Is CO a Special Ligand in Organometallic Chemistry? Theoretical Investigation of AB, Fe(CO)4AB, and Fe(AB)5(AB = N2, CO, BF, SiO)

Femtosecond Core-Level Spectroscopy Reveals Involvement of Triplet States in the Gas-Phase Photodissociation of Fe(CO)5

Excitation of iron pentacarbonyl [Fe(CO)5], a prototypical photocatalyst, at 266 nm causes the sequential loss of two CO ligands in the gas phase, creating catalytically active, unsaturated iron carbonyls. Despite numerous studies, major aspects of its ultrafast photochemistry remain unresolved because the early excited-state dynamics have so far eluded spectroscopic observation. This has led to the long-held assumption that ultrafast dissociation of gas-phase Fe(CO)5 proceeds exclusively on the singlet manifold. Herein, we present a combined experimental-theoretical study employing ultrafast extreme ultraviolet transient absorption spectroscopy near the Fe M2,3-edge, which features spectral evolution on 100 fs and 3 ps time scales, alongside high-level electronic structure theory, which enables characterization of the molecular geometries and electronic states involved in the ultrafast photodissociation of Fe(CO)5. We assign the 100 fs evolution to spectroscopic signatures associated with intertwined structural and electronic dynamics on the singlet metal-centered states during the first CO loss and the 3 ps evolution to the competing dissociation of Fe(CO)4 along the lowest singlet and triplet surfaces to form Fe(CO)3. Calculations of transient spectra in both singlet and triplet states as well as spin-orbit coupling constants along key structural pathways provide evidence for intersystem crossing to the triplet ground state of Fe(CO)4. Thus, our work presents the first spectroscopic detection of transient excited states during ultrafast photodissociation of gas-phase Fe(CO)5 and challenges the long-standing assumption that triplet states do not play a role in the ultrafast dynamics.

Beryllium as a Base: Complexes of Be(CO)3 with HX (X=F, Cl, Br, CN, NC, CCH, OH)

Beryllium chemistry is typically governed by its electron deficient character, but in some compounds it can act as a base. In order to understand better the unusual basicity of Be, we have systematically explored the complexes of one such compound, Be(CO)3, towards several hydrogen bond donors HX (X=F, Cl, Br, CN, NC, CCH, OH). For all complexes we find three different minima, two hydrogen bonded minima (to the Be or O atoms), and one weak beryllium bonded minimum. Further characterization of the interactions using a topological analysis of the electron density and Symmetry Adapted Perturbation Theory (SAPT) provide insight into the nature of these interactions. Overall these results highlight the capability of certain beryllium compounds to act as either a weak Lewis acid or, unconventionally, a Lewis base whose basicity towards hydrogen bonding is comparable to that of π systems.

Theoretical Investigation of Transient Species Following Photodissociation of Ironpentacarbonyl in Ethanol Solution

Photodissociation of ironpentacarbonyl [1Fe(CO)5] in solution generates transient species in different electronic states, which we studied theoretically. From ab initio molecular dynamics simulations in ethanol solution, the closed-shell parent compound 1Fe(CO)5 is found to interact weakly with the solvent, whereas the irontetracarbonyl [Fe(CO)4] species, formed after photodissociation, has a strongly spin-dependent behavior. It coordinates a solvent molecule tightly in the singlet state [1Fe(CO)4] and weakly in the triplet state [3Fe(CO)4]. From the simulations, we have gained insights into intersystem crossing in solvated irontetracarbonyl based on the distinct structural differences induced by the change in multiplicity. Alternative forms of coordination between 1Fe(CO)4 and functional groups of the ethanol molecule are simulated, and a quantum chemical investigation of the energy landscape for the coordinated irontetracarbonyl gives information about the interconversion of different transient species in solution. Furthermore, insights from the simulations, in which we find evidence of a solvent exchange mechanism, challenge the previously proposed mechanism of chain walking for under-coordinated metal carbonyls in solution.

