Understanding the Mechanisms of Unusually Fast HH,CH,and CC Bond Reductive Eliminations from Gold(III) Complexes

Carbon–carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co‐workers have demonstrated extremely rapid C?C reductive elimination from cis‐[AuPPh₃(4‐F‐C₆H₄)₂Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy‐atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis‐[Au(PPh₃)₂(4‐F‐C₆H₄)₂]⁺ was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for H?H, C?H, and C?C bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp³ carbon atoms. Metal–carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis‐[AuPPh₃(H)CH₃]⁺ predict that at ?52 °C, about 82 % of the reaction occurs by hydrogen‐atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.     

Methane Activation by Diatomic Molybdenum Carbide Cations

Metal carbide species have been proposed as a new type of chemical entity to activate methane in both gas‐phase and condensed‐phase studies. Herein, methane activation by the diatomic cation MoC⁺ is presented. MoC⁺ ions have been prepared and mass‐selected by a quadrupole mass filter and then allowed to interact with methane in a hexapole reaction cell. The reactant and product ions have been detected by a reflectron time‐of‐flight mass spectrometer. Bare metal Mo⁺ and MoC₂H₂⁺ ions have been observed as products, suggesting the occurrence of ethylene elimination and dehydrogenation reactions. The branching ratio of the C₂H₄ elimination channel is much larger than that of the dehydrogenation channel. Density functional theory calculations have been performed to explore in detail the mechanism of the reaction of MoC⁺ with CH₄. The computed results indicate that the ethylene elimination process involves the occurrence of spin conversions in the C?C coupling (doublet→quartet) and hydrogen atom transfer (quartet→sextet) steps. The carbon atom in MoC⁺ plays a key role in methane activation because it becomes sp³ hybridized in the initial stages of the ethylene elimination reaction, which leads to much lower energy barriers and more stable intermediates. This study provides insights into the C?H bond activation and C?C coupling involved in methane transformation over molybdenum carbide‐based catalysts.     

Mechanistic Details of the Fe+-Mediated C?C and C?H Bond Activations in Propane: A Theoretical Investigation

Quantum-chemical calculations employing a density-functional theory/Hartree-Fock hybrid method (B3LYP) have been used to explore the mechanistic details of the C? C and C? H bond-activation processes in propane mediated by a bare Fe⁺ ion. While the theoretically predicted results are in complete accord with all available experimental data, they give rise to a different mechanistic picture than envisaged previously. In contrast to earlier speculation, the activation barriers for the initial insertion steps of Fe⁺ into a C? H or C? C bond are found to be significantly below the Fe⁺ + C₃H₈ channel. The rate-determining steps for both, the C? C and the C? H bond activation branches of the [FeC₃H₈]⁺ potential-energy surface rather occur late on the respective reaction coordinates and are connected with saddle points of concerted rearrangement processes. The C? C bond activation, which leads to the exothermic reductive elimination of methane, occurs via the C? C inserted species and not as a side channel originating from a C? H inserted ion, as assumed hitherto. For the C? H bond-activation processes, which finally results in the exothermic expulsion of molecular hydrogen, two energetically similar reaction channels for an [1,2]-elimination exist. The results clearly show, that an [1,3]-H₂-elimination mechanism cannot compete with the [1,2]-elimination paths, in line with the experimental findings. Overall, a lower energy demand for the reductive elimination of methane compared to the loss of H₂ is obtained, straightforwardly explaining the preference of the former process observed experimentally.     

On the Activation of Methane and Carbon Dioxide by [HTaO]+ and [TaOH]+ in the Gas Phase: A Mechanistic Study

The thermal reactions of [Ta,O,H]⁺ with methane and carbon dioxide have been investigated experimentally and theoretically by using electrospray ionization mass spectrometry (ESI MS) and density functional theory calculations. Although the activation of methane proceeds by liberation of H₂, the activation of CO₂ gives rise to the formation of [OTa(OH)]⁺ under the elimination of CO. Computational studies of the reactions of methane and carbon dioxide with the two isomers of [Ta,O,H]⁺, namely, [HTaO]⁺ and [Ta(OH)]⁺, have been performed to elucidate mechanistic aspects and to explain characteristic reaction patterns.     

