Norrish-type-II reaction on a semiquinone radical: Stable semiquinone radicals serve as novel molecular platforms on which a Norrish-type-II photoreaction can be initiated. A detailed reaction scheme involving a 1,5-hydrogen transfer followed by a cyclization step that finally leads to a new C—C bond formation could be verified. Transient absorption spectroscopy and DFT calculations trace convincingly the intermediates and transition states along the reaction path (see scheme).

The emission spectra of the solids [n-Bu₄N]₂Tc₂X₈ (X = Cl, Br) have been investigated at room temperature and 77 K. In each case, the emission originates in the ¹Î´â€“δ* excited state, as with the rhenium homologues, but has a shorter lifetime.

The only operating mechanism in the oxidation of water to dioxygen catalyzed by the mononuclear cis-[Ru^II(bpy)₂(H₂O)₂]²⁺ complex when treated with excess Ce^IV was unambiguously established. Theoretical calculations together with ¹⁸O-labeling experiments (see plot) revealed that it is the nucleophilic attack of water on a Ru=O group.

Recent studies of organouranium chemistry have provided unusual pairs of similar polymetallic molecules containing (N)^3− and (O)^2− ligands, namely [(C₅Me₅)U(μ-I)₂]₃(μ₃-N), 1, and [(C₅Me₅)U(μ-I)₂]₃(μ₃-O), 2, and chair and boat conformations of [(C₅Me₅)₂U(μ-N)U(μ-N₃)(C₅Me₅)₂]₄, 3. These compounds were analyzed by density functional theory and multiconfigurational quantum chemical studies to differentiate nitride versus oxide in molecules for which the crystallographic data were not definitive and to provide insight into the electronic structure and unique chemical bonding of these polymetallic compounds. Calculations were also performed on [(C₅Me₅)₂UN₃(μ-N₃)]₃, 4, and [(C₆F₅)₃BNU(N[Me]Ph)₃], 5, for comparison with 1 and 3. On the basis of these results, the complex, [(C₅Me₅)U(μ₃-E)]₈, 6, for which only low-quality X-ray crystallographic data are available, was analyzed to predict if E is nitride or oxide.

The compounds Tc₂Cl₄(PMe₃)₄ and Tc₂Br₄(PMe₃)₄ were formed from the reaction between (n-Bu₄N)₂Tc₂X₈ (X = Cl, Br) and trimethylphosphine. The Tc(II) dinuclear species were characterized by single-crystal XRD, UV−visible spectroscopy, and cyclic voltammetry techniques, and the results are compared to those obtained from density functional theory and multiconfigurational (CASSCF/CASPT2) quantum chemical studies. The compound Tc₂Cl₄(PMe₃)₄ crystallizes in the monoclinic space group C2/c [a = 17.9995(9) Ã…, b = 9.1821(5) Ã…, c = 17.0090(9) Ã…, β = 115.4530(10)°] and is isostructural to M₂Cl₄(PMe₃)₄ (M = Re, Mo, W) and to Tc₂Br₄(PMe₃)₄. The metal−metal distance (2.1318(2) Ã…) is similar to the one found in Tc₂Br₄(PMe₃)₄ (2.1316(5) Ã…). The calculated molecular structures of the ground states are in excellent agreement with the structures determined experimentally. Calculations of effective bond orders for Tc₂X₈^2− and Tc₂X₄(PMe₃)₄ (X = Cl, Br) indicate stronger Ï€ bonds in the Tc₂⁴⁺ core than in Tc₂⁶⁺ core. The electronic spectra were recorded in benzene and show a series of low intensity bands in the range 10000−26000 cm−1. Assignment of the bands as well as computing their excitation energies and intensities were performed at both TD-DFT and CASSCF/CASPT2 levels of theory. Calculations predict that the lowest energy band corresponds to the δ* → σ* transition, the difference between calculated and experimental values being 228 cm^−1 for X = Cl and 866 cm^−1 for X = Br. The next bands are attributed to δ* → Ï€*, δ → σ*, and δ → Ï€* transitions. The cyclic voltammograms exhibit two reversible waves and indicate that Tc₂Br₄(PMe₃)₄ exhibits more positive oxidation potentials than Tc₂Cl₄(PMe₃)₄. This phenomenon is discussed and ascribed to stronger metal (d) to halide (d) back bonding in the bromo complex. Further analysis indicates that Tc(II) dinuclear species containing Ï€-acidic phosphines are more difficult to oxidize, and a correlation between oxidation potential and phosphine acidity was established.