Pressure Induced Alteration and Stabilization of Intermolecular Stacking in Square-planar Osmium tetracarbonyl

Osmium carbonyls are well known to form stable 18-electron complexes like Os(CO)5 , Os2 (CO)9 and Os3 (CO)12 having both bridging and terminal carbonyls. For osmium tetra-carbonyl, Os(CO)4 solid-state packing significantly alters the ground-state structure. The gas-phase stable see-saw geometry converts to a square-planar structure in solid state. Highly efficient intermolecular stacking between Os(CO)4 units assists this transformation. Each Os(CO)4 molecule is stacked in a staggered orientation with respect to each other. Pressure induces a [Xe]4f14 5d6 6s2 (S=2)→[Xe]4f14 5d8 (S=0) electronic transition in osmium stabilize a square planar osmium tetra-carbonyl. Under the influence of isotropic pressure, the molecules not only come closer to each other but their relative orientations also get significantly altered. Calculations show that at P=1 GPa and above, the eclipsed orientation for the intermolecular stacking gets preferred over the staggered form. The staggered→eclipsed intermolecular stacking orientation under pressure is shown to be controlled by London dispersion interactions.

Identification of Ge≡O Triple Bond in Ge6O- Cluster: Anion Photoelectron Spectroscopy and Theoretical Calculations

Unlike C≡O, which is common in coordination chemistry and organometallic chemistry, little is known about Si≡O or Ge≡O compounds. Here we report a Ge₆O^- cluster featuring a Ge≡O triple bond. The structural and chemical bonding properties of Ge₆O^-/0 are investigated using anion photoelectron spectroscopy and theoretical calculations. Two nearly degenerate isomers have been found for Ge₆O^-. The lowest-energy structure (6A) can be viewed as an O atom bonding with a tetragonal bipyramidal Ge₆. The second one (6B) can be considered as an O atom interacting with a capped trigonal bipyramidal Ge₆. Chemical bonding analyses reveal that Ge₆O^- (6A) can be viewed as a Ge≡O unit interacting with a σ antiaromatic C_2v symmetric tetragonal pyramidal Ge₅^3- moiety. Comparisons of the chemical bonding in Ge₆O^- (6A) with that in Ge₅CO^- and Ge₅MnO^- indicate the similar behavior of Ge≡O to C≡O and Mn≡O in its bonding to the Ge₅^3- and Ge₅^4- moieties.

Terminal end-on coordination of dinitrogen versus isoelectronic CO: A comparison using the charge displacement analysis

The metal dinitrogen bonding in a wide series of terminal end-on dinitrogen complexes is investigated with the charge displacement analysis based on natural orbitals of chemical valence (CD-NOCV). The effect of the σ donation and π backdonation on the NN bond are discussed and compared with the observations for a series of carbonyl complexes, published in 2016 by Tarantelli et al. The σ donation is relative invariant over the series of dinitrogen complexes and has no significant effect on the NN bond strength, whereas the π backdonation causes a considerable elongation of the NN bond. Some uncommon examples of weakly bound dinitrogen with blue-shifted stretching frequency compared to free N2 (ν = 2330 cm-1 ) are known. The dinitrogen bonding in these complexes is simulated with a point charge. Apparently, electrostatics account for the shortened N─N bond in these systems.

1,2-Dihydro-1,3,2-diazaborinine tautomer as an electron-pair donor in hydrogen-bonded complexes

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to investigate 1,2-dihydro-1,3,2-diazaborinine:HX complexes for HX = H+, HF, HCl, H2O, HCN, NH3, HCP, and HCCH. Most complexes are stabilized by linear, traditional hydrogen bonds except for those with H2O and NH3, which have bridging structures and nonlinear hydrogen bonds. H-atom transfer from N to B can occur in complexes with HF and HCl, with formation of a traditional F–H···N bond and a proton-shared Cl···H···N bond. The binding energies of the uncharged complexes range from 25 to 88 kJ mol−1. Spin-spin coupling constants have been used to characterize these hydrogen-bonded complexes.

