Stabilizing Monoatomic Two-Coordinate Bismuth(I) and Bismuth(II) Using a Redox Noninnocent Bis(germylene) Ligand.

The formation of isolable monatomic BiI complexes and BiII radical species is challenging due to the pronounced reducing nature of metallic bismuth. Here, we report a convenient strategy to tame BiI and BiII atoms by taking advantage of the redox noninnocent character of a new chelating bis(germylene) ligand. The remarkably stable novel BiI cation complex 4, supported by the new bis(iminophosphonamido-germylene)xanthene ligand [(P)GeII(Xant)GeII(P)] 1, [(P)GeII(Xant)GeII(P) = Ph2P(NtBu)2GeII(Xant)GeII(NtBu)2PPh2, Xant = 9,9-dimethyl-xanthene-4,5-diyl], was synthesized by a two-electron reduction of the cationic BiIIII2 precursor complex 3 with cobaltocene (Cp2Co) in a molar ratio of 1:2. Notably, owing to the redox noninnocent character of the germylene moieties, the positive charge of BiI cation 4 migrates to one of the Ge atoms in the bis(germylene) ligand, giving rise to a germylium(germylene) BiI complex as suggested by DFT calculations and X-ray photoelectron spectroscopy (XPS). Likewise, migration of the positive charge of the BiIIII2 cation of 3 results in a bis(germylium)BiIIII2 complex. The delocalization of the positive charge in the ligand engenders a much higher stability of the BiI cation 4 in comparison to an isoelectronic two-coordinate Pb0 analogue (plumbylone; decomposition below -30 °C). Interestingly, 4[BArF] undergoes a reversible single-electron transfer (SET) reaction (oxidation) to afford the isolable BiII radical complex 5 in 5[BArF]2. According to electron paramagnetic resonance (EPR) spectroscopy, the unpaired electron predominantly resides at the BiII atom. Extending the redox reactivity of 4[OTf] employing AgOTf and MeOTf affords BiIII(OTf)2 complex 7 and BiIIIMe complex 8, respectively, demonstrating the high nucleophilic character of BiI cation 4.

Stepwise conversion of the Cys6[4Fe-3S] to a Cys4[4Fe-4S] cluster and its impact on the oxygen tolerance of [NiFe]-hydrogenase.

The membrane-bound [NiFe]-hydrogenase of Cupriavidus necator is a rare example of a truly O2-tolerant hydrogenase. It catalyzes the oxidation of H2 into 2e- and 2H+ in the presence of high O2 concentrations. This characteristic trait is intimately linked to the unique Cys6[4Fe-3S] cluster located in the proximal position to the catalytic center and coordinated by six cysteine residues. Two of these cysteines play an essential role in redox-dependent cluster plasticity, which bestows the cofactor with the capacity to mediate two redox transitions at physiological potentials. Here, we investigated the individual roles of the two additional cysteines by replacing them individually as well as simultaneously with glycine. The crystal structures of the corresponding MBH variants revealed the presence of Cys5[4Fe-4S] or Cys4[4Fe-4S] clusters of different architecture. The protein X-ray crystallography results were correlated with accompanying biochemical, spectroscopic and electrochemical data. The exchanges resulted in a diminished O2 tolerance of all MBH variants, which was attributed to the fact that the modified proximal clusters mediated only one redox transition. The previously proposed O2 protection mechanism that detoxifies O2 to H2O using four protons and four electrons supplied by the cofactor infrastructure, is extended by our results, which suggest efficient shutdown of enzyme function by formation of a hydroxy ligand in the active site that protects the enzyme from O2 binding under electron-deficient conditions.

Structural Determinants of the Catalytic Nia-L Intermediate of [NiFe]-Hydrogenase.

[NiFe]-hydrogenases catalyze the reversible cleavage of H2 into two protons and two electrons at the inorganic heterobimetallic NiFe center of the enzyme. Their catalytic cycle involves at least four intermediates, some of which are still under debate. While the core reaction, including H2/H- binding, takes place at the inorganic cofactor, a major challenge lies in identifying those amino acid residues that contribute to the reactivity and how they stabilize (short-lived) intermediate states. Using cryogenic infrared and electron paramagnetic resonance spectroscopy on the regulatory [NiFe]-hydrogenase from Cupriavidus necator, a model enzyme for the analysis of catalytic intermediates, we deciphered the structural basis of the hitherto elusive Nia-L intermediates. We unveiled the protonation states of a proton-accepting glutamate and a Ni-bound cysteine residue in the Nia-L1, Nia-L2, and the hydride-binding Nia-C intermediates as well as previously unknown conformational changes of amino acid residues in proximity of the bimetallic active site. As such, this study unravels the complexity of the Nia-L intermediate and reveals the importance of the protein scaffold in fine-tuning proton and electron dynamics in [NiFe]-hydrogenase.

