylene glycol (EG). Due to the thermodynamic restriction for the direct MeOH-2-EG, plasma-assisted catalysis was introduced, and platinum group metals were identified as prospective transition metal catalysts that can achieve the formation of strong metal hydride bonds and guarantee the controlled C–C coupling of two plasma-activated hydroxymethyl radicals (*CH2OH) from methanol, both of which are essential for the single-step MeOH-2-EG.

Reaction of a Bulky β‐Diketiminato Magnesium Hydride with CO2 Yielding a Polynuclear Magnesium Cage Compound

The reaction of a sterically shielded Mg hydride complex, bearing a bulky β‐diketiminato (BDI) ligand, with CO2 leads to the formation of polynuclear complexes. CO2 not only inserts into the metal hydride bond yielding formate ions, but also adds onto the γ‐C of the BDI ligand. This reactivity gives rise to the formation of tri‐ and tetranuclear structures in THF, leading to a hexameric structure associated with a change in the coordination mode of the BDI ligand at 60 °C in THF within four days. The hexameric complex was isolated in high reproducible yield from the solution and characterized by X‐ray diffraction analysis. The reaction affording the hexameric complex was shown to be completed within 1 h at room temperature in toluene. Similar reactions of CO2 with a BDI calcium hydride complex proceeds less cleanly and a pure product could not be isolated.

Kinetic Isotope Effects and the Mechanism of CO2 Insertion into the Metal-Hydride Bond of fac-(bpy)Re(CO)3H.

The 1,2-insertion reaction of CO2 into metal-hydride bonds of d6-octahedral complexes to give κ1-O-metal-formate products is the key step in various CO2 reduction schemes and as a result has attracted extensive mechanistic investigations. For many octahedral catalysts, CO2 insertion follows an associative mechanism in which CO2 interacts directly with the coordinated hydride ligand instead of the more classical dissociative mechanism that opens an empty coordination site to bind the substrate to the metal prior to a hydride migration step. To better understand the associative mechanism, we conducted a systematic quantum chemical investigation on the reaction between CO2 and fac-(bpy)Re(CO)3H (1-Re-H; bpy = 2,2'-bipyridine) starting with the gas phase and then moving to THF and other solvents with increased dielectric constants. Detailed analyses of the potential energy surfaces (PESs) and intrinsic reaction coordinates (IRCs) reveal that the reaction is enabled in all media by an initial stage of making a 3c-2e bond between the carbon of CO2 and the metal-hydride bond that is most consistent with an organometallic bridging hydride Re-H-CO2 species. Once CO2 is bent and anchored to the metal-hydride bond, the reaction proceeds by a rotation motion via a cyclic transition state TS2 that interchanges Re-H-CO2 and Re-O-CHO coordination. The combined stages provide an asynchronous-concerted pathway for CO2 insertion on the Gibbs free energy surface with TS2 as the highest energy point. Consideration of TS2 as a rate-determining TS gives activation barriers, inverse KIEs, substituent effects, and solvent effects that agree with the experimental data available in this system. An important new insight revealed by the analyses of the results is that the initial stage of the reaction is not a hydride transfer step as has been assumed in some studies. In fact, the loose vibration of the TS that can be identified for the first stage of the reaction in solution (TS1) does not involve the Re-H stretching vibrational mode. Accordingly, the imaginary frequency of TS1 is insensitive to deuteration, and therefore, TS1 leads to no significant KIE.

Reactivity of metal hydrides with CO2: going beyond formate with a high-valent cationic pentahydride Mo(vi) complex.

The cationic molybdenum pentahydride complex [MoH5(depe)2]+ (depe = 1,2-bis(diethylphosphino)ethane) is shown to undergo two consecutive reactions with carbon dioxide. In the initial, room-temperature process, classical insertion of CO2 into a metal-hydride bond is observed, resulting in the formation of the expected formate complex, [MoH2(HCOO)(depe)2]+. Further reactivity is triggered at temperature above 100 °C. Complete conversion into two new complexes is indeed observed, resulting from the formal cleavage of a C-O bond of carbon dioxide, [MoH(CO)2(depe)2]+ and [MoO(HCOO)(depe)2]+. Unprecedented in the absence of ligand assistance, such metal hydride reactivity has been comprehensively studied by a combination of experimental and computational means with the aim of elucidating the underlying mechanism that governs this process.