Variational Energy Decomposition Analysis of Charge-Transfer Interactions between Metals and Ligands in Carbonyl Complexes

Variational energy decomposition analyses have been presented to quantify the σ-dative, ligand-to-metal forward charge transfer (CT) and the π-conjugative, metal-to-ligand backward charge delocalization on a series of isolelectronic transition-metal carbonyl complexes M(CO)₆, including M = Ti^2-, V^-, Cr, Mn⁺, and Fe²⁺. Although the qualitative features of these energy terms are understood, well-defined quantitative studies have been scarce. Consistent with early findings, electrostatic and Pauli exchange effects play a key role in σ-donation, resulting in blue shifts in ligand vibrational frequency in the complex geometries. Excluding chemical bonding interactions between the CO ligand and the metal fragments in the energy decomposition analysis, we found that loosely bound electrostatic complexes can be formed at a longer metal-to-ligand distance due to the exponential decay of Pauli exchange. In all complexes, the overall binding stabilization can be attributed to CT effects, with opposing trends between σ-donation and π-back bonding that follows an order of Ti^2- (4.4) > V^1- (2.6) > Cr (1.5) > Mn¹⁺ (1.1) > Fe²⁺ (0.5) in π-to-σ CT ratio. These electronic and energetic features are mirrored in the vibrational frequency shifts induced by different factors. The present investigation may help stimulate the use of energy decomposition techniques to understand the structure and activity of metallocatalysts using density functional theory.

The cooperative role of innocent ligand in N-heterocyclic carbene manganese catalyzed carbon dioxide hydrogenation

The concept of Lewis acidic π* cooperation was proposed for innocent CO ligand in NHC–Mn catalyzed CO2 hydrogenation by systematic DFT studies.

Intermetallic transfer of unsymmetrical borylene fragments: isolation of the second early-transition-metal terminal borylene complex and other rare species

Transition metal borylene complexes of the type [(OC)5M[double bond, length as m-dash]BN(SiMe3)(tBu)] (M = Cr, Mo, W) have been synthesised by salt elimination of the corresponding dibromoborane and the dianionic metallates Na2[M(CO)5]. The borylene complexes have been characterised by multinuclear solution-state NMR spectroscopy and solid-state molecular structure determination. The group 6 borylene complexes can be used to effectively transfer the borylene ligand to other transition metal complexes by replacing one or two carbonyl ligands upon irradiation of the reaction mixture with UV light. This borylene transfer reaction led to the formation of new terminal and bridging borylene complexes which cannot be formed by the corresponding salt elimination reactions, including a rare example of a bis(terminal borylene) complex and only the second reported terminal borylene complex of an early transition metal (vanadium).

Structure and bonding in triorganotin chlorides: a perspective from energy decomposition analysis

The Sn-Cl chemical bond of four organotin halides (Me₃SnCl, Et₃SnCl, Bu₃SnCl, and Ph₃SnCl) was studied by using relativistic density functional theory in combination with a quantitative energy decomposition analysis to explain the formation of charged species. The σ orbital is the dominant contributor to the stabilization of the Sn-Cl bond, and the π-orbital interactions also have a significant contribution to the stabilization of Ph₃Sn⁺ cation when the aromatic groups are bonded to the tin atom. The aromaticity of the phenyl groups delocalizes the positive charge, donating electrons to tin atom by conjugation. Although Me₃SnCl and Ph₃SnCl are constituted by groups which the size of the substituents is different, the interaction energies obtained with the energy decomposition analysis present similar values, which also occur with the thermodynamic parameters. Graphical abstract Organotin compounds have widely studied as a potential antitumoral agent. The mechanism in triorganotin compounds includes the formation of cation species, R₃Sn⁺. This article studies the influence of the R groups on the rupture of Sn-Cl bond using the fragment analysis and quantitative energy decomposition analysis.

Chemical interactions and dynamics with femtosecond X-ray spectroscopy and the role of X-ray free-electron lasers

X-ray free-electron lasers with intense, tuneable and short-pulse X-ray radiation are transformative tools for the investigation of transition-metal complexes and metalloproteins. This becomes apparent in particular when combining the experimental observables from X-ray spectroscopy with modern theoretical tools for calculations of electronic structures and X-ray spectra from first principles. The combination gives new insights into how charge and spin densities change in chemical reactions and how they determine reactivity. This is demonstrated for the investigations of structural dynamics with metal K-edge absorption spectroscopy, spin states in excited-state dynamics with metal 3p-3d exchange interactions, the frontier-orbital interactions in dissociation and substitution reactions with metal-specific X-ray spectroscopy, and studies of metal oxidation states with femtosecond pulses for 'probe-before-destroy' spectroscopy. The role of X-ray free-electron lasers is addressed with thoughts about how they enable 'bringing back together' different aspects of the same problem and this is thought to go beyond a conventional review paper where these aspects are formulated in italic font type in a prequel, an interlude and in a sequel. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Rocha MV, Hamlin TA, Poater J, Bickelhaupt FM. Understanding the differences between iron and palladium in cross-coupling reactions

