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.

A recent EPR study (M. Perrez Navarro et al., Proc. Natl. Acad. Sci.­ 2013, 110, 15561) provided evidence that ammonia binding to the oxygen-evolving complex (OEC) of photosystem II in its S₂ state takes place at a terminal-water binding position (W1) on the “dangler” manganese center Mn_A. This contradicted earlier interpretations of ¹⁴N electron-spin-echo envelope modulation (ESEEM) and extended X-ray absorption fine-structure (EXAFS) data, which were taken to indicate replacement of a bridging oxo ligand by an NH₂ unit. Here we have used systematic broken-symmetry density functional theory calculations on large (ca. 200 atom) model clusters of an extensive variety of substitution patterns and core geometries to examine these contradictory pieces of evidence. Computed relative energies clearly favor the terminal substitution pattern over bridging-ligand arrangements (by about 20–30 kcal mol⁻¹) and support W1 as the preferred binding site. Computed ¹⁴N EPR nuclear-quadrupole coupling tensors confirm previous assumptions that the appreciable asymmetry may be accounted for by strong, asymmetric hydrogen bonding to the bound terminal NH₃ ligand (mainly by Asp61). Indeed, bridging NH₂ substitution would lead to exaggerated asymmetries. Although our computed structures confirm that the reported elongation of an Mn–Mn distance by about 0.15 Å inferred from EXAFS experiments may only be reproduced by bridging NH₂ substitution, it seems possible that the underlying EXAFS data were skewed by problems due to radiation damage. Overall, the present data clearly support the suggested terminal NH₃ coordination at the W1 site. The finding is significant for the proposed mechanistic scenarios of OEC catalysis, as this is not a water substrate site, and effects of this ammonia binding on catalysis thus must be due to more indirect influences on the likely substrate binding site at the O5 bridging-oxygen position.

The conformational energy landscape and the associated electronic structure and spectroscopic properties (UV/Vis/near-infrared (NIR) and IR) of three formally d⁵/d⁶ mixed-valence diruthenium complex cations, [{Ru(dppe)Cp*}₂(μ-C≡CC₆H₄C≡C)]⁺, [1]⁺, [trans-{RuCl(dppe)₂}₂(μ-C≡CC₆H₄C≡C)]⁺, [2]⁺, and the Creutz–Taube ion, [{Ru(NH₃)₅}₂(μ-pz)]⁵⁺, [3]⁵⁺ (Cp=cyclopentadienyl; dppe=1,2-bis(diphenylphosphino)ethane; pz=pyrazine), have been studied using a nonstandard hybrid density functional BLYP35 with 35 % exact exchange and continuum solvent models. For the closely related monocations [1]⁺ and [2]⁺, the calculations indicated that the lowest-energy conformers exhibited delocalized electronic structures (or class III mixed-valence character). However, these minima alone explained neither the presence of shoulder(s) in the NIR absorption envelope nor the presence of features in the observed vibrational spectra characteristic of both delocalized and valence-trapped electronic structures. A series of computational models have been used to demonstrate that the mutual conformation of the metal fragments—and even more importantly the orientation of the bridging ligand relative to those metal centers—influences the electronic coupling sufficiently to afford valence-trapped conformations, which are of sufficiently low energy to be thermally populated. Areas in the conformational phase space with variable degrees of symmetry breaking of structures and spin-density distributions are shown to be responsible for the characteristic spectroscopic features of these two complexes. The Creutz–Taube ion [3]⁵⁺ also exhibits low-lying valence-trapped conformational areas, but the electronic transitions that characterize these conformations with valence-localized electronic structures have low intensities and do not influence the observed spectroscopic characteristics to any notable extent.