Role of alkaline-earth metal in catalysed imine hydrogenations

Alkaline-earth metal (Ae) catalysts have been recently developed for challenging imine and alkene hydrogenation at moderate reaction conditions, providing a sustainable alternative to transition metal catalysis. The understanding of catalytic hydrogenations mediated by group 2 metals is underdeveloped and mechanistic studies are scarce from experimental and computational sides. Herein, we examine the role of the metal on the catalytic hydrogenation of imines by Ae[N(SiMe3)2]2 (Ae = Mg, Ca, Sr, Ba) using state-of-the-art computational techniques. Trends in energy barriers and turnover frequencies agree remarkably well with the experimentally observed increase in catalytic activity upon descending group 2 (Mg ≪ Ca < Sr < Ba). Structural and chemical bonding differences in the key intermediates were found to be the main driving force behind the enhanced reactivity of heavier Ae catalysts. More specifically, the N-Ae-H^ bond angle is drastically reduced in the Ca, Sr, and Ba catalytic species driven by the participation of the d-orbitals in the chemical bonding. The activation strain model reveals that these catalytic reactions are strain controlled and the higher activation barriers for the Mg catalyst originates from unfavourable bond angles in the Mg hydride species featuring linear structures and a more covalent metal-hydride bond. Further decomposition of the interaction energy reveals that stronger repulsive interactions destabilize the Mg species, indicating that the steric congestion due to the small Mg centre impedes reaction kinetics. Overall, the different aspects to be considered in the Ae catalyst design for imine hydrogenations are the strength and flexibility of the Ae-H bond, the bond ionicity, the N-Ae-H^ angle and the strength of the noncovalent interactions in the TOF-determining intermediate.

Mechanism of formic acid dehydrogenation catalysed by Cp*Co(III) and Cp*Rh(III) complexes with N,N’-bidentate imidazoline-based ligands: A DFT exploration

Homogenous formic acid dehydrogenation (FAD) is largely accomplished with the aid of Ir and Rh based catalysts. The use of earth-abundant transition-metal based catalysts for FAD are rather scarcely reported. This work computationally explores the mechanistic aspects of FAD catalysed by Cp*Co(III) and Cp*Rh(III) -based catalysts with pyridyl-imidazoline (L1) and 2,2’-biimidazoline (L2) ligand architectures. The computed free energy profiles substantiate that the β-hydride elimination is the slowest step in the FAD process. Further mechanistic analysis revealed that H3O+ is the energetically favoured proton source compared to formic acid (FA) for generating H2. NBO analysis signified that the proton from H3O+ ion is relatively more electrophilic than FA and that the metal-hydride bond in the transition state is more negatively charged in the transition state involving H3O+ ion. These factors make the protonation process easier and decreases the barrier for hydronium ion mediated protonation process. All the four designed catalysts were found to possess a moderate range of computed activation barrier (13.5 kcal/mol to 16.2 kcal/mol) at the rate-determining step which are comparable with their corresponding iridium analogues reported in the earlier studies. The Co and Rh catalysts designed in the present work with L1 ligand system were found to be relatively better candidates than L2. Particularly, the Co catalysts and the mechanistic details derived there in would help in the development of efficient earth-abundant transition metal-based catalysts for FAD.

Experimental and Theoretical Study of N2 Adsorption on Hydrogenated Y2C4H- and Dehydrogenated Y2C4- Cluster Anions at Room Temperature.

The adsorption of atmospheric dinitrogen (N2) on transition metal sites is an important topic in chemistry, which is regarded as the prerequisite for the activation of robust N≡N bonds in biological and industrial fields. Metal hydride bonds play an important part in the adsorption of N2, while the role of hydrogen has not been comprehensively studied. Herein, we report the N2 adsorption on the well-defined Y2C4H0,1− cluster anions under mild conditions by using mass spectrometry and density functional theory calculations. The mass spectrometry results reveal that the reactivity of N2 adsorption on Y2C4H− is 50 times higher than that on Y2C4− clusters. Further analysis reveals the important role of the H atom: (1) the presence of the H atom modifies the charge distribution of the Y2C4H− anion; (2) the approach of N2 to Y2C4H− is more favorable kinetically compared to that to Y2C4−; and (3) a natural charge analysis shows that two Y atoms and one Y atom are the major electron donors in the Y2C4− and Y2C4H− anion clusters, respectively. This work provides new clues to the rational design of TM-based catalysts by efficiently doping hydrogen atoms to modulate the reactivity towards N2.

Ferralactone formation from iron acetylides and carbon dioxide

CO2 reacts rapidly with the acetylide iron hydride trans- [FeH(C≡CH)(dmpe)2] by insertion into the metal hydride bond. However, this reaction is reversible and the final products of the reaction are the ferralactones cis- [Fe(OC(=O)CHCH-κ2C,O)(dmpe)2] and cis- [Fe(OC(=O)C(C(=O)OH)CH-κ2C,O)(dmpe)2] resulting from single and double CO2 addition to the β-carbon of the coordinated acetylene respectively, followed by cyclization. Reaction of the t-butylacetylide complex trans- [FeH(C≡CtBu)(dmpe)2] with CO2 affords the t‑butyl‑substituted ferralactone cis- [Fe(OC(=O)C(tBu)CH-κ2C,O)(dmpe)2]. Reaction of CO2 with trans- [FeH(C≡CPh)(dmpe)2] forms the phenyl-substituted ferralactone cis- [Fe(OC(=O)C(Ph)CH-κ2C,O)(dmpe)2] where the phenyl substituent is attached to the carbon adjacent to the carbonyl group of the metalalactone. Remarkably, reaction of CO2 with the side-on-bound complex Fe(η2-HC≡CPh)(dmpe)2] forms a different phenyl-substituted metalalactone cis- [Fe(OC(=O)CHC(Ph)-κ2C,O)(dmpe)2] where the phenyl substituent is attached to the metal-bound carbon of the metalalactone ring.

Hydricity of 3d Transition Metal Complexes from Density Functional Theory: A Benchmarking Study.

A range of modern density functional theory (DFT) functionals have been benchmarked against experimentally determined metal hydride bond strengths for three first-row TM hydride complexes. Geometries were found to be produced sufficiently accurately with RI-BP86-D3(PCM)/def2-SVP and further single-point calculations with PBE0-D3(PCM)/def2-TZVP were found to reproduce the experimental hydricity accurately, with a mean absolute deviation of 1.4 kcal/mol for the complexes studied.

Cobalt‐Catalyzed Markovnikov‐Type Selective Hydroboration of Terminal Alkynes

AbstractA cobalt‐catalyzed Markovnikov‐type hydroboration of terminal alkynes with HBpin to access α‐alkenyl boronates with good regioselectivity and atom economy is reported. A new ligand has been developed for the cobalt hydride catalyst that has been used for a unique Markovnikov selective insertion of terminal alkynes into metal hydride bond. This operationally simple protocol exhibits excellent functional group tolerance to deliver valuable alkene derivatives.

Cobalt-Catalyzed Markovnikov-Type Selective Hydroboration of Terminal Alkynes.

A cobalt-catalyzed Markovnikov-type hydroboration of terminal alkynes with HBpin to access α-alkenyl boronates with good regioselectivity and atom economy is reported. A new ligand has been developed for the cobalt hydride catalyst that has been used for a unique Markovnikov selective insertion of terminal alkynes into metal hydride bond. This operationally simple protocol exhibits excellent functional group tolerance to deliver valuable alkene derivatives.

Praseodymium cation (Pr+) reactions with H2, D2, and HD: PrH+ bond energy and mechanistic insights from guided ion beam and theoretical studies.

Guided ion beam tandem mass spectrometry was used to study the reactions of the atomic lanthanide praseodymium cation (Pr+) with H2, D2, and HD as a function of collision energy. Modeling the kinetic-energy-dependent endothermic reactions to form PrH+ (PrD+) yields a 0 K bond dissociation energy (BDE) of 2.10 ± 0.05 eV for PrH+. Quantum chemical calculations were performed for PrH+ at the B3LYP, BHLYP, PBE0, and coupled-cluster with single, double, and perturbative triple levels of theory, and they overestimate the PrH+ experimental BDE by 0.06 -0.28 eV. The branching ratio of the PrH+ and PrD+ products in the HD reaction suggests that the reaction occurs via a direct reaction mechanism with short-lived intermediates. This is consistent with the theoretical calculations for the relaxed potential energy surfaces of PrH2 +, where no strongly bound dihydride intermediates were found. The reactivity and PrH+ BDE are compared with previous results for lanthanide metal cations (La+, Ce+, Sm+, Gd+, and Lu+). Periodic trends across the lanthanide series and insights into the role of the electronic configuration on metal-hydride bond strength are discussed.

Mechanistic insights into the reaction Cp2Nb(CO)H (Cp = η5-C5H5) with acetylenedicarboxylic acid (ADCA): DFT studies

The insertions of unsaturated molecules into metal-hydride bonds are very common steps in industrial catalytic reactions such as olefinic isomerization, hydrogenation and hydroformylation. In this paper, the reaction mechanism of Cp2Nb(CO)H (Cp = η5-C5H5) with acetylenedicarboxylic (ADCA) has been investigated based on density functional theory calculations. Kinetic calculation and topological analyses of electron density have been conducted. The calculated results show that the reaction can proceed easily even at very low temperature. (Z)-pro Cp2Nb(CO)(η1-(Z)–HOOCC = CHCOOH) and the isomer (E)-pro can be got via two parallel steps. The yield rate of (E)-pro is larger than that of (Z)-pro because of its lower energy barrier. In addition, the carboxylato-containing niobocene compound Cp2Nb(CO)(k1-OOCC(H)= CHCOOH) can also be got as the most stable product through a proton transfer process. Along the reaction pathway, there is a η2-(C, C)-Nb agostic structure. The results provide useful informations and theoretical predictions on much more insertion reactions.

Scandium Tetrahedron Supported by H Anion and CN Pentaanion inside Fullerene C80.

Endohedral metallofullerenes have greatly expanded the range of the fullerene family due to their nesting structure and unusual encapsulated clusters protected by a fullerene cage. Herein, we report a metallofullerene Sc4CNH@Ih-C80, which has a scandium tetrahedron supported by H and CN anions inside fullerene C80. Sc4CNH@Ih-C80 has a rare multilayer nesting structure, and the internal Sc4CNH is the most complex endohedral cluster disclosed to date. Sc4CNH@Ih-C80 has so many bonding types (metal-carbide, metal-nitride, and metal-hydride), which weave a polyhedron of Sc4CNH clusters. This work shows that the endohedral metallofullerenes have the potential to build inorganic nesting polyhedra that have distinctive architectures and unique electronic properties. Sc4CNH@Ih-C80 was synthesized by means of the arc-discharge method using scandium and graphite under the mixed atmosphere of hydrogen, nitrogen, and helium. It is the first time to disclose an unprecedented metal-hydride bond in a fullerene cage. This result shows that the endohedral fullerenes bearing hydrogen species can be synthesized by the arc-discharge technique under an atmosphere of hydrogen. This work demonstrates that a fullerene cage can be an ample carrier to encapsulate unusual cluster moieties.