We aim at developing design principles, based on quantum chemical analyses, for a novel type of iron-based catalysts that mimic the behavior of their well-known palladium analogs in the bond activation step of cross coupling reactions. To this end, we have systematically explored C-X bond activation via oxidative addition of CH3X substrates (X = H, Cl, CH3) to model catalysts mFe(CO)4q (q = 0, -2; m = singlet, triplet) and, for comparison, Pd(PH3)2 and Pd(CO)2, using relativistic density functional theory at the ZORA-OPBE/TZ2P level. We find that the neutral singlet iron catalyst 1Fe(CO)4 activates all three C-X bonds via barriers that are lower than those for Pd(PH3)2 and Pd(CO)2. This is a direct consequence of the capability of the iron complex to engage not only in π-backdonation, but also in comparably strong σ-donation. Interestingly, whereas the palladium complexes favor C-Cl activation, 1Fe(CO)4 shows a strong preference for activating the C-H bond, with a barrier as low as 10.4 kcal mol-1. Our results suggest a high potential for iron to feature in palladium-type cross-coupling reactions.

Terminal coordination of diatomic boron monofluoride to iron

Boron monofluoride (BF) is a diatomic molecule with 10 valence electrons, isoelectronic to carbon monoxide (CO). Unlike CO, which is a stable molecule at room temperature and readily serves as both a bridging and terminal ligand to transition metals, BF is unstable below 1800°C in the gas phase, and its coordination chemistry is substantially limited. Here, we report the isolation of the iron complex Fe(BF)(CO)₂(CNAr^Tripp2)₂ [Ar^Tripp2, 2,6-(2,4,6-(i-Pr)₃C₆H₂]₂C₆H₃; i-Pr, iso-propyl], featuring a terminal BF ligand. Single-crystal x-ray diffraction as well as nuclear magnetic resonance, infrared, and Mössbauer spectroscopic studies on Fe(BF)(CO)₂(CNAr^Tripp2)₂ and the isoelectronic dinitrogen (N₂) and CO complexes Fe(N₂)(CO)₂(CNAr^Tripp2)₂ and Fe(CO)₃(CNAr^Tripp2)₂ demonstrate that the terminal BF ligand possesses particularly strong σ-donor and π-acceptor properties. Density functional theory and electron-density topology calculations support this conclusion.

Diffusion network of CO in FeFe-Hydrogenase

FeFe-hydrogenase is an efficient enzyme to produce H₂ under optimal conditions. However, the activity of this enzyme is highly sensitive to the presence of inhibitory gases CO and O₂ that cause irreversible damage to the active site. Therefore, a detailed knowledge of the diffusion pathways of these inhibitory gases is necessary to develop strategies for designing novel enzymes that are tolerant to these gases. In this work, we studied the diffusion pathways of CO in the CpI FeFe-hydrogenase from Clostridium pasteurianum. Specifically, we used several enhanced sampling and free-energy simulation methods to reconstruct a three-dimensional free-energy surface for CO diffusion which revealed 45 free-energy minima forming an interconnected network of pathways. We discovered multiple pathways of minimal free-energy as diffusion portals for CO and found that previously suggested hydrophobic pathways are not thermodynamically favorable for CO diffusion. We also observed that the global minimum in the free-energy surface is located in the vicinity of the active-site metal cluster, the H-cluster, which suggests a high-affinity for CO near the active site. Among 19 potential residues that we propose as candidates for future mutagenesis studies, 11 residues are shared with residues that have been previously proposed to increase the tolerance of this enzyme for O₂. We hypothesize that these shared candidate residues are potentially useful for designing new variants of this enzyme that are tolerant to both inhibitory gases.

Fostering the Basic Instinct of Boron in Boron-Beryllium Interactions

A set of complexes L₂HB···BeX₂ (L = CNH, CO, CS, N₂, NH₃, NCCH₃, PH₃, PF₃, PMe₃, OH₂; X = H, F) containing a boron-beryllium bond is described at the M06-2X/6-311+G(3df,2pd)//M062-2X/6-31+G(d) level of theory. In this quite unusual bond, boron acts as a Lewis base and beryllium as a Lewis acid, reaching binding energies up to -283.3 kJ/mol ((H₂O)₂HB···BeF₂). The stabilization of these complexes is possible thanks to the σ-donor role of the L ligands in the L₂HB···BeX₂ structures and the powerful acceptor nature of beryllium. According to the topology of the density, these B-Be interactions present positive laplacian values and negative energy densities, covering different degrees of electron sharing. ELF calculations allowed measuring the population in the interboundary B-Be region, which varies between 0.20 and 2.05 electrons upon switching from the weakest ((CS)₂HB···BeH₂) to the strongest complex ((H₂O)₂HB···BeF₂). These B-Be interactions can be considered as beryllium bonds in most cases.

The strongest CO binding and the highest C-O stretching frequency

A coupled-cluster study is performed on CO bound BeY complexes (Y = O, CO₃, SO₄, NH, NCN, and NBO) to understand the effect of attached ligands (Y) on the CO binding ability and C-O stretching frequency (ν_CO). Herein, we report that BeNCN has the highest CO binding ability (via both C- and O-side binding) among the studied neutral Be-based clusters, whereas OCBeSO₄ has the highest ν_CO among the neutral carbonyls. The nature and extent of shift in ν_CO compared to free CO are explained in terms of change in polarization in the bonding orbitals of CO and relative contribution from OC→BeY or CO→BeY σ-donation, and OC←BeY or CO←BeY π-back-donation. The largest blue-shift in OCBeSO₄ and the largest red-shift in COBeNH are consequences of the smallest OC←BeSO₄ π-back-donation and the largest CO←BeNH π-back-donation, respectively.

Encoding of coordination complexes with XML

An in-silico system to encode structure, bonding and properties of coordination complexes is developed. The encoding is achieved through a semantic XML markup frame. Composition of the coordination complexes is captured in terms of central atom and ligands. Structural information of central atom is detailed in terms of electron status of valence electron orbitals. The ligands are encoded with specific reference to the electron environment of ligand centre atoms. Behaviour of ligands to form low or high spin complexes is accomplished by assigning a Ligand Centre Value to every ligand based on the electronic environment of ligand centre atom. Chemical ontologies are used for categorization purpose and to control different hybridization schemes. Complexes formed by the central atoms of transition metal, non-transition elements belonging to s-block, p-block and f-block are encoded with a generic encoding platform. Complexes of homoleptic, heteroleptic and bridged types are also covered by this encoding system. Utility of the encoded system to predict redox electron transfer reaction in the coordination complexes is demonstrated with a simple application.

Synthesis, Characterization and Thermal Properties of Poly(ethylene oxide), PEO, Polymacromonomers via Anionic and Ring Opening Metathesis Polymerization

Branched polymers are a valuable class of polymeric materials. In the present study, anionic polymerization techniques were employed for the synthesis of low molecular weight poly(ethylene oxide) (PEO) macromonomers bearing norbornenyl end groups. The macromonomers were characterized by SEC, MALDI-TOF and NMR spectroscopy. Subsequent ring opening metathesis polymerization (ROMP) of the macromonomers using ruthenium catalysts (Grubbs catalysts of the 1st, 2nd and 3rd generations) afforded the corresponding polymacromonomers. The effects of the macromonomer molecular weight, the type of the catalyst, the nature of the solvent, the monomer concentration and the polymerization temperature on the molecular characteristics of the branched polymers were examined in detail. The crystallization behavior of the macromonomers and the corresponding polymacromonomers were studied by Differential Scanning Calorimetry (DSC). The thermal stability and the kinetics of the thermal decomposition of the samples were also studied by Thermogravimetric Analysis (TGA). The activation energies of the thermal decomposition were analyzed using the Ozawa–Flynn–Wall and Kissinger methodologies.

Computational Study on the Mechanisms of Multiple Complexation of CO and Isonitrile Ligands to Boron

The recent experimental realization of compound Tripp-B(CO)₂ (denoted as 2a), where Tripp is 2,6-di(2,4,6-triisopropylphenyl)-phenyl), breaks through conventional knowledge that only transition metals can bind more than one CO to form multicarbonyl adducts. Compound 2a is stable in air but liberates CO under light. The B-CO bonds of 2a are considered to be similar to donor-acceptor bonds of transition metal complexes. To address the formation mechanism and chemical bonding of this novel type of boron compounds, we present a density functional theory study on the formation and photolysis of 2a and similar compounds. The results suggest that the formation of 2a is facile by three consecutive additions of CO to the terminal borylene metal complex, that is, the boron source of the synthesis. These CO additions can be practically accomplished via two different paths: CO direct addition and CO migration followed by addition. Such mechanisms can be excellently rationalized by the donor-acceptor bonding model of the terminal borylene complex, which in turn suggests that using donor-acceptor bonds for 2a is natural for understanding the mechanisms. Liberation of CO from 2a and its similar compounds has higher energy barriers at the ground states than that at the triplet states by 40 kcal/mol. These energy barriers explain the experimentally observed air stability and photolysis of these compounds. The results for the first time provide mechanistic insights for the unprecedented chemical processes; they allow evaluation of the applicability of donor-acceptor bonding in main-group compounds from the new perspective of chemical reactions.

Boron as an Electron-Pair Donor for B⋅⋅⋅Cl Halogen Bonds

MP2/aug'-cc-pVTZ calculations were performed to investigate boron as an electron-pair donor in halogen-bonded complexes (CO)₂ (HB):ClX and (N₂ )₂ (HB):ClX, for X=F, Cl, OH, NC, CN, CCH, CH₃ , and H. Equilibrium halogen-bonded complexes with boron as the electron-pair donor are found on all of the potential surfaces, except for (CO)₂ (HB):ClCH₃ and (N₂ )₂ (HB):ClF. The majority of these complexes are stabilized by traditional halogen bonds, except for (CO)₂ (HB):ClF, (CO)₂ (HB):ClCl, (N₂ )₂ (HB):ClCl, and (N₂ )₂ (HB):ClOH, which are stabilized by chlorine-shared halogen bonds. These complexes have increased binding energies and shorter B-Cl distances. Charge transfer stabilizes all complexes and occurs from the B lone pair to the σ* Cl-A orbital of ClX, in which A is the atom of X directly bonded to Cl. A second reduced charge-transfer interaction occurs in (CO)₂ (HB):ClX complexes from the Cl lone pair to the π* C≡O orbitals. Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin-spin coupling constants, ^1x J(B-Cl), across the halogen bonds are also indicative of the changing nature of this bond. ^1x J(B-Cl) values for both series of complexes are positive at long distances, increase as the distance decreases, and then decrease as the halogen bonds change from traditional to chlorine-shared bonds, and begin to approach the values for the covalent bonds in the corresponding ions [(CO)₂ (HB)-Cl]⁺ and [(N₂ )₂ (HB)-Cl]⁺ . Changes in ¹¹ B chemical shieldings upon complexation correlate with changes in the charges on B.

Evidence for a chemical clock in oscillatory formation of UiO-66

Chemical clocks are often used as exciting classroom experiments, where an induction time is followed by rapidly changing colours that expose oscillating concentration patterns. This type of reaction belongs to a class of nonlinear chemical kinetics also linked to chaos, wave propagation and Turing patterns. Despite its vastness in occurrence and applicability, the clock reaction is only well understood for liquid-state processes. Here we report a chemical clock reaction, in which a solidifying entity, metal–organic framework UiO-66, displays oscillations in crystal dimension and number, as shown by X-ray scattering. In rationalizing this result, we introduce a computational approach, the metal–organic molecular orbital methodology, to pinpoint interaction between the tectonic building blocks that construct the metal–organic framework material. In this way, we show that hydrochloric acid plays the role of autocatalyst, bridging separate processes of condensation and crystallization.

Reactions with non-linear kinetics, such as chemical clocks, are reasonably common but only well understood in the liquid phase. Here, the authors report and rationalize a chemical clock reaction taking place in a solidifying metal-organic framework.