Binding of exogenous cyanide reveals new active-site states in [FeFe] hydrogenases.

[FeFe] hydrogenases are highly efficient metalloenyzmes for hydrogen conversion. Their active site cofactor (the H-cluster) is composed of a canonical [4Fe-4S] cluster ([4Fe-4S]H) linked to a unique organometallic di-iron subcluster ([2Fe]H). In [2Fe]H the two Fe ions are coordinated by a bridging 2-azapropane-1,3-dithiolate (ADT) ligand, three CO and two CN- ligands, leaving an open coordination site on one Fe where substrates (H2 and H+) as well as inhibitors (e.g. O2, CO, H2S) may bind. Here, we investigate two new active site states that accumulate in [FeFe] hydrogenase variants where the cysteine (Cys) in the proton transfer pathway is mutated to alanine (Ala). Our experimental data, including atomic resolution crystal structures and supported by calculations, suggest that in these two states a third CN- ligand is bound to the apical position of [2Fe]H. These states can be generated both by "cannibalization" of CN- from damaged [2Fe]H subclusters as well as by addition of exogenous CN-. This is the first detailed spectroscopic and computational characterisation of the interaction of exogenous CN- with [FeFe] hydrogenases. Similar CN--bound states can also be generated in wild-type hydrogenases, but do not form as readily as with the Cys to Ala variants. These results highlight how the interaction between the first amino acid in the proton transfer pathway and the active site tunes ligand binding to the open coordination site and affects the electronic structure of the H-cluster.

Stepwise assembly of the active site of [NiFe]-hydrogenase.

[NiFe]-hydrogenases are biotechnologically relevant enzymes catalyzing the reversible splitting of H2 into 2e- and 2H+ under ambient conditions. Catalysis takes place at the heterobimetallic NiFe(CN)2(CO) center, whose multistep biosynthesis involves careful handling of two transition metals as well as potentially harmful CO and CN- molecules. Here, we investigated the sequential assembly of the [NiFe] cofactor, previously based on primarily indirect evidence, using four different purified maturation intermediates of the catalytic subunit, HoxG, of the O2-tolerant membrane-bound hydrogenase from Cupriavidus necator. These included the cofactor-free apo-HoxG, a nickel-free version carrying only the Fe(CN)2(CO) fragment, a precursor that contained all cofactor components but remained redox inactive and the fully mature HoxG. Through biochemical analyses combined with comprehensive spectroscopic investigation using infrared, electronic paramagnetic resonance, Mössbauer, X-ray absorption and nuclear resonance vibrational spectroscopies, we obtained detailed insight into the sophisticated maturation process of [NiFe]-hydrogenase.

Reversible Glutamate Coordination to High-Valent Nickel Protects the Active Site of a [NiFe] Hydrogenase from Oxygen.

NAD+-reducing [NiFe] hydrogenases are valuable biocatalysts for H2-based energy conversion and the regeneration of nucleotide cofactors. While most hydrogenases are sensitive toward O2 and elevated temperatures, the soluble NAD+-reducing [NiFe] hydrogenase from Hydrogenophilus thermoluteolus (HtSH) is O2-tolerant and thermostable. Thus, it represents a promising candidate for biotechnological applications. Here, we have investigated the catalytic activity and active-site structure of native HtSH and variants in which a glutamate residue in the active-site cavity was replaced by glutamine, alanine, and aspartate. Our biochemical, spectroscopic, and theoretical studies reveal that at least two active-site states of oxidized HtSH feature an unusual architecture in which the glutamate acts as a terminal ligand of the active-site nickel. This observation demonstrates that crystallographically observed glutamate coordination represents a native feature of the enzyme. One of these states is diamagnetic and characterized by a very high stretching frequency of an iron-bound active-site CO ligand. Supported by density-functional-theory calculations, we identify this state as a high-valent species with a biologically unprecedented formal Ni(IV) ground state. Detailed insights into its structure and dynamics were obtained by ultrafast and two-dimensional infrared spectroscopy, demonstrating that it represents a conformationally strained state with unusual bond properties. Our data further show that this state is selectively and reversibly formed under oxic conditions, especially upon rapid exposure to high O2 levels. We conclude that the kinetically controlled formation of this six-coordinate high-valent state represents a specific and precisely orchestrated stereoelectronic response toward O2 that could protect the enzyme from oxidative damage.

High-Yield Production of Catalytically Active Regulatory [NiFe]-Hydrogenase From Cupriavidus necator in Escherichia coli.

Hydrogenases are biotechnologically relevant metalloenzymes that catalyze the reversible conversion of molecular hydrogen into protons and electrons. The O2-tolerant [NiFe]-hydrogenases from Cupriavidus necator (formerly Ralstonia eutropha) are of particular interest as they maintain catalysis even in the presence of molecular oxygen. However, to meet the demands of biotechnological applications and scientific research, a heterologous production strategy is required to overcome the low production yields in their native host. We have previously used the regulatory hydrogenase (RH) from C. necator as a model for the development of such a heterologous hydrogenase production process in E. coli. Although high protein yields were obtained, the purified enzyme was inactive due to the lack of the catalytic center, which contains an inorganic nickel-iron cofactor. In the present study, we significantly improved the production process to obtain catalytically active RH. We optimized important factors such as O2 content, metal availability, production temperature and time as well as the co-expression of RH-specific maturase genes. The RH was successfully matured during aerobic cultivation of E. coli by co-production of seven hydrogenase-specific maturases and a nickel permease, which was confirmed by activity measurements and spectroscopic investigations of the purified enzyme. The improved production conditions resulted in a high yield of about 80 mg L-1 of catalytically active RH and an up to 160-fold space-time yield in E. coli compared to that in the native host C. necator [<0.1 U (L d) -1]. Our strategy has important implications for the use of E. coli K-12 and B strains in the recombinant production of complex metalloenzymes, and provides a blueprint for the production of catalytically active [NiFe]-hydrogenases in biotechnologically relevant quantities.

Crucial Role of the Chaperonin GroES/EL for Heterologous Production of the Soluble Methane Monooxygenase from Methylomonas methanica MC09.

Methane is a widespread energy source and can serve as an attractive C1 building block for a future bioeconomy. The soluble methane monooxygenase (sMMO) is able to break the strong C-H bond of methane and convert it to methanol. The high structural complexity, multiplex cofactors, and unfamiliar folding or maturation procedures of sMMO have hampered the heterologous production and thus biotechnological applications. Here, we demonstrate the heterologous production of active sMMO from the marine Methylomonas methanica MC09 in Escherichia coli by co-synthesizing the GroES/EL chaperonin. Iron determination, electron paramagnetic resonance spectroscopy, and native gel immunoblots revealed the incorporation of the non-heme diiron centre and homodimer formation of active sMMO. The production of recombinant sMMO will enable the expansion of the possibilities of detailed studies, allowing for a variety of novel biotechnological applications.

Catalytic and Spectroscopic Properties of the Halotolerant Soluble Methane Monooxygenase Reductase from Methylomonas methanica MC09.

The soluble methane monooxygenase receives electrons from NADH via its reductase MmoC for oxidation of methane, which is itself an attractive C1 building block for a future bioeconomy. Herein, we present biochemical and spectroscopic insights into the reductase from the marine methanotroph Methylomonas methanica MC09. The presence of a flavin adenine dinucleotide (FAD) and [2Fe2S] cluster as its prosthetic group were revealed by reconstitution experiments, iron determination and electron paramagnetic resonance spectroscopy. As a true halotolerant enzyme, MmoC still showed 50 % of its specific activity at 2 M NaCl. We show that MmoC produces only trace amounts of superoxide, but mainly hydrogen peroxide during uncoupled turnover reactions. The characterization of a highly active reductase is an important step for future biotechnological applications of a halotolerant sMMO.

Optimization of Culture Conditions for Oxygen-Tolerant Regulatory [NiFe]-Hydrogenase Production from Ralstonia eutropha H16 in Escherichia coli.