Mechanistic Studies on the Gas‐Phase Dehydrogenation of Alkanes at Cyclometalated Platinum Complexes

In the ion/molecule reactions of the cyclometalated platinum complexes [Pt(L? H)]⁺ (L=2,2′‐bipyridine (bipy), 2‐phenylpyridine (phpy), and 7,8‐benzoquinoline (bq)) with linear and branched alkanes C_nH_2n+2 (n=2–4), the main reaction channels correspond to the eliminations of dihydrogen and the respective alkenes in varying ratios. For all three couples [Pt(L? H)]⁺/C₂H₆, loss of C₂H₄ dominates clearly over H₂ elimination; however, the mechanisms significantly differs for the reactions of the “rollover”‐cyclometalated bipy complex and the classically cyclometalated phpy and bq complexes. While double hydrogen‐atom transfer from C₂H₆ to [Pt(bipy? H)]⁺, followed by ring rotation, gives rise to the formation of [Pt(H)(bipy)]⁺, for the phpy and bq complexes [Pt(L? H)]⁺, the cyclometalated motif is conserved; rather, according to DFT calculations, formation of [Pt(L? H)(H₂)]⁺ as the ionic product accounts for C₂H₄ liberation. In the latter process, [Pt(L? H)(H₂)(C₂H₄)]⁺ (that carries H₂ trans to the nitrogen atom of the heterocyclic ligand) serves, according to DFT calculation, as a precursor from which, due to the electronic peculiarities of the cyclometalated ligand, C₂H₄ rather than H₂ is ejected. For both product‐ion types, [Pt(H)(bipy)]⁺ and [Pt(L? H)(H₂)]⁺ (L=phpy, bq), H₂ loss to close a catalytic dehydrogenation cycle is feasible. In the reactions of [Pt(bipy? H)]⁺ with the higher alkanes C_nH_2n+2 (n=3, 4), H₂ elimination dominates over alkene formation; most probably, this observation is a consequence of the generation of allyl complexes, such as [Pt(C₃H₅)(bipy)]⁺. In the reactions of [Pt(L? H)]⁺ (L=phpy, bq) with propane and n‐butane, the losses of the alkenes and dihydrogen are of comparable intensities. While in the reactions of “rollover”‐cyclometalated [Pt(bipy? H)]⁺ with C_nH_2n+2 (n=2–4) less than 15 % of the generated product ions are formed by C? C bond‐cleavage processes, this value is about 60 % for the reaction with neo‐pentane. The result that C? C bond cleavage gains in importance for this substrate is a consequence of the fact that 1,2‐elimination of two hydrogen atoms is no option; this observation may suggest that in the reactions with the smaller alkanes, 1,1‐ and 1,3‐elimination pathways are only of minor importance.     

“Roll‐over” Cyclometalation of 2,2′‐Bipyridine Platinum(II) Complexes in the Gas Phase: A Combined Experimental and Computational Study

In a combined experimental/computational investigation, the gas‐phase behavior of cationic [Pt(bipy)(CH₃)((CH₃)₂S)]⁺ ( 1 ) (bipy=2,2′‐bipyridine) has been explored. Losses of CH₄ and (CH₃)₂S from 1 result in the formation of a cyclometalated 2,2′‐bipyrid‐3‐yl species [Pt(bipy?H)]⁺ ( 2 ). As to the mechanisms of ligand evaporation, detailed labeling experiments complemented by DFT‐based computations reveal that the reaction follows the mechanistically intriguing “roll‐over” cyclometalation path in the course of which a hydrogen atom from the C(3)‐position is combined with the Pt‐bound methyl group to produce CH₄. Activation of a C? H‐bond of the (CH₃)₂S ligand occurs as well, but is less favored (35 % versus 65 %) as compared to the C(3)? H bond activation of bipy. In addition, the thermal ion/molecule reactions of [Pt(bipy?H)]⁺ with (CH₃)₂S have been examined, and for the major pathway, that is, the dehydrogenative coupling of the two methyl groups to form C₂H₄, a mechanism is suggested that is compatible with the experimental and computational findings. A hallmark of the gas‐phase chemistry of [Pt(bipy?H)]⁺ with the incoming (CH₃)₂S ligand is the exchange of one (and only one) hydrogen atom of the bipy fragment with the C? H bonds of dimethylsulfide in a reversible “roll‐over” cyclometalation reaction. The Pt^II‐mediated conversion of (CH₃)₂S to C₂H₄ may serve as a model to obtain mechanistic insight in the dehydrosulfurization of sulfur‐containing hydrocarbons.     