Cerium Cation (Ce+) Reactions with H2, D2, and HD: CeH+ Bond Energy and Mechanistic Insights from Guided Ion Beam and Theoretical Studies

Reactions of the atomic lanthanide cerium cation (Ce+) with H2, D2, and HD were studied using guided ion beam tandem mass spectrometry. Analysis of the kinetic-energy-dependent endothermic reactions to form CeH+ (CeD+) lead to a 0 K bond dissociation energy (BDE) for CeH+ of 2.19 ± 0.09 eV. Theoretical calculations for CeH+ were performed at the B3LYP, BHLYP, and PBE0 levels of theory and overestimate the experimental BDE. In contrast, extrapolation to the complete basis set limit using coupled-cluster with single, double, and perturbative triple excitations, CCSD(T), gave a value (2.33 eV) in reasonable agreement with the experimental BDE. The branching ratio of the CeH+ and CeD+ products in the HD reaction suggests that the reaction occurs via a statistical mechanism involving a long-lived intermediate. Relaxed potential energy surfaces for CeH2+ were computed and are consistent with the availability of such an intermediate, but the crossing point between quartet and doublet surfaces helps explain the inefficiency of the association reaction observed in the literature. The reactivity and CeH+ BDE are compared with previous results for group 4 transition metal cations (Ti+, Zr+, Hf+), other lanthanides (La+, Sm+, Gd+, Lu+), and the isovalent actinide Th+. Periodic trends and insight into the role of the electronic configuration on metal-hydride bond strength are discussed.

Solution and Solid‐State Structure of the First NHC‐Substituted Rhenium Heptahydrides

The structure of three new bis(N‐heterocyclic carbene) heptahydride rhenium complexes ([ReH7(NHC)2] with NHC= IMes (2b) (1,3‐bis(2,4,6‐trimethyl phenyl) imidazol‐2‐ylidene), IMesCl2 (4b) (4,5‐dichloro‐1,3‐dihydro‐1,3‐bis(2,4,6‐trimethyl phenyl)‐2H‐imidazol‐2‐ylidene) BIMes (5b) (1,3‐Bis(2,4,6‐trimethylphenyl)‐benzo imidazole ylidene), and of the known complex [ReH7(PPh3)2] (1) (bis(triphenyl phosphine)‐rhenium heptahydride), and the one of their labelled counterparts 2c‐d7, 4c‐d7, and 5c‐d7, were established in solution. The compounds exchanged with HD (2 bar) at 60 °C in deuterated THF to produce statistically distributed isotopomers. This exchange pointed to the intermediacy of H2 complexes and dynamic systems with fast equilibration processes of all hydride positions. T1min measurements were run on 2b, 4b, 5b and 1 in deuterated methylcyclohexane: 2b T1min = 144 ms, 5b T1min = 127 ms, both at 203 K. For 4b two T1min values were obtained corresponding to the 1H‐NMR signals of the hydrides: T1min = 119 ms and T1min = 128 ms both at 203 K. The T1min values determined are typical of terminal rhenium polyhydrides and for 1 was reported a comparably short value of T1min (70 ms, [D8]toluene, at –73 °C), typical of complexes that contain dihydrogen ligands. D‐NMR T1min measurements were also run for the fully deuterated compounds 2c‐d7, 4c‐d7, 5c‐d7 and of 1c‐d7 allowing to calculate the ionicities of the metal hydride bonds. The Re‐D ionicity values were obtained from the Deuterium Quadrupole Coupling Constant (DQCC) determinations. They were in the range of 0.61–0.71 for the fully labelled complexes: 2c‐d7: T1min = 15 ms, i = 61 %; 4c‐d7: (A) T1min = 20 ms, i = 67 %, (B) T1min = 41 ms, i = 77 %; 5c‐d7: T1min = 24 ms, i= 70 %; and 1c‐d7: T1min = 26 ms, i = 71 %. Single crystal X‐ray diffraction studies on 4b enabled us to refine the hydride positions with only restraints in the bond lengths and thermal parameters. The X‐ray solid structure resembles a pseudo‐capped square antiprismatic geometry.