Hydrogenases are abundant metalloenzymes that catalyze the reversible conversion of molecular H2 into protons and electrons. Important achievements have been made over the past two decades in the understanding of these highly complex enzymes. However, most hydrogenases have low production yields requiring many efforts and high costs for cultivation limiting their investigation. Heterologous production of these hydrogenases in a robust and genetically tractable expression host is an attractive strategy to make these enzymes more accessible. In the present study, we chose the oxygen-tolerant H2-sensing regulatory [NiFe]-hydrogenase (RH) from Ralstonia eutropha H16 owing to its relatively simple architecture compared to other [NiFe]-hydrogenases as a model to develop a heterologous hydrogenase production system in Escherichia coli. Using screening experiments in 24 deep-well plates with 3 mL working volume, we investigated relevant cultivation parameters, including inducer concentration, expression temperature, and expression time. The RH yield could be increased from 14 mg/L up to >250 mg/L by switching from a batch to an EnPresso B-based fed-batch like cultivation in shake flasks. This yield exceeds the amount of RH purified from the homologous host R. eutropha by several 100-fold. Additionally, we report the successful overproduction of the RH single subunits HoxB and HoxC, suitable for biochemical and spectroscopic investigations. Even though both RH and HoxC proteins were isolated in an inactive, cofactor free apo-form, the proposed strategy may powerfully accelerate bioprocess development and structural studies for both basic research and applied studies. These results are discussed in the context of the regulation mechanisms governing the assembly of large and small hydrogenase subunits.

Changing the Reactivity of Zero- and Mono-Valent Germanium with a Redox Non-Innocent Bis(silylenyl)carborane Ligand.

Using the chelating C,C'-bis(silylenyl)-ortho-dicarborane ligand, 1,2-(RSi)2 -1,2-C2 B10 H10 [R=PhC(NtBu)2 ], leads to the monoatomic zero-valent Ge complex ("germylone") 3. The redox non-innocent character of the carborane scaffold has a drastic influence on the reactivity of 3 towards reductants and oxidants. Reduction of 3 with one molar equivalent of potassium naphthalenide (KC10 H8 ) causes facile oxidation of Ge0 to GeI along with a two-electron reduction of the C2 B10 cluster core and subsequent GeI -GeI coupling to form the dianionic bis(silylene)-supported Ge2 complex 4. In contrast, oxidation of 3 with one molar equivalent of [Cp2 Fe][B{C6 H3 (CF3 )2 }4 ] as a one-electron oxidant furnishes the dicationic bis(silylene)-supported Ge2 complex 5. The Ge0 atom in 3 acts as donor towards GeCl2 to form the trinuclear mixed-valent Ge0 →GeII ←Ge0 complex 6, from which dechlorination with KC10 H8 affords the neutral Ge2 complex 7 as a diradical species.

Exploring Structure and Function of Redox Intermediates in [NiFe]-Hydrogenases by an Advanced Experimental Approach for Solvated, Lyophilized and Crystallized Metalloenzymes.

To study metalloenzymes in detail, we developed a new experimental setup allowing the controlled preparation of catalytic intermediates for characterization by various spectroscopic techniques. The in situ monitoring of redox transitions by infrared spectroscopy in enzyme lyophilizate, crystals, and solution during gas exchange in a wide temperature range can be accomplished as well. Two O2 -tolerant [NiFe]-hydrogenases were investigated as model systems. First, we utilized our platform to prepare highly concentrated hydrogenase lyophilizate in a paramagnetic state harboring a bridging hydride. This procedure proved beneficial for 57 Fe nuclear resonance vibrational spectroscopy and revealed, in combination with density functional theory calculations, the vibrational fingerprint of this catalytic intermediate. The same in situ IR setup, combined with resonance Raman spectroscopy, provided detailed insights into the redox chemistry of enzyme crystals, underlining the general necessity to complement X-ray crystallographic data with spectroscopic analyses.

Bis(silylene)-Stabilized Monovalent Nitrogen Complexes.

The first series of bis(silylene)-stabilized nitrogen(I) compounds is described. Starting from the 1,2-bis(N-heterocyclic silylenyl) 1,2-dicarba-closo-dedocaborane(12) scaffold 1, [1,2-(LSi)2 C2 B10 H10 ; L=PhC(Nt Bu)2 ], reaction with adamantyl azide (AdN3 ) affords the terminal N-µ2 -bridged zwitterionic carborane-1,2-bis(silylium) AdN3 adduct 2 with an open-cage dianionic nido-C2 B10 cluster core. Remarkably, upon one-electron reduction of 2 with C8 K and liberation of N2 and adamantane, the two silylene subunits are regenerated to furnish the isolable bis(silylene)-stabilized NI complex as an anion of 3 with the nido-C2 B10 cluster cage. On the other hand, one-electron oxidation of 2 with silver(I) yields the monocationic bis(silylene) NI complex 4 with the closo-C2 B10 cluster core. Moreover, the corresponding neutral NI radical complex 5 results from single-electron transfer from 3 to 4.