The purpose of this systematic experimental and theoretical study is to deeply understand the unique bonding situation in ferrocene-stabilized silylium ions as a function of the substituents at the silicon atom and to learn about the structure parameters that determine the ²⁹Si NMR chemical shift and electrophilicity of these strong Lewis acids. For this, ten new members of the family of ferrocene-stabilized silicon cations were prepared by a hydride abstraction reaction from silanes with the trityl cation and characterized by multinuclear ¹H and ²⁹Si NMR spectroscopy. A closer look at the NMR spectra revealed that additional minor sets of signals were not impurities but silylium ions with substitution patterns different from that of the initially formed cation. Careful assignment of these signals furnished experimental proof that sterically less hindered silylium ions are capable of exchanging substituents with unreacted silane precursors. Density functional theory calculations provided mechanistic insight into that substituent transfer in which the migrating group is exchanged between two silicon fragments in a concerted process involving a ferrocene-bridged intermediate. Moreover, the quantum-chemical analysis of the ²⁹Si NMR chemical shifts revealed a linear relationship between δ(²⁹Si) values and the Fe⋅⋅⋅Si distance for subsets of silicon cations. An electron localization function and electron localizability indicator analysis shows a three-center two-electron bonding attractor between the iron, silicon, and C′_ipso atoms, clearly distinguishing the silicon cations from the corresponding carbenium ions and boranes. Correlations between ²⁹Si NMR chemical shifts and Lewis acidity, evaluated in terms of fluoride ion affinities, are seen only for subsets of silylium ions, sometimes with non-intuitive trends, indicating a complicated interplay of steric and electronic effects on the degree of the Fe⋅⋅⋅Si interaction.

Herein, we report the preparation of a new unsymmetrical, bis(thiophosphinoyl)-substituted dilithio methandiide and its application for the synthesis of zirconium- and palladium-carbene complexes. These complexes were found to exhibit remarkably shielded ¹³C NMR shifts, which are much more highfield-shifted than those of “normal” carbene complexes. DFT calculations were performed to determine the origin of these observations and to distinguish the electronic structure of these and related carbene complexes compared with the classical Fischer and Schrock-type complexes. Various methods show that these systems are best described as highly polarized Schrock-type complexes, in which the metal–carbon bond possesses more electrostatic contributions than in the prototype Schrock systems, or even as “masked” methandiides. As such, geminal dianions represent a kind of “extreme” Schrock-type ligands favoring the ionic resonance structure M⁺CR₂⁻ as often used in textbooks to explain the nucleophilic nature of Schrock complexes.

A unique heterobimetallic disulfur monoradical, complex 2, with a diamond-shaped {NiS₂Pt} core has been synthesized by two-electron reduction of a supersulfido-(nacnac)nickel(II) complex (nacnac=β-diketiminato) with [Pt(Ph₃P)₂(η²-C₂H₄)] as a platinum(0) source and isolated in 82 % yield. Strikingly, the results of DFT calculations in accordance with spectroscopic (EPR, paramagnetic NMR) and structural features of the complex revealed that the bonding situation of the S₂ ligand is between the elusive “half-bonded” S₂ radical trianion () and two separated S²⁻ ligands. Accordingly, the Ni^II center is partially oxidized, whereas the Pt^II site is redox innocent. The complex can be reversibly oxidized to the corresponding Ni,Pt-disulfido monocation, compound 3, with a SS single bond, and reacts readily with O₂ to form the corresponding superoxonickel(II) and disulfidoplatinum(II) (4) complexes. These compounds have been isolated in crystalline form and fully characterized, including IR and multi-nuclear NMR spectroscopy as well as ESI mass spectrometry. The molecular structures of compounds 2–4 have been confirmed by single-crystal X-ray crystallography.

TeX₃[Al(OR^F)₄] (X=Cl, Br, I; R^F=C(CF₃)₃) were synthesized by the reaction of Ag[Al(OR^F)₄] and TeX₄ or the reaction of AuX, Ag[Al(OR^F)₄], and elemental tellurium in liquid SO₂. The compounds were characterized by ¹²⁵Te NMR in solution and by X-ray diffraction, Raman, and IR spectroscopy in the solid state. The vibrational spectra and the crystal structure show very weak secondary interactions, indicating “pseudo gas phase conditions” in the condensed phase. The observed trend of the ¹²⁵Te NMR chemical shifts along the [TeX₃]⁺ series follows neither the monotonous decrease known as “normal halogen dependence” nor the increase known as “inverse halogen dependence”. By relativistic two-component calculations based on the ZORA approach, we find that this “abnormal halogen dependence” results from an interplay of relativistic and solvent effects, where non-negligible scalar relativistic effects and intermediate-sized spin-orbit effects compensate to some extent. The reasons for these trends are evaluated in the context of the Te s-orbital character of the TeX bonds and compared with the halogen dependence(s) within the isoelectronic [SeX₃]⁺ and PX₃ series and related trihalomethyl [CX₃]⁺ cations.