σ-Aromatic cyclic M3+ (M = Cu, Ag, Au) clusters and their complexation with dimethyl imidazol-2-ylidene, pyridine, isoxazole, furan, noble gases and carbon monoxide

The structure, stability, bonding and σ-aromaticity in dimethyl imidazol-2-ylidene, pyridine, isoxazole, furan, noble gas and carbon monoxide bound M₃⁺ (M = Cu, Ag, Au) complexes are analyzed.

Phase Diagram and Transformations of Iron Pentacarbonyl to nm Layered Hematite and Carbon-Oxygen Polymer under Pressure

We present the phase diagram of Fe(CO)₅, consisting of three molecular polymorphs (phase I, II and III) and an extended polymeric phase that can be recovered at ambient condition. The phase diagram indicates a limited stability of Fe(CO)₅ within a pressure-temperature dome formed below the liquid- phase II- polymer triple point at 4.2 GPa and 580 K. The limited stability, in turn, signifies the temperature-induced weakening of Fe-CO back bonds, which eventually leads to the dissociation of Fe-CO at the onset of the polymerization of CO. The recovered polymer is a composite of novel nm-lamellar layers of crystalline hematite Fe₂O₃ and amorphous carbon-oxygen polymers. These results, therefore, demonstrate the synthesis of carbon-oxygen polymer by compressing Fe(CO)₅, which advocates a novel synthetic route to develop atomistic composite materials by compressing organometallic compounds.

Pressure-imposed changes of benzoic acid crystals

Structural and energetic properties of benzoic acid crystals at pressure elevated from ambient condition up to 2.21 GPa were characterized. The directly observed variations of cell parameters and consequently cell volume are associated with many other changes including energetic, geometric, and electronic characteristics. First of all the non-monotonous change of lattice energy are noticed with the rise of pressure since the increase of stabilization up to 1GPa is followed by systematic decrease of lattice energies after extending the hydrostatic compression. There is also an observed increase of C₂²(8) synthon stabilization interaction with increase of pressure. The lattice response rather than interaction within synthons are source of observed pressure-related trend of lattice energy changes. The energy decomposition analysis revealed that the total steric interactions determine the overall trend of lattice energy change with the rise of pressure. Besides geometric aromaticity index was used as a measure of geometric changes. Serious discrepancies were noticed between HOMA values computed with the use of experimental and optimized geometries of the ring. Even inclusion of uncertainties of experimental geometries related to limited precision of X-ray diffraction measurements does not cancel mentioned discrepancies. Although HOMA exhibit similar trends at modest pressures the diversity became surprisingly high at more extreme conditions. This might suggest limitations of periodic DFT computations at elevated pressures and the experimentally observed breaking of molecules at very high pressures will probably not be accounted properly in this approach. Also limitation of direct use of experimental geometries were highlighted.

The siliconyl, boronyl, and iminoboryl ligands as analogues of the well-known carbonyl ligand: predicted reactivity towards dipolar cyclooligomerization in iron/cobalt carbonyl complexes

The Fe(CO)₄(SiO), Co(CO)₄(BO), and Co(CO)₄(BNSiMe₃), complexes akin to the well-known Fe(CO)₅ are predicted by density functional theory to undergo exothermic oligomerization to give the oligomers containing Si_nO_n/B_nO_n/B₂N₂ rings with single bonds.

P3F9(2-): an all-pseudo-π* 2π-aromatic

A qualitative MO analysis suggests (PH3)3(2-) as a candidate for an all-pseudo-π* 2π-aromatic; however computational studies rule out its existence. Fluorine substitution which increases the contribution of p orbitals on P in the pseudo-π* MO makes (PF3)3(2-) a minimum and an aromatic. The 2π aromaticity arising from the bonding combination of the three pseudo-π* fragment MOs is comparable to that in C3O3(2-) and is another example for the analogy between CO and PF3. The dianion (PF3)3(2-) forms the first example of a three-membered ring with all the vertices constituted by pentacoordinate phosphorus. The ability of PF3 to form the all-pseudo-π* 2π-aromatic system is not shared by the heavier analogues, AsF3 and SbF3.