Visible-light-activated radical photoinitiators are pivotal in the efficient construction of complex molecular architectures and the precision synthesis of advanced polymeric materials. At the heart of their function lies the formation of reactive radical species that drive selective homolytic bond cleavage, but elucidating the fundamental mechanisms of these processes is notoriously difficult due to the fleeting nature of key intermediates. In this study, time-resolved infrared (TRIR) spectroscopy provides a powerful window into the complete reaction profile of the versatile photocatalyst [Mn2(CO)10], including the observation of [Mn(O2)(CO)5], which is a long-lived (ms) reservoir of the reactive 17-electron complex [Mn(CO)5]. We give unprecedented structural and mechanistic insights concerning the formation of [Mn(CO)5] in electronically and vibrationally excited states (fs-ps), its quenching by O2 (ns), regeneration of the ground-state catalyst, and C-I bond activation (ms). New avenues for the rational design of next-generation metal-metal photocatalysts are provided, as [Mn(O2)(CO)5] significantly extends the catalyst longevity.

The thermal dissociation of carbon monoxide is a fundamental entry step to many catalytic cycles involving metal carbonyl (MCO) complexes as it reveals a vacant coordination site at the metal center, enabling substrate coordination. Overcoming the dissociation barrier requires sufficient accumulation of energy in vibrational modes with displacement vectors along the reaction coordinate (M-CO distance). Hence, understanding the energy transfer to and from these vibrations is essential in developing a detailed understanding of a precatalyst's activation pathway. Here, the intramolecular vibrational energy redistribution (IVR) within the heteroleptic metal carbonyl complex [Mn(ppy)(CO)4] (1, ppy = cyclometalated 2-phenylpyridine) in dichloromethane solution has been studied using dual-frequency, two-dimensional infrared spectroscopy. The responses of three vibrational modes localized on the ppy-ligand were monitored following the photoexcitation of carbonyl ligand stretching modes. A rise of signal strength by a factor of 4.7 within the first 35 ps, followed by a decay within ca. 150 ps exemplify the IVR from the metal carbonyl to the organic moiety, and the intermolecular energy transfer (IET) to the solvent, respectively. Moreover, pronounced changes of the spectral shape within the first 10 ps indicate the population of distinct vibrationally excited states. Direct anharmonic coupling between the pumped and probed modes gives rise to the initial spectral features, which include an uncommon, negative anharmonic coupling. These features do not decay single-exponentially as expected, but rise during the first 23 ps instead. This finding is assigned to the population of low-frequency modes (<250 cm-1), which are predicted to have a similar coupling pattern toward the measured bands as the CO stretching modes, based on density functional theory (DFT) calculations. The stronger signals predominant at late waiting times have uniform anharmonic shifts of ca. -2.5 cm-1 arising from the coupling to medium-frequency modes (250-1200 cm-1), which are strongly localized on the ppy-ligand. Due to the distinct signal positions, the time-dependent populations of low- and medium-frequency modes can be evaluated independently. Rate constants for the rise of their populations were found to be 1/52 ps-1 and 1/58 ps-1, respectively, while the rate constants of depopulation via IET to the solvent are 1/19 ps-1 and 1/25 ps-1.

Infrared (IR) absorption and time resolved IR (IRpump-IRprobe, 2D-IR) spectroscopies have been combined to study the vibrational dynamics and solvent interactions of the carbonyl ligand stretching vibrational modes of the photocatalyst dimanganese decacarbonyl, [Mn2(CO)10], in solvents with varying physical properties (heptane, cyclohexane, THF, MeCN, DMSO, iPrOH, and MeOH). The presence of a solvent-mediated symmetry breaking mechanism leading to a gain in oscillator strength of formally symmetry-forbidden modes was observed in all solvents, although the effect was more marked in polar solvents. Ultrafast vibrational energy dissipation was found to occur via two solvent dependent relaxation pathways, rapid intramolecular vibrational energy redistribution (IVR ∼ 0.3-1 ps) and relaxation to the ground vibrational state (T1 ∼ 80-250 ps). Accelerating factors for vibrational relaxation included hydrogen bonding and the presence of solvent vibrational modes resonant with the carbonyl modes of [Mn2(CO)10], while IVR timescales displayed an anticorrelation with vibrational relaxation times. Overall, a simple association of dynamic behavior with solvent properties could not be identified for any of the measured parameters. Rather, specific solvent properties were found to contribute to different extents in each case. Thus, our results highlight the need for careful consideration of solvent factors when attempting a rational selection of catalyst/solvent combinations or the implementation of sustainable replacement solvents.