Various indolenine squarylium dyes with additional electron-donating amine redox centres have been synthesised and their redox chemistry has been studied. A combination of cyclic voltammetry, spectro-electrochemistry and DFT calculations has been used to characterise the electronic structure of the mono-, di- and, in one case, trications. All monocations still retain the cyanine-like, delocalised character due to the relatively low redox potential of the squaraine bridge and are therefore compounds of Robin–Day class III. Thus we extended previous studies on organic mixed-valence systems by using the indolenine squaraine moiety as very electron-rich bridge between two electron-donating amine redox centres to provoke a strong coupling between the additional redox centres. We synthesised TA3, which has an N–N distance of 26 bonds between the triarylamine redox centres and is to our knowledge the longest bis(triarylamine) radical cation that is completely delocalised. We furthermore show that altering the symmetry of a squaraine dye by substitution of a squaric ring oxygen atom by a dicyanomethylene group has a direct impact on the optical properties of the monocations. In case of the dications, it turned out that the energetically most stable state of dianisylamine-substituted squaraines is an anti-ferromagnetically coupled open-shell singlet state.

The unusual bridging and semi-bridging binding mode of tertiary phosphanes, arsanes, and stibanes in dinuclear low-valent Group 9 complexes have been studied by density functional methods and bonding analyses. The influence of various parameters (bridging and terminal ligands, metal atoms) on the structural preferences and bonding of dinuclear complexes of the general composition [A¹ M¹(μ-CH₂)₂(μ-EX₃)M² A²] (M¹, M²=Co, Rh, Ir; A¹, A²=F, Cl, Br, I, κ²-acac; E=P, As, Sb, X=H, F, CH₃) has been analyzed. A number of factors have been identified that favor bridging or semi-bridging modes for the phosphane ligands and their homologues. A more symmetrical position of the bridging ligand EX₃ is promoted by more polar EX bonding, but by less electronegative (softer) terminal anionic ligands. Among the Group 9 metal elements Co, Rh, and Ir, the computations clearly show that the 4d element rhodium exhibits the largest preference for a {M¹(μ-EX₃)M²} bridge, in agreement with experimental observation. Iridium complexes should be valid targets, whereas cobalt does not seem to support well a symmetric bridging mode. Analyses of the Electron Localization Function (ELF) indicate a competition between a delocalized three-center bridge bond and direct metal–metal bonding.

Based on broken-symmetry density functional calculations, the ⁵⁵Mn hyperfine tensors of a series of exchange-coupled, mixed-valence, dinuclear Mn^IIIMn^IV complexes have been computed. We go beyond previous quantum chemical work by fully including the effects of local zero-field splitting (ZFS) interactions in the spin projection, following the first-order perturbation formalism of Sage et al. [J. Am. Chem. Soc.1989, 111, 7239]. This allows the ZFS-induced transfer of hyperfine anisotropy from the Mn^III site to the Mn^IV site to be described with full consideration of the orientations of local hyperfine and ZFS tensors. After scaling to correct for systematic deficiencies in the quantum chemically computed local ZFS tensors, good agreement with experimental ⁵⁵Mn anisotropies at the Mn^IV site is obtained. The hyperfine coupling anisotropies on the Mn^III site depend sensitively on structural distortions for a d⁴ ion. The latter are neither fully reproduced by using a DFT-optimized coordination environment nor by using experimental structures. For very small exchange-coupling constants, the perturbation treatment breaks down and a dramatic sensitivity to the scaling of the local ZFS tensors is observed. These results are discussed with respect to ongoing work to elucidate the structure of the oxygen-evolving complex of photosystem II by analysis of the EPR spectra.