Dinitrogen Fixation and Reduction by Ta3N3H0,1– Cluster Anions at Room Temperature: Hydrogen-Assisted Enhancement of Reactivity

Dinitrogen activation and reduction is one of the most challenging and important subjects in chemistry. Herein, we report the N2 binding and reduction at the well-defined Ta3N3H- and Ta3N3- gas-phase clusters by using mass spectrometry (MS), anion photoelectron spectroscopy (PES), and quantum-chemical calculations. The PES and calculation results show clear evidence that N2 can be adsorbed and completely activated by Ta3N3H- and Ta3N3- clusters, yielding to the products Ta3N5H- and Ta3N5-, but the reactivity of Ta3N3H- is five times higher than that of the dehydrogenated Ta3N3- clusters. The detailed mechanistic investigations further indicate that a dissociative mechanism dominates the N2 activation reactions mediated by Ta3N3H- and Ta3N3-; two and three Ta atoms are active sites and also electron donors for the N2 reduction, respectively. Although the hydrogen atom in Ta3N3H- is not directly involved in the reaction, its very presence modifies the charge distribution and the geometry of Ta3N3H-, which is crucial to increase the reactivity. The mechanisms revealed in this gas-phase study stress the fundamental rules for N2 activation and the important role of transition metals as active sites as well as the new significant role of metal hydride bonds in the process of N2 reduction, which provides molecular-level insights into the rational design of tantalum nitride-based catalysts for N2 fixation and activation or NH3 synthesis.

Experimental and Computational Investigation of the Aerobic Oxidation of a Late Transition Metal-Hydride.

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 IrIII hydride complex (dmPhebox)Ir(OAc)(H) (1) in the presence of acetic acid, which proceeds to form the IrIII complex (dmPhebox)Ir(OAc)2(OH2) (2), is described. The evidence supports a multifaceted mechanism wherein a small amount of an initially formed metal hydroperoxide proceeds to generate a metal-oxyl species that then initiates a radical chain reaction to rapidly convert the remaining IrIII-H. Insight into the initiation step was gained through kinetic and mechanistic studies of the radical chain inhibition by BHT (butylated hydroxytoluene). Computational studies were employed to contribute to a further understanding of initiation and propagation in this system.

Understanding the Individual and Combined Effects of Solvent and Lewis Acid on CO2 Insertion into a Metal Hydride.

The insertion of CO2 into a metal hydride bond to form a metal formate is a key elementary step in many catalytic cycles for CO2 conversion. Similarly, the microscopic reverse reaction, the decarboxylation of a metal formate to form a metal hydride and CO2, is important in both organic synthesis and strategies for hydrogen storage using organic liquids. There are however few experimental studies probing the mechanism of these reactions and identifying the effects of specific variables such as Lewis acid (LA) additives or solvent, which have been shown to significantly impact catalytic performance. In this study, we use a rapid mixing stopped-flow instrument to study the kinetics of CO2 insertion into the cationic ruthenium hydride [Ru(tpy)bpy)H]PF6 (tpy = 2,2':6',2″-terpyridine, bpy = 2,2'-bipyridine) in various solvents, both in the presence and in the absence of a LA. We show that LAs can increase the observed rate of this reaction and determine the first quantitative trends for the rate enhancement observed for CO2 insertion in the presence of cationic LAs, Li+ ≫ Na+ > K+ > Rb+. Furthermore, we show that the rate enhancement observed with LAs is solvent dependent. Specifically, as the acceptor number (AN) of the solvent increases, the effect of the LA becomes smaller. Last, we demonstrate that there is a significant solvent effect on CO2 insertion in the absence of a LA. Although the AN of the solvent has been previously used to predict the rate of CO2 insertion, this work shows that the best model for the rate of insertion is based on the Dimroth-Reichardt ET(30) value of the solvent, a parameter that better accounts for specific solute/solvent interactions.

Switching between Stepwise and Concerted Proton-Coupled Electron Transfer Pathways in Tungsten Hydride Activation.

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.