[FeFe] hydrogenases are Nature's most efficient catalysts for the cleavage and evolution of molecular hydrogen. Despite decades of research, key aspects of the catalytic cycle and the underlying geometrical and electronic properties of the active-site cofactor, called the H-cluster, are not fully understood. Spectroscopic techniques have played a central role in establishing the current state of knowledge on [FeFe] hydrogenases, and further advances in the field depend critically on novel techniques that yield so-far inaccessible insights into structural and mechanistic aspects. Infrared (IR) absorption spectroscopy represents a well-established and versatile technique that can identify and characterize all active and inactive states of the H-cluster by means of structurally sensitive and spectrally isolated CO and CN stretching vibrations. However, the amount of information that can be extracted from these linear experiments is inherently limited. Here we introduce experimental and computational two-dimensional (2D-)IR spectroscopy for the characterization of [FeFe] hydrogenases. Utilizing the Hinact state of the H-cluster as a model system, we demonstrate that this nonlinear technique yields direct information about the nature and interactions of the CO and CN stretching vibrations. These insights allow, for the first time, to quantitatively describe the character of these widely used reporter vibrations, their spatial localization, and the way they change upon structural variation of the H-cluster. The strength of this approach is demonstrated by correctly identifying the proposed structure of the Hinact state, in solution and at ambient temperature. In conclusion, the introduced combination of experimental and computational 2D-IR spectroscopy represents a powerful approach for studying [FeFe] hydrogenases and other complex organometallic targets.

The reaction of [Ir(IPr)2H2][BArF 4] (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; BArF 4 = B{C6H3(3,5-CF3)2}4) with ZnMe2 proceeds with CH4 elimination to give [Ir(IPr)(IPr ')(ZnMe)2H][BArF 4] (3, where (IPr ') is a cyclometalated IPr ligand). 3 reacts with H2 to form tetrahydride [Ir(IPr)2(ZnMe)2H4][BArF 4], 4, that loses H2 under forcing conditions to form [Ir(IPr)2(ZnMe)2H2][BArF 4], 5. Crystallization of 3 also results in the formation of its noncyclometalated isomer, [Ir(IPr)2(ZnMe)2][BArF 4], 2, in the solid state. Reactions of 1 and CdMe2 form [Ir(IPr)2(CdMe)2][BArF 4], 6, and [Ir(IPr)(IPr ')(CdMe)2H][BArF 4], 7, which reacts with H2 to give [Ir(IPr)2(CdMe)2H4][BArF 4], 8, and [Ir(IPr)2(CdMe)2H2][BArF 4], 9. Structures of 2-8 are determined crystallographically. Computational analyses show the various hydrides in 3-5 sit on a terminal to bridging continuum, with bridging hydrides exhibiting greater Zn delta+H delta- electrostatic interaction. The isolobal analogy between H and ZnMe ligands holds when both are present as terminal ligands. However, the electrostatic component to the Zn delta+H delta- unit renders it significantly different to a nominally isolobal HH moiety. Thus, H2 addition to 3 is irreversible, whereas H2 addition to 1 reversibly forms highly fluxional [Ir(IPr)2(eta 2-H2)2H2][BArF 4], 11. Computed mechanisms for cyclometalation and H2 addition showcase the role of the bridging Zn delta+H delta- moiety in promoting reactivity. In this, the Lewis acidic ZnMe ligand plays a dual role: as a terminal Z-type ligand that can stabilize electron-rich Ir centers through direct Ir-ZnMe bonding, or by stabilizing strongly hydridic character via Zn delta+H delta- interactions.

The [CpFe(CO)(CN)2]- unit is an excellent structural model for the Fe(CO)(CN)2 moiety of the active site found in [NiFe] hydrogenases. Ultrafast infrared (IR) pump-probe and 2D-IR spectroscopy have been used to study K[CpFe(CO)(CN)2] (M1) in a range of protic and polar solvents and as a dry film. Measurements of anharmonicity, intermode vibrational coupling strength, vibrational relaxation time, and solvation dynamics of the CO and CN stretching modes of M1 in H2O, D2O, methanol, dimethyl sulfoxide, and acetonitrile reveal that H-bonding to the CN ligands plays an important role in defining the spectroscopic characteristics and relaxation dynamics of the Fe(CO)(CN)2 unit. Comparisons of the spectroscopic and dynamic data obtained for M1 in solution and in a dry film with those obtained for the enzyme led to the conclusion that the protein backbone forms an important part of the bimetallic active site environment via secondary coordination sphere interactions.