Caught in the Hinact : Crystal Structure and Spectroscopy Reveal a Sulfur Bound to the Active Site of an O2 -stable State of [FeFe] Hydrogenase.

[FeFe] hydrogenases are the most active H2 converting catalysts in nature, but their extreme oxygen sensitivity limits their use in technological applications. The [FeFe] hydrogenases from sulfate reducing bacteria can be purified in an O2 -stable state called Hinact . To date, the structure and mechanism of formation of Hinact remain unknown. Our 1.65 Šcrystal structure of this state reveals a sulfur ligand bound to the open coordination site. Furthermore, in-depth spectroscopic characterization by X-ray absorption spectroscopy (XAS), nuclear resonance vibrational spectroscopy (NRVS), resonance Raman (RR) spectroscopy and infrared (IR) spectroscopy, together with hybrid quantum mechanical and molecular mechanical (QM/MM) calculations, provide detailed chemical insight into the Hinact state and its mechanism of formation. This may facilitate the design of O2 -stable hydrogenases and molecular catalysts.

Shedding Light on Proton and Electron Dynamics in [FeFe] Hydrogenases.

[FeFe] hydrogenases are highly efficient catalysts for reversible dihydrogen evolution. H2 turnover involves different catalytic intermediates including a recently characterized hydride state of the active site (H-cluster). Applying cryogenic infrared and electron paramagnetic resonance spectroscopy to an [FeFe] model hydrogenase from Chlamydomonas reinhardtii (CrHydA1), we have discovered two new hydride intermediates and spectroscopic evidence for a bridging CO ligand in two reduced H-cluster states. Our study provides novel insights into these key intermediates, their relevance for the catalytic cycle of [FeFe] hydrogenase, and novel strategies for exploring these aspects in detail.

The large subunit of the regulatory [NiFe]-hydrogenase from Ralstonia eutropha - a minimal hydrogenase?

Chemically synthesized compounds that are capable of facilitating the reversible splitting of dihydrogen into protons and electrons are rare in chemists' portfolio. The corresponding biocatalysts - hydrogenases - are, however, abundant in the microbial world. [NiFe]-hydrogenases represent a major subclass and display a bipartite architecture, composed of a large subunit, hosting the catalytic NiFe(CO)(CN)2 cofactor, and a small subunit whose iron-sulfur clusters are responsible for electron transfer. To analyze in detail the catalytic competence of the large subunit without its smaller counterpart, we purified the large subunit HoxC of the regulatory [NiFe]-hydrogenase of the model H2 oxidizer Ralstonia eutropha to homogeneity. Metal determination and infrared spectroscopy revealed a stoichiometric loading of the metal cofactor. This enabled for the first time the determination of the UV-visible extinction coefficient of the NiFe(CO)(CN)2 cofactor. Moreover, the absence of disturbing iron-sulfur clusters allowed an unbiased look into the low-spin Fe2+ of the active site by Mössbauer spectroscopy. Isolated HoxC was active in catalytic hydrogen-deuterium exchange, demonstrating its capacity to activate H2. Its catalytic activity was drastically lower than that of the bipartite holoenzyme. This was consistent with infrared and electron paramagnetic resonance spectroscopic observations, suggesting that the bridging position between the active site nickel and iron ions is predominantly occupied by water-derived ligands, even under reducing conditions. In fact, the presence of water-derived ligands bound to low-spin Ni2+ was reflected by the absorption bands occurring in the corresponding UV-vis spectra, as revealed by time-dependent density functional theory calculations conducted on appropriate in silico models. Thus, the isolated large subunits indeed represent simple [NiFe]-hydrogenase models, which could serve as blueprints for chemically synthesized mimics. Furthermore, our data point to a fundamental role of the small subunit in preventing water access to the catalytic center, which significantly increases the H2 splitting capacity of the enzyme.