Thermal Ethane Activation by Bare [V2O5]+ and [Nb2O5]+ Cluster Cations: on the Origin of Their Different Reactivities

The gas‐phase reactivity of [V₂O₅]⁺ and [Nb₂O₅]⁺ towards ethane has been investigated by means of mass spectrometry and density functional theory (DFT) calculations. The two metal oxides give rise to the formation of quite different reaction products; for example, the direct room‐temperature conversions C₂H₆→C₂H₅OH or C₂H₆→CH₃CHO are brought about solely by [V₂O₅]⁺. In distinct contrast, for the couple [Nb₂O₅]⁺/C₂H₆, one observes only single and double hydrogen‐atom abstraction from the hydrocarbon. DFT calculations reveal that different modes of attack in the initial phase of C?H bond activation together with quite different bond‐dissociation energies of the M?O bonds cause the rather varying reactivities of [V₂O₅]⁺ and [Nb₂O₅]⁺ towards ethane. The gas‐phase generation of acetaldehyde from ethane by bare [V₂O₅]⁺ may provide mechanistic insight in the related vanadium‐catalyzed large‐scale process.     

An electron impact study of HCN elimination from indolizine by use of 13C labelling

The study of the loss of HCN from the molecular ions of [1-¹³C]-, [2-¹³C]- and [3-¹³C]-indolizine shows that, if the C-3 atom is eliminated predominantly, as may be expected, the C-2 atom, and (a) carbon atom(s) of the hexagonal ring are also involved. The losses of ¹³CCH₃^. and C₂H₃^. from the [M? H¹²CN]^+˙ ions of the three compounds point to the interference of distinct mechanisms of HCN elimination, leading to different structures for the [C₇H₆]^+˙ ions.     

Ab initio study of C4H3 potential energy surface and reaction of ground‐state carbon atom with propargyl radical

The potential energy surface for the reaction of the ground‐state carbon atom [C(³P_j)] with the propargyl radical [HCCCH₂(X²B₁)] is investigated using the G2M(RCC,MP2) method. Numerous local minima and transition states for various isomerization and dissociation pathways of doublet C₄H₃ are studied. The results show that C(³P_j) attacks the π system of the propargyl radical at the acetylenic carbon atom and yields the n‐C₄H₃(²A′) isomer i3 after an 1,2‐H atom shift. This intermediate either splits a hydrogen atom and produces singlet diacetylene, [HCCCCH ( p1 )+H] or undergoes (to a minor amount) a 1,2‐H migration to i‐C₄H₃(²A′) i5 , which in turn dissociates to p1 plus an H atom. Alternatively, atomic carbon adds to the triple C?C bond of the propargyl radical to form a three‐member ring C₄H₃ isomer i1 , which ring opens to i3 . Diacetylene is concluded to be a nearly exclusive product of the C(³P_j)+HCCCH₂ reaction. At the internal energy of 10.0 kcal/mol above the reactant level, Rice–Ramsperger–Kassel–Marcus calculations show about 91.7% of HCCCCH comes from fragmentation of i3 and 8.3% from i5 . The other possible minor channels are identified as HCCCC+H₂ and C₂H+HCCH. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1522–1535, 2001     

The Unique Gas‐Phase Chemistry of the [AuO]+/CH4 Couple: Selective Oxygen‐Atom Transfer to,Rather than Hydrogen‐Atom Abstraction from,Methane

The thermal reaction of [AuO]⁺ with methane has been explored using FT‐ICR mass spectrometry complemented by high‐level quantum chemical calculations. In contrast to the previously studied congener [CuO]⁺, and to [AgO]⁺, [AuO]⁺ reacts with CH₄ exclusively via oxygen‐atom transfer to form CH₃OH, and a novel mechanistic scenario for this selective oxidation process has been revealed. Also, the origin of the inertness of the [AgO]⁺/CH₄ couple has been addressed computationally.     