The solution phase structure, vibrational spectroscopy, and ultrafast relaxation dynamics of the precatalyst species [Mn(ppy)(CO)4] (1) in solution have been investigated using ultrafast two-dimensional infrared (2D-IR) spectroscopy. By comparing 2D-IR data with the results of anharmonic density functional theory (DFT) calculations, we establish an excellent agreement between measured and predicted inter-mode couplings of the carbonyl stretching vibrational modes of 1 that relates to the atomic displacements of axial and equatorial ligands in the modes and the nature of the molecular orbitals involved in M-CO bonding. Measurements of IR pump-probe spectra and 2D-IR spectra as a function of waiting time reveal the presence of ultrafast (few ps) intramolecular vibrational energy redistribution between carbonyl stretching modes prior to vibrational relaxation. The vibrational relaxation times of the CO-stretching modes of 1 are found to be relatively solvent-insensitive, suggestive of limited solvent-solute interactions in the ground electronic state. Overall, these data provide a detailed picture of the complex potential energy surface, bonding and vibrational dynamics of 1, establishing a fundamental basis for the next steps in understanding and modulating precatalyst behavior.

Systems incorporating the cis-Mo(O)2 motif catalyse a range of important thermal homogeneous and heterogeneous oxygen atom transfer (OAT) reactions spanning biological oxidations to platform chemical synthesis. Analogous light-driven processes could offer a more sustainable approach. The cis-Mo(O)2 complexes reported here photocatalyse OAT under visible light irradiation, and operate via a non-emissive excited state with substantial ligand-to-metal charge-transfer (LMCT) character, in which a Mo[double bond, length as m-dash]O π*-orbital is populated via transfer of electron density from a chromophoric salicylidene-aminophenol (SAP) ligand. SAP ligands can be prepared from affordable commercially-available precursors. The respective cis-Mo(O)2-SAP catalysts are air stable, function in the presence of water, and do not require additional photosensitisers or redox mediators. Benchmark OAT between phosphines and sulfoxides shows that electron withdrawing groups (e.g. C(O)OMe, CF3) are necessary for photocatalytic activity. The photocatalytic system described here is mechanistically distinct from both thermally catalysed OAT by the cis-Mo(O)2 motif, as well as typical photoredox systems that operate by outer sphere electron transfer mediated by long-lived emissive states. Both photoactivated and thermally activated OAT steps are coupled to establish a catalytic cycle, offering new opportunities for the development of photocatalytic atom transfer based on readily-available, high-valent metals, such as molybdenum.

Time-resolved temperature-jump/drop infrared (IR) spectroscopy has been used to measure the impact of stem base sequence on the melting and refolding dynamics of ribonucleic acid (RNA) tetraloops. A series of three 12-nucleotide RNA hairpin sequences were studied, each featuring a UACG tetraloop motif and a double-stranded stem containing four base pairs. In each case, the stem comprised three GC pairs plus a single AU base pair inserted at the closing point of the loop (RNAloop), in the middle of the stem (RNAmid), or at the stem terminus (RNAend). Results from analogous DNA tetraloop (TACG) sequences were also obtained. Inclusion of AU or AT base pairs in the stem leads to faster melting of the stem-loop structure compared to a stem sequence featuring four GC base pairs while refolding times were found to be slower, consistent with a general reduction in stem-loop stability caused by the AU/AT pair. Independent measurement of the dynamic timescales for melting and refolding of ring vibrational modes of guanine (GR) and adenine (AR) provided position-specific insight into hairpin dynamics. The GR-derived data showed that DNA sequences melted more quickly (0.5 ± 0.1 to 0.7 ± 0.1 μs at 70 °C) than analogous RNA sequences (4.3 ± 0.4 to 4.4 ± 0.3 μs at 70 °C). Position-sensitive data from the AR modes suggests that DNA hairpins begin melting from the terminal end of the stem toward the loop while RNA sequences begin melting from the loop. Refolding timescales for both RNA and DNA hairpins were found to be similar (250 ± 50 μs at 70 °C) except for RNAend and DNAloop which refolded much more slowly (746 ± 36 and 430 ± 31 μs, respectively), showing that the refolding pathway is significantly impaired by the placement of AU/AT pairs at different points in the stem. We conclude that conformational changes of analogous pairs of RNA and DNA tetraloops proceed by different mechanisms.