Hydroxy-bridged resting states of a [NiFe]-hydrogenase unraveled by cryogenic vibrational spectroscopy and DFT computations.

The catalytic mechanism of [NiFe]-hydrogenases is a subject of extensive research. Apart from at least four reaction intermediates of H2/H+ cycling, there are also a number of resting states, which are formed under oxidizing conditions. Although not directly involved in the catalytic cycle, the knowledge of their molecular structures and reactivity is important, because these states usually accumulate in the course of hydrogenase purification and may also play a role in vivo during hydrogenase maturation. Here, we applied low-temperature infrared (cryo-IR) and nuclear resonance vibrational spectroscopy (NRVS) to the isolated catalytic subunit (HoxC) of the heterodimeric regulatory [NiFe]-hydrogenase (RH) from Ralstonia eutropha. Cryo-IR spectroscopy revealed that the HoxC protein can be enriched in almost pure resting redox states suitable for NRVS investigation. NRVS analysis of the hydrogenase catalytic center is usually hampered by strong spectral contributions of the FeS clusters of the small, electron-transferring subunit. Therefore, our approach to investigate the FeS cluster-free, 57Fe-labeled HoxC provided an unprecedented insight into the [NiFe] site modes, revealing their contributions in a spectral range otherwise superimposed by FeS cluster-derived bands. Rationalized by density functional theory (DFT) calculations, our data provide structural descriptions of the previously uncharacterized hydroxy- and water-containing resting states. Our work highlights the relevance of cryogenic vibrational spectroscopy and DFT to elucidate the structure of barely defined redox states of the [NiFe]-hydrogenase active site.

In Vitro Assembly as a Tool to Investigate Catalytic Intermediates of [NiFe]-Hydrogenase.

[NiFe]-hydrogenases catalyze the reversible reaction H2 ⇄ 2H+ + 2e-. Their basic module consists of a large subunit, coordinating the NiFe(CO)(CN)2 center, and a small subunit that carries electron-transferring iron-sulfur clusters. Here, we report the in vitro assembly of fully functional [NiFe]-hydrogenase starting from the isolated large and small subunits. Activity assays complemented by spectroscopic measurements revealed a native-like hydrogenase. This approach was used to label exclusively the NiFe(CO)(CN)2 center with 57Fe, enabling a clear view of the catalytic site by means of nuclear resonance vibrational spectroscopy. This strategy paves the way for in-depth studies of [NiFe]-hydrogenase catalytic intermediates.

X-ray Crystallography and Vibrational Spectroscopy Reveal the Key Determinants of Biocatalytic Dihydrogen Cycling by [NiFe] Hydrogenases.

[NiFe] hydrogenases are complex model enzymes for the reversible cleavage of dihydrogen (H2 ). However, structural determinants of efficient H2 binding to their [NiFe] active site are not properly understood. Here, we present crystallographic and vibrational-spectroscopic insights into the unexplored structure of the H2 -binding [NiFe] intermediate. Using an F420 -reducing [NiFe]-hydrogenase from Methanosarcina barkeri as a model enzyme, we show that the protein backbone provides a strained chelating scaffold that tunes the [NiFe] active site for efficient H2 binding and conversion. The protein matrix also directs H2 diffusion to the [NiFe] site via two gas channels and allows the distribution of electrons between functional protomers through a subunit-bridging FeS cluster. Our findings emphasize the relevance of an atypical Ni coordination, thereby providing a blueprint for the design of bio-inspired H2 -conversion catalysts.

Single-Electron Transfer Reactions in Frustrated and Conventional Silylium Ion/Phosphane Lewis Pairs.

Silylium ions undergo a single-electron reduction with phosphanes, leading to transient silyl radicals and the corresponding stable phosphoniumyl radical cations. As supported by DFT calculations, phosphanes with electron-rich 2,6-disubstituted aryl groups are sufficiently strong reductants to facilitate this single-electron transfer (SET). Frustration as found in kinetically stabilized triarylsilylium ion/phosphane Lewis pairs is not essential, and silylphosphonium ions, which are generated by conventional Lewis adduct formation of solvent-stabilized trialkylsilylium ions and phosphanes, engage in the same radical mechanism. The trityl cation, a Lewis acid with a higher electron affinity, even oxidizes trialkylphosphanes, such as tBu3 P, which does not react with either B(C6 F5 )3 or silylium ions.