On the Origin of the Distinctly Different Reactivity of Ruthenium in [MO]+/CH4 Systems (M=Fe,Ru, Os)

The thermal gas‐phase reactions of [RuO]⁺ with methane have been explored by FT‐ICR mass spectrometry and high‐level quantum‐chemical calculations. In contrast to the previously studied [FeO]⁺/CH₄ and [OsO]⁺/CH₄ couples, which undergo oxygen/hydrogen atom transfers and dehydrogenation, respectively, the [RuO]⁺/CH₄ system produces selectively [Ru(CH)₂]⁺ and H₂O, albeit with much lower efficiency. Various mechanistic scenarios were uncovered, and the associated electronic origins were revealed by high‐level quantum‐chemical calculations. The reactivity differences observed for the [MO]⁺/CH₄ couples (M=Fe, Ru, Os) are due to the subtle interplay of the spin–orbit coupling efficiency, orbital overlap, and relativistic effects.     

Functional group interaction in the fragmentation of protonated 2,7-octanedione

The fragmentation of 2,7-octanedione, induced by chemical ionization with methane as a reagent gas (CI (CH₄)), is shown to be extensively governed by the interaction of the two carbonyl groups. Tandem mass spectrometry reveals that a sequential loss of H₂O and C₂H₄O from the [M + H]⁺ ion competes with sequential loss of H₂O and C₆H₁₀, and that both processes occur via the same [MH - H₂O]⁺ intermediate. This intermediate is likely to be formed via intramolecular gas-phase aldol condensation and subsequent dehydration. The resulting C(1) protonated 1-acetyl-2-methylcyclopentene structure readily accounts for the observed further decomposition to CH₃C?O⁺ and 1-methylcyclopentene (C₆H₁₀) or, alternatively, to [C₆H₉]⁺ (e. g. 1-methylcyclopentenylium) ions and acetaldehyde (C₂H₄O). Support for this mechanistic rationale is derived from deuterium isotope labelling and low-energy collision-induced dissociation (CID) of the [MH - H₂O]⁺ ion. The common intermediate shows a CID behaviour indistinguishable by these techniques from that of reference ions, which are produced by gas-phase protonation of the authentic cyclic aldol or by gas-phase addition of an acetyl cation to 1-methylcyclopentene in a CI (CH₃COOCH₃) experiment.     

Chemical ionization of cyclopropyl ethers II—the nature of the alkyl group fragmentationt

Oxygen-alkyl cleavage is ruled out in the methane chemical ionization- and electron mpact-induced decomposition of cyclopropyl ethers by the finding that for trans,trans-2,3-diethylmethoxycyclopropane the [M ? C₂H₅·]+ ion is more intense than the [M ? CH₃·]+ ion. The possibility for [M + H ? C₂H₆]⁺ is discounted by comparison with the methane chemical ionization nass spectrum of tran,tran-2,3-dimethylmethoxycyclopropane. The isobutane chemical ionization nass spectrum of the diethylcyclopropyl methyl ether affords nearly exclusive electrocyclic methanol fragmentation, i.e. [M + H ? CH₃OH]⁺.     

Reactive Dimerization of an N‐Heterocyclic Plumbylene: C−H Activation with PbII

The N‐heterocyclic plumbylene [Fe{(η⁵‐C₅H₄)NSiMe₃}₂Pb:] is in equilibrium with an unprecedented dimer in solution, whose formation involves the cleavage of a strong C?H bond and concomitant formation of a Pb?C and an N?H bond. According to a mechanistic DFT assessment, dimer formation does not involve direct Pb^II insertion into a cyclopentadienyl C?H bond, but is best described as an electrophilic substitution. The bulkier plumbylene [Fe{(η⁵‐C₅H₄)NSitBuMe₂}₂Pb:] shows no dimerization, but compensates its electrophilicity by the formation of an intramolecular Fe?Pb bond.     