Vibrational spectroscopy is an important tool in chemical and biological analysis. A key issue when applying vibrational spectroscopy to dilute liquid samples is the inherently low sensitivity caused by short interaction lengths and small extinction coefficients, combined with low target molecule concentrations. Here, we introduce a novel type of surface-enhanced infrared absorption spectroscopy based on the resonance of a dielectric metasurface. We demonstrate that the method is suitable for probing vibrational bands of dilute analytes with a range of spectral linewidths. We observe that the absorption signal is enhanced by 1-2 orders of magnitude and show that this enhancement leads to a lower limit of detection compared to attenuated total reflection (ATR). Overall, the technique provides an important addition to the spectroscopist's toolkit especially for probing dilute samples.

Ultrafast two-dimensional infrared (2D-IR) spectroscopy of Escherichia coli Hyd-1 (EcHyd-1) reveals the structural and dynamic influence of the protein scaffold on the Fe(CO)(CN)2 unit of the active site. Measurements on as-isolated EcHyd-1 probed a mixture of active site states including two, which we assign to Nir-SI/II, that have not been previously observed in the E. coli enzyme. Explicit assignment of carbonyl (CO) and cyanide (CN) stretching bands to each state is enabled by 2D-IR. Energies of vibrational levels up to and including two-quantum vibrationally excited states of the CO and CN modes have been determined along with the associated vibrational relaxation dynamics. The carbonyl stretching mode potential is well described by a Morse function and couples weakly to the cyanide stretching vibrations. In contrast, the two CN stretching modes exhibit extremely strong coupling, leading to the observation of formally forbidden vibrational transitions in the 2D-IR spectra. We show that the vibrational relaxation times and structural dynamics of the CO and CN ligand stretching modes of the enzyme active site differ markedly from those of a model compound K[CpFe(CO)(CN)2] in aqueous solution and conclude that the protein scaffold creates a unique biomolecular environment for the NiFe site that cannot be represented by analogy to simple models of solvation.

[NiFe] hydrogenases are metalloenzymes that catalyze the reversible cleavage of dihydrogen (H2), a clean future fuel. Understanding the mechanism of these biocatalysts requires spectroscopic techniques that yield insights into the structure and dynamics of the [NiFe] active site. Due to the presence of CO and CN− ligands at this cofactor, infrared (IR) spectroscopy represents an ideal technique for studying these aspects, but molecular information from linear IR absorption experiments is limited. More detailed insights can be obtained from ultrafast nonlinear IR techniques like IRpump-IRprobe and two-dimensional (2D-)IR spectroscopy. However, fully exploiting these advanced techniques requires an in-depth understanding of experimental observables and the encoded molecular information. To address this challenge, we present a descriptive and predictive computational approach for the simulation and analysis of static 2D-IR spectra of [NiFe] hydrogenases and similar organometallic systems. Accurate reproduction of experimental spectra from a first-coordination-sphere model suggests a decisive role of the [NiFe] core in shaping the enzymatic potential energy surface. We also reveal spectrally encoded molecular information that is not accessible by experiments, thereby helping to understand the catalytic role of the diatomic ligands, structural differences between [NiFe] intermediates, and possible energy transfer mechanisms. Our studies demonstrate the feasibility and benefits of computational spectroscopy in the 2D-IR investigation of hydrogenases, thereby further strengthening the potential of this nonlinear IR technique as a powerful research tool for the investigation of complex bioinorganic molecules.

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.

Determining the structural dynamics of RNA and DNA is essential to understanding their cellular function, but direct measurement of strand association or folding remains experimentally challenging. Here we illustrate a temperature-jump/drop method able to reveal refolding dynamics. Time-resolved temperature-jump/drop infrared spectroscopy is used to measure the melting and refolding dynamics of a 12-nucleotide RNA sequence comprising a UACG tetraloop and a four-base-pair double-stranded GC stem, comparing them to an equivalent DNA (TACG) sequence. Stem-loop melting occurred an order of magnitude more slowly in RNA than DNA (6.0 ± 0.1 μs versus 0.8 ± 0.1 μs at 70 °C). In contrast, the refolding dynamics of both sequences occurred on similar time scales (200 μs). While the melting and refolding dynamics of RNA and DNA hairpins both followed Arrhenius temperature dependences, refolding was characterized by an apparent negative activation energy, consistent with a mechanism involving multiple misfolded intermediates prior to zipping of the stem base pairs.

Ultrafast 2D-IR spectroscopy has proved to be a powerful analytical tool for the detection and differentiation of Bacillus spores as dry films on surfaces. Here, we expand on these findings by employing 2D-IR spectroscopy to study spores from B. atrophaeus (BG) in aqueous solution. Specific vibrational modes attributable to the calcium dipicolinate trihydrate biomarker for spore formation were observed alongside distinctive off-diagonal spectral features that can be used to differentiate spores from different Bacillus species, indicating that 2D-IR has potential for use as a sensing platform with both solid and liquid phase samples. The ability of 2D-IR to enhance the protein amide I band relative to the overlapping water bending vibration was exploited to compare the nature of the protein component of spores to that of solution phase protein molecules. The vibrational lifetime for the amide I band of the BG spore in H2O was 1.4 ± 0.1 ps, longer than those reported for the proteins in H2O solution. The nature of a band at 1710 cm-1 was also investigated. Collectively these results show the potential advantages of 2D-IR spectroscopy, with successful detection and classification of spores under different conditions being based on detailed molecular understanding of the spore state.

Ultrafast 2D-IR spectroscopy is a powerful tool for understanding the spectroscopy and dynamics of biological molecules in the solution phase. A number of recent studies have begun to explore the utility of the information-rich 2D-IR spectra for analytical applications. Here, we report the application of ultrafast 2D-IR spectroscopy for the detection and classification of bacterial spores. 2D-IR spectra of Bacillus atrophaeus and Bacillus thuringiensis spores as dry films on CaF2 windows were obtained. The sporulated nature of the bacteria was confirmed using 2D-IR diagonal and off-diagonal peaks arising from the calcium dipicolinate CaDP·3H2O biomarker for sporulation. Distinctive peaks, in the protein amide I region of the spectrum were used to differentiate the two types of spore. The identified marker modes demonstrate the potential for the use of 2D-IR methods as a direct means of spore classification. We discuss these new results in perspective with the current state of analytical 2D-IR measurements, showing that the potential exists to apply 2D-IR spectroscopy to detect the spores on surfaces and in suspensions as well as in dry films. The results demonstrate how applying 2D-IR screening methodologies to spores would enable the creation of a library of spectra for classification purposes.

The reaction of a series of N,N-di-substituted aroylthioureas (HL1-3; L-1 = H, L-2 = Cl, L-3 = OMe) with the Pd(II) centre in K2PdCl4 leads to cis-[Pd(L1-3-kappa S,O)(2)] complexes exclusively which then undergoes photo-induced isomerism to the trans-[Pd(L1-3-kappa S,O)(2)] isomer upon broadband UV irradiation. The photochemical iso-merization allows these unstable trans-[Pd(L1-3-kappa S,O)(2)] products to be isolated in pure forms if the conversion is completed in acetonitrile after slow evaporation of the solvent. The molecular structures of both cis-[Pd(L-2- kappa S,O)(2)] and trans-[Pd(L-2-kappa S,O)(2)] isomers have been determined by single-crystal X-ray diffraction. Upon dis-solving trans-[Pd(L1-3-kappa S,O)(2)] in chloroform in the dark, spontaneous trans -> cis isomerism occurs. In-situ laser photolysis in conjunction with NMR spectroscopy allows the time course of both these photochemical and thermal isomerisations to be monitored. The results indicate that a photostationary state is reached, that the back reaction is promoted both photochemically and thermally, and that the established photostationary state reverts back to the initial composition once photolysis stops. The presence of an electron-withdrawing chloro substituent on the para position of the phenyl ring increases the rate of the thermally initiated trans -> cis iso-merisation step. Furthermore, the addition of trace amounts of the HL1-3 ligands, a coordinating solvent or a two electron donor such as pyridine also speeds up this process in accordance with an associative mechanism. Mixed-ligand complexes were detected by reverse phase (RP) chromatography when cross-reactions were performed in accordance with this hypothesis. All the experimental evidence fits with a mechanism involving a competitive photochemical and associative ligand exchange.