Since the discovery of a formal quintuple bond in Ar′CrCrAr′ (CrCr = 1.835 Ã…) by Power and co-workers in 2005, many efforts have been dedicated to isolating dichromium species featuring quintuple-bond character. In the present study we investigate the electronic configuration of several, recently synthesized dichromium species with ligands using nitrogen to coordinate the metal centers. The bimetallic bond distances of Power’s compound and Cr₂-diazadiene (1) (CrCr = 1.803 Ã…) are compared to those found for Cr₂(μ-η²-ArNC(R)NAr)₂ (2) (CrCr = 1.746 Ã…; R = H, Ar = 2,6-Et₂C₆H₃), Cr₂(μ-η²-Ar^XylNC(H)NAr^Xyl)₃ (3) (CrCr = 1.740^reduced/1.817^neutral Ã…; Ar^Xyl= 2,6-C₆H₃-(CH₃)₂), Cr₂(μ-η²-TippPyNMes)₂ (4) (CrCr = 1.749 Ã…; TippPyNMes = 6-(2,4,6-triisopropylphenyl)pyridin-2-yl (2,4,6-trimethylphenyl)amide), and Cr₂(μ-η²-DippNC(NMe₂)N-Dipp)₂ (5) (CrCr = 1.729 Ã…, Dipp = 2,6-i-Pr₂C₆H₃). We show that the correlation between the CrCr bond length and the effective bond order (EBO) is strongly affected by the nature of the ligand, as well as by the steric hindrance due to the ligand structure (e.g., the nature of the coordinating nitrogen). A linear correlation between the EBO and CrCr bond distance is established within the same group of ligands. As a result, the CrCr species based on the amidinate, aminopyridinate, and guanidinate ligands have bond patterns similar to the Ar′CrCrAr′ compound. Unlike these latter species, the dichromium diazadiene complex is characterized by a different bonding pattern involving Cr−NÏ€ interactions, resulting in a lower bond order associated with the short metal−metal bond distance. In this case the short CrCr distance is most probably the result of the constraints imposed by the diazadiene ligand, implying a Cr₂N₄ core with a closer CrCr interaction.

Playing with a full deck: Single-crystal X-ray and neutron diffraction data show that the Th center in the title complex 1 (see structure; Th orange, B beige, N purple, C black, H blue) forms bonds with 15 H atoms, thus making 1 the first crystallographically characterized example of a complex with a Werner coordination number of fifteen. DFT calculations suggest that 1 adopts the fully symmetric 16-coordinate structure in the gas phase.

A thorough characterization of the Ru−Hbpp (in,in-{[Ru^II(trpy)(H₂O)]₂(μ-bpp)}³⁺ (trpy is 2,2′:6′,2′′-terpyridine, bpp is bis(2-pyridyl)-3,5-pyrazolate)) water oxidation catalyst has been carried out employing structural (single crystal X-ray), spectroscopic (UV−vis and NMR), kinetic, and electrochemical (cyclic voltammetry) analyses. The latter reveals the existence of five different oxidation states generated by sequential oxidation of an initial II,II state to an ultimate, formal IV,IV oxidation state. Each of these oxidation states has been characterized by UV−vis spectroscopy, and their relative stabilities are reported. The electron transfer kinetics for individual one-electron oxidation steps have been measured by means of stopped flow techniques at temperatures ranging from 10 to 40 °C and associated second-order rate constants and activation parameters (ΔH_‡ and ΔS_‡) have been determined. Room-temperature rate constants for substitution of aqua ligands by MeCN as a function of oxidation state have been determined using UV−vis spectroscopy. Complete kinetic analysis has been carried out for the addition of 4 equiv of oxidant (Ce^IV) to the initial Ru−Hbpp catalyst in its II,II oxidation state. Subsequent to reaching the formal oxidation state IV,IV, an intermediate species is formed prior to oxygen evolution. Intermediate formation and oxygen evolution are both much slower than the preceding ET processes, and both are first order with regard to the catalyst; rate constants and activation parameters are reported for these steps. Theoretical modeling at density functional and multireference second-order perturbation theory levels provides a microscopic mechanism for key steps in intermediate formation and oxygen evolution that are consistent with experimental kinetic data and also oxygen labeling experiments, monitored via mass spectrometry (MS), that unambiguously establish that oxygen−oxygen bond formation proceeds intramolecularly. Finally, the Ru−Hbpp complex has also been studied under catalytic conditions as a function of time by means of manometric measurements and MS, and potential deactivation pathways are discussed.