Dichloro­[(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)methane]zinc(II) and di‐μ‐chloro‐bis­{chloro­[(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)methane]cadmium(II)}

The Zn atom in dichloro­[(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)­methane]zinc(II), [ZnCl₂(C₁₁H₁₆N₄)], (I), is tetra­hedrally coordinated by two N atoms from one bis­(3,5‐dimethyl­pyrazol­yl)methane ligand and two terminal Cl atoms. The mol­ecule has no crystallographic symmetry. One H atom of the CH₂ group of the bis­(3,5‐dimethyl­pyrazol­yl)methane ligand inter­acts with a Cl atom of an adjacent mol­ecule to yield inter­molecular C—H⋯Cl contacts, thereby forming a one‐dimensional zigzag chain extending along the b axis. On the other hand, in di‐μ‐chloro‐bis­{chloro­[(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)methane]cadmium(II)}, [Cd₂Cl₄(C₁₁H₁₆N₄)₂], (II), each of the two crystallographically equivalent Cd atoms is penta­coordinated by two N atoms from one bis­(3,5‐dimethyl­pyrazol­yl)methane ligand, and by one terminal and two bridging Cl⁻ anions. The mol­ecule has a crystallographic centre of symmetry located at the mid‐point of the Cd⋯Cd line. One H atom of the CH₂ group of the bis­(3,5‐dimethyl­pyrazolyl)­methane ligand inter­acts with a Cl atom of an adjacent mol­ecule to produce pairwise inter­molecular C—H⋯Cl contacts, thereby affording chains of mol­ecules running along the c axis.     

Loss of methyl from ionized 2-hydroxy-1,3-butadiene

The loss of methyl from unstable, metastable and collisionally activated [CH₂?CH? C(OH)?CH₂]⁺˙ ions (1⁺˙) was examined by means of deuterium and ¹³C labelling, appearance energy measurements and product identification. High-energy, short-lived 1⁺˙ lose methyl groups incorporating the original enolic methene (C(1)) and the hydroxyl hydrogen atom (H(0)). The eliminations of C(1)H(1)H(1)H(4) and C(4)H(4)H(4)H(0) are less frequent in high-energy ions. Metastable 1⁺˙ eliminate mainly C(1)H(1)H(1)H(4), the elimination being accompanied by incomplete randomization of the five carbon-bound hydrogen atoms. The resulting [C₃H₃O]⁺ ions have been identified as the most stable CH₂?CH? CO⁺ species. The appearance energy for the loss of methyl from 1 was measured as AE[C₃H₃O]⁺ = 10.47 ± 0.05 eV. The critical energy for 1⁺˙ → [C₃H₃O]⁺ + CH₃˙ is assessed as E_c ? 173 kJ mol^?1. Reaction mechanisms are proposed and discussed.     

An investigation of the mechanism of single and double hydrogen atom transfer reactions in alkyl benzoates by the ortho effect

Single and double hydrogen atom transfers in reactions (1) and (2) in the mass spectra of ethyl benzoate, isopropyl benzoate, and isobutyl benzoate have been investigated with reference to the ortho effect: (1) [C₆H₅CO₂R]⁺? → [C₆H₅CO₂H]⁺? (m/z 122) + (R-H); (2) [C₆H₅CO₂R]⁺? → [C₆H₅CO₂H₂]⁺ (m/z 123) + · (R-2H). It is demonstrated that the intermediate ion [C₆H₅CO₂H₂]⁺ has the protonated benzoic acid structure with the hydrogen atom on the carbonyl group.     

A Reaction-Induced Localization of Spin Density Enables Thermal C−H Bond Activation of Methane by Pristine FeC4+

The reactivity of the cationic metal-carbon cluster FeC₄⁺ towards methane has been studied experimentally using Fourier-transform ion cyclotron resonance mass spectrometry and computationally by high-level quantum chemical calculations. At room temperature, FeC₄H⁺ is formed as the main ionic product, and the experimental findings are substantiated by labeling experiments. According to extensive quantum chemical calculations, the C−H bond activation step proceeds through a radical-based hydrogen-atom transfer (HAT) mechanism. This finding is quite unexpected because the initial spin density at the terminal carbon atom of FeC₄⁺, which serves as the hydrogen acceptor site, is low. However, in the course of forming an encounter complex, an electron from the doubly occupied sp-orbital of the terminal carbon atom of FeC₄⁺ migrates to the singly occupied π*-orbital; the latter is delocalized over the entire carbon chain. Thus, a highly localized spin density is generated in situ at the terminal carbon atom. Consequently, homolytic C−H bond activation occurs without the obligation to pay a considerable energy penalty that is usually required for HAT involving closed-shell acceptor sites. The mechanistic insights provided by this combined experimental/computational study extend the understanding of methane activation by transition-metal carbides and add a new facet to the dizzying mechanistic landscape of hydrogen-atom transfer.     