We present a quantitative analysis of the timescales of reactivity that are accessible to a laser pump, NMR probe spectroscopy method using para-hydrogen induced polarisation (PHIP) and identify three kinetic regimes: fast, intermediate and slow. These regimes are defined by the relative rate of reaction, k, compared to δω, the frequency of the NMR signal oscillations associated with the coherent evolution of the hyperpolarised 1H NMR signals created after para-hydrogen (p-H2) addition during the pump-probe delay. The kinetic regimes are quantitatively defined by a NMR dephasing parameter, ε = δω/k. For the fast regime, where k ≫ δω and ε tends to zero, the observed NMR signals are not affected by the chemical evolution of the system and so only an upper bound on k can be determined. In the slow regime, where k ≪ δω and ε tends to infinity, destructive interference leads to the complete dephasing of the coherent NMR signal intensity oscillations. As a result, the observed NMR signal evolution during the pump-probe delay reflects only the chemical change of the system and NMR relaxation. Finally, in the intermediate regime, where k ∼ δω, characteristic partial dephasing of the NMR signal oscillations is predicted. In the limit where the dephasing parameter is small but non-zero, chemical evolution manifests itself as a phase shift in the NMR signal oscillation that is equal to the dephasing parameter. As this phase shift is predicted to persist for pump-probe delays much longer than the timescale of the formation of the product molecules, it provides a route to measure reactivity on micro-to-millisecond timescales through NMR detection. We predict that the most significant fundamental limitations of the accessible reaction timescales are the duration of the NMR excitation pulse (∼1 μs) and the chemical shift difference (in Hz) between the p-H2-derived protons in the product molecule.

Upon ex situ UV-visible light irradiation, complex cis-bis(N,N-diethyl-N'-naphthoylthioureato)-palladium(ii), cis-[Pd(L-κS,O)2], undergoes isomerization in acetonitrile-d3 and chloroform-d to yield trans-[Pd(L-κS,O)2] which then rearranges thermally to novel trans-[Pd(L-κS,N)2] prior to reverting thermally to the cis isomer in the absence of light. The thermal isomerization rate is highly solvent dependent and harnessed to enable each of these three geometric isomers to be isolated and characterized by 1H NMR spectroscopy, X-ray crystallography, melting point and thermal analysis. The formation of the trans-[Pd(L-κS,N)2] isomer as part of this isomerization has only been observed with the sterically demanding cis-bis(N,N-diethyl-N'-(naphthoylthioureato)palladium(ii) precursor based on our knowledge to date. In situ irradiation with monochromatic laser light (λ = 355 nm) coupled to 1H NMR spectroscopy of solutions of cis-[Pd(L-κS,O)2] in acetonitrile-d3 supports the ex situ photo-induced isomerization experiments.

Photochemical reactions of germane and diphenylgermane with Ru(PP)(2)H-2 (PP = R2PCH2CH2PR2 or DuPhos, R = Ph dppe, R = Et depe, R = Me dmpe) are reported. Reaction with GeH4 generates a mixture of cis and trans isomers of Ru(PP)(2)(GeH3)H except for the DuPhos complex which yields the product only in the cis form. In situ laser photolysis (355 nm) demonstrates that the initial product is the cis isomer that undergoes thermal isomerization to the trans isomer. The complex cis-[Ru(dppe)(2)(GeH3)H] crystallizes selectively, allowing determination of its X-ray structure as a germyl hydride with a long Ru-H center dot center dot center dot Ge separation of 2.64(3) angstrom indicating that no residual interaction between the RuH and Ge is present. DFT calculations are also consistent with full oxidative addition. The structure of cis-[Ru(DuPhos)(2)(GeH3)H] reveals significant distortion from an octahedral geometry. The major species in the crystal (95%) exhibits a structure with a Ru-H center dot center dot center dot Ge distance of 2.42(5) angstrom suggesting negligible interaction between these centers. DFT calculations of the structure are consistent with the experimental determination. The reactions of Ru(PP)(2)H-2 with diphenylgermane yield cis-[Ru(PP)(2)(GePh2H)H] exclusively for PP = dmpe and depe, while the cis isomer is dominant in the case of dppe. A photochemical competition reaction between Ru(dppe)(2)(H)(2) and the two substrates Ph2SiH2 and Ph2GeH2 results in both Si- H and Ge-H oxidative addition activation with a kinetic preference (0.18:1) for the germyl hydride product. Thermal conversion of Ru(dppe)(2)(SiPh2H)H to Ru(dppe)(2)(GePh2H)H is observed on heating.