Deviations from statistical binding, that is cooperativity, in self-assembled polynuclear complexes partly result from intermetallic interactions ΔE^M,M, whose magnitudes in solution depend on a balance between electrostatic repulsion and solvation energies. These two factors have been reconciled in a simple point-charge model, which suggests severe and counter-intuitive deviations from predictions based solely on the Coulomb law when considering the variation of ΔE^M,M with metallic charge and intermetallic separation in linear polynuclear helicates. To demonstrate this intriguing behaviour, the ten microscopic interactions that define the thermodynamic formation constants of some twenty-nine homometallic and heterometallic polynuclear triple-stranded helicates obtained from the coordination of the segmental ligands L1-L11 with Zn²⁺ (a spherical d-block cation) and Lu³⁺ (a spherical 4f-block cation), have been extracted by using the site binding model. As predicted, but in contrast with the simplistic coulombic approach, the apparent intramolecular intermetallic interactions in solution are found to be i) more repulsive at long distance (ΔE^Lu,Lu_{1-4 >} ΔE^Lu,Lu_1-2), ii) of larger magnitude when Zn²⁺ replaces Lu³⁺ (ΔE^Zn,Lu_1-2 > ΔE^Lu,Lu_1-2) and iii) attractive between two triply charged cations held at some specific distance (ΔE^Lu,Lu_1-3 < 0). The consequences of these trends are discussed for the design of polynuclear complexes in solution.

A method is suggested which allows truncation of the virtual space in Cholesky decomposition-based multiconfigurational perturbation theory (CD-CASPT2) calculations with systematic improvability of the results. The method is based on a modified version of the frozen natural orbital (FNO) approach used in coupled cluster theory. The idea is to exploit the near-linear dependence among the eigenvectors of the virtual-virtual block of the second-order Møller–Plesset density matrix. It is shown that FNO-CASPT2 recovers more than 95% of the full CD-CASPT2 correlation energy while requiring only a fraction of the total virtual space, especially when large atomic orbital basis sets are in use. Tests on various properties commonly investigated with CASPT2 demonstrate the reliability of the approach and the associated reduction in computational cost and storage demand of the calculations.

Multiconfigurational quantum chemical methods (CASSCF/CASPT2) have been used to study the chemiionization reactions Ce + O → CeO⁺ + e^- and Ce + O₂ → CeO₂⁺ + e^-. Selected spectroscopic constants for CeO_n and CeO_n⁺ (n = 1, 2), as well as reaction enthalpies of the chemiionization reactions of interest, have been computed and compared with experimental values. In contrast to the lanthanum case, for both Ce + O₂(X³Î£_g^-) and Ce + O₂( a¹Î”_g), the Ce + O₂ → CeO₂⁺ + e^- reaction is shown to be exothermic, and thus, contributes to the experimental chemielectron spectra. The apparent discrepancy between the computed reaction enthalpies and the high kinetic energy offset values measured in the chemielectron spectra is rationalized by arguing that chemielectrons are produced mainly via two sequential reactions (Ce + O₂ → CeO + O, followed by Ce + O → CeO⁺ + e^-) as in the case of lanthanum. For Ce + O₂ (a¹Î”_g), a chemielectron band with higher kinetic energy than that recorded for Ce + O₂( X³Î£_g^-) is obtained. This is attributed to production of O( ¹D) from the reaction Ce + O₂( a¹Î”_g) → CeO + O( ¹D), followed by chemiionization via the reaction Ce + O( ¹D) → CeO⁺ + e^-. Accurate potential energy curves for the ground and a number of excited states of CeO and CeO⁺ have been computed, and a mechanism for the chemiionization reactions investigated experimentally was proposed.

The La + O and La + O₂ chemiionization reactions have been investigated with quantum chemical methods. For La + O₂(X³Î£_g) and La + O₂(a¹Î”_g), the chemiionization reaction La + O₂ → LaO₂⁺ + e^− has been shown to be endothermic and does not contribute to the experimental chemielectron spectra. For the La + O₂(X³Î£_g) reaction conditions, chemielectrons are produced by La + O₂ → LaO + O, followed by La + O → LaO⁺ + e^−. This is supported by the same chemielectron band, arising from La + O → LaO⁺ + e^−, being observed from both the La + O(³P) and La + O₂(X³Î£_g) reaction conditions. For La + O₂(a¹Î”_g), a chemielectron band with higher electron kinetic energy than that obtained from La + O₂(X³Î£_g) is observed. This is attributed to production of O(¹D) from the reaction La + O₂(a¹Î”_g) → LaO + O(¹D), followed by chemiionization via the reaction La + O(¹D) → LaO⁺ + e^−. Potential energy curves are computed for a number of states of LaO, LaO* and LaO⁺ to establish mechanisms for the observed La + O → LaO⁺ + e^− chemiionization reactions.