Transition‐Metal Complexes Based on the 1,3,5‐Triethynyl Benzene Linking Unit

The synthesis of a unique series of heteromultinuclear transition metal compounds is reported. Complexes 1‐I‐3‐Br‐5‐(FcC≡C)‐C₆H₃ ( 4 ), 1‐Br‐3‐(bpy‐C≡C)‐5‐(FcC≡C)‐C₆H₃ ( 6 ), 1,3‐(bpy‐C≡C)₂‐5‐(FcC≡C)‐C₆H₃ ( 7 ), 1‐(XC≡C)‐3‐(bpy‐C≡C)‐5‐(FcC≡C)‐C₆H₃ ( 8 , X = SiMe₃; 9 , X = H), 1‐(HC≡C)‐3‐[(CO)₃ClRe(bpy‐C≡C)]‐5‐(FcC≡C)‐C₆H₃ ( 11 ), 1‐[(Ph₃P)AuC≡C]‐3‐[(CO)₃ClRe(bpy‐C≡C)]‐5‐(FcC≡C)‐C₆H₃ ( 13 ), 1‐[(Ph₃P)AuC≡C]‐3‐(bpy‐C≡C)‐5‐(FcC≡C)‐C₆H₃ ( 14 ), [1‐[(Ph₃PAuC≡C]‐3‐[{[Ti](C≡CSiMe₃)₂}Cu(bpy‐C≡C)]‐5‐(FcC≡C)‐C₆H₃]PF₆ ( 16 ), and [1,3‐[(tBu₂bpy)₂Ru(bpy‐C≡C)]₂‐5‐(FcC≡C)‐C₆H₃](PF₆)₄ ( 18 ) (Fc = (η⁵‐C₅H₄)(η⁵‐C₅H₅)Fe, bpy = 2,2′‐bipyridiyl‐5‐yl, [Ti] = (η⁵‐C₅H₄SiMe₃)₂Ti) were prepared by using consecutive synthesis methodologies including metathesis, desilylation, dehydrohalogenation, and carbon–carbon cross‐coupling reactions. In these complexes the corresponding metal atoms are connected by carbon‐rich bridging units comprising 1,3‐diethynyl‐, 1,3,5‐triethynylbenzene and bipyridyl units. They were characterized by elemental analysis, IR and NMR spectroscopy, and partly by ESI‐TOF mass spectrometry., The structures of 4 and 11 in the solid state are reported. Both molecules are characterized by the central benzene core bridging the individual transition metal complex fragments. The corresponding acetylide entities are, as typical, found in a linear arrangement with representative M–C, C–C_C≡C and C≡C bond lengths.     

Snapshots of a Migrating H‐Atom: Characterization of a Reactive Iron(III) Indenide Hydride and its Nearly Isoenergetic Ring‐Protonated Iron(I) Isomer

We report the characterization of an S= iron π‐complex, [Fe(η⁶‐IndH)(depe)]⁺ (Ind=Indenide (C₉H₇^?), depe=1,2‐bis(diethylphosphino)ethane), which results via C?H elimination from a transient Fe^III hydride, [Fe(η³:η²‐Ind)(depe)H]⁺. Owing to weak M?H/C?H bonds, these species appear to undergo proton‐coupled electron transfer (PCET) to release H₂ through bimolecular recombination. Mechanistic information, gained from stoichiometric as well as computational studies, reveal the open‐shell π‐arene complex to have a BDFE_C‐H value of ≈50 kcal mol^?1, roughly equal to the BDFE_Fe‐H of its Fe^III?H precursor (ΔG°≈0 between them). Markedly, this reactivity differs from related Fe(η⁵‐Cp/Cp*) compounds, for which terminal Fe^III?H cations are isolable and have been structurally characterized, highlighting the effect of a benzannulated ring (indene). Overall, this study provides a structural, thermochemical, and mechanistic foundation for the characterization of indenide/indene PCET precursors and outlines a valuable approach for the differentiation of a ring‐ versus a metal‐bound H‐atom by way of continuous‐wave (CW) and pulse EPR (HYSCORE) spectroscopic measurements.