Ultra-incompressible Ternary Diborides Re_（0.5）Ir_（0.5）B_2,Re_（0.5）Tc_（0.5）B_2,Os_（0.5）W_（0.5）B_2 and Os_（0.5）Ru_（0.5）B_2：Density Functional Calculation

In this paper, density functional computations have been applied to the structural, elastic and electronic properties of ternary transition metal diborides Re0.5Ir0.5B2, Re0.5Tc0.5B2, Os0.5W0.5B2 and Os0.5Ru0.5B2 in hexagonal (P63/mmc) and orthorhombic (Pmmn) structures with both local density approximation and generalized gradient approximation. LDA gives smaller lattice parameters and larger elastic moduli than GGA. Both results show that the hexagonal ones are more stable than orthorhombic ones except Os0.5Ru0.5B2. Moreover, the hexagonal structure has superior elastic property than orthorhombic one. Generally speaking, the calculated elastic moduli of Re0.5Ir0.5B2 and Os0.5Ru0.5B2 are smaller than those values of Re0.5Tc0.5B2 and Os0.5W0.5B2 within the same structure because of the filling of antibonding states. The relativistic effects result in weaker bonds of Tc-B (Ru-B) than those of Re-B (Os-B). All the diborides are ultra-incompressible. Re0.5Tc0.5B2 has the largest shear modulus and it is a promising superhard diboride like Os0.5W0.5B2. The elastic properties are in high correlation with the bond strength. The shear moduli are more sensitive than the bulk moduli to the bond strength.     

A boundary for reaction mechanisms within the transition metals interacting with nitrogen oxides is observed in DSIMS experiments

Summary Dynamic secondary ion mass spectrometry (DSIMS) investigations have been carried out with Cr, Mn, Fe, Co, Mo, Rh, W, Re, Os and Ir under 4 mPa N₂O, NO and 3 mPa NO₂ as reactant gases. Results indicate similar behaviour in adsorption for Cr, Mn, Fe, Mo, W on the one hand and for Co, Rh, Os and Ir on the other. For the first group of metals the nitrogen oxide molecules are always totally destroyed in adsorption whereas the second group shows evidence for surface compounds such as MeNO (Me=metal) indicating only a partial dissociation in the case of N₂O and NO₂, and molecular adsorption under NO respectively. Re does not belong uniquely to either group because it reacts with N₂O and NO₂ dissociatively whereas under NO only partial dissociation is observed.Abbreviations SIMS Secondary ion mass spectrometry - SSIMS Static SIMS - AES Auger electron spectroscopy - EELS Electron energy-loss spectroscopy - LEED Low energy electron diffraction - TDS Thermal desorption spectroscopy - XPS, UPS X-ray/Ultraviolet photoelectron spectroscopy     

Probes of spin conservation in heavy metal reactions: experimental and theoretical studies of the reactions of Re+ with H2, D2, and HD

A guided ion beam tandem mass spectrometer is used to examine the kinetic energy dependence of reactions of the third-row transition metal cation, Re(+), with molecular hydrogen and its isotopologues. A flow tube ion source produces Re(+) in its (7)S(3) electronic ground state. Reaction with H(2), D(2), and HD forms Re H(+)(Re D(+)) in endothermic processes. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energy of D(0)(Re(+)-H)=2.29+/-0.07 eV (221+/-6 kJ/mol). The experimental thermochemistry is consistent with ab initio calculations, performed here and in the literature. Theory also provides the electronic structures of these species and is used to examine the reactive potential energy surfaces. Results from reactions with HD provide insight into the reaction mechanisms and indicate that the late metal ion, Re(+), reacts largely via a statistical mechanism. This is consistent with the potential energy surfaces which locate a stable Re H(2) (+)((5)B(2)) complex. Results for this third-row transition metal system are compared with the first-row congener (Mn(+)) and found to have much higher reactivity towards dihydrogen and stronger M(+)-H bonds. These differences can be attributed to efficient coupling among surfaces of different spin along with lanthanide contraction and relativistic effects.     

Guided ion beam and theoretical study of the reactions of Os+ with H2, D2, and HD

Reactions of the third-row transition metal cation Os(+) with H(2), D(2), and HD to form OsH(+) (OsD(+)) were studied using a guided ion beam tandem mass spectrometer. A flow tube ion source produces Os(+) in its (6)D (6s(1)5d(6)) electronic ground state level. Corresponding state-specific reaction cross sections are obtained. The kinetic energy dependences of the cross sections for the endothermic formation of OsH(+) and OsD(+) are analyzed to give a 0 K bond dissociation energy of D(0)(Os(+)-H) = 2.45 ± 0.10 eV. Quantum chemical calculations are performed here at several levels of theory, with B3LYP approaches generally overestimating the experimental bond energy whereas results obtained using BHLYP and CCSD(T), coupled-cluster with single, double, and perturbative triple excitations, levels show good agreement. Theory also provides the electronic structures of these species and the potential energy surfaces for reaction. Results from the reactions with HD provide insight into the reaction mechanism and indicate that Os(+) reacts via a direct reaction. We also compare this third-row transition metal system with the first-row and second-row congeners, Fe(+) and Ru(+), and find that Os(+) reacts more efficiently with dihydrogen, forming a stronger M(+)-H bond. These differences can be attributed to the lanthanide contraction and relativistic effects.     

Influence of polyoxometalate ligands on the nature of high-valent transition metal nitrido species. A theoretical analysis of experimentally known and unprecedented compounds

The electronic structure of group 6-8 transition metal (TM) nitrido derivatives [PW(11)O(39){TM(VI)N}](4-) is studied computationally and the potential reactivity of the polyoxoanions is discussed. The observed electrophilic reactivity for the Ru(VI) nitrido derivative is rationalized from frontier molecular orbital analysis. When we move to the left or down in the periodic table (TM = Os, Tc, Re, Mo and W) the electrophilic character of the polyoxometalate decreases or the cluster should be better regarded as a nucleophile. The DFT analysis of the redox properties suggests that the still unknown high-valent Mn(VI)N and Fe(VI)N units could be stabilized by the porphyrin-like ligand [PW(11)O(39)](7-) and their electronic structure indicates that these anions should have a high potential reactivity towards nucleophiles.     

Single d-metal atoms on F(s) and F(s+) defects of MgO(001): a theoretical study across the periodic table

Single d-metal atoms on oxygen defects F(s) and F(s+) of the MgO(001) surface were studied theoretically. We employed an accurate density functional method combined with cluster models, embedded in an elastic polarizable environment, and we applied two gradient-corrected exchange-correlation functionals. In this way, we quantified how 17 metal atoms from groups 6-11 of the periodic table (Cu, Ag, Au; Ni, Pd, Pt; Co, Rh, Ir; Fe, Ru, Os; Mn, Re; and Cr, Mo, W) interact with terrace sites of MgO. We found bonding with F(s) and F(s+) defects to be in general stronger than that with O2- sites, except for Mn-, Re-, and Fe/F(s) complexes. In M/F(s) systems, electron density is accumulated on the metal center in a notable fashion. The binding energy on both kinds of O defects increases from 3d- to 4d- to 5d-atoms of a given group, at variance with the binding energy trend established earlier for the M/O2- complexes, 4d < 3d < 5d. Regarding the evolution of the binding energy along a period, group 7 atoms are slightly destabilized compared to their group 6 congeners in both the F(s) and F(s+) complexes; for later transition elements, the binding energy increases gradually up to group 10 and finally decreases again in group 11, most strongly on the F(s) site. This trend is governed by the negative charge on the adsorbed atoms. We discuss implications for an experimental detection of metal atoms on oxide supports based on computed core-level energies.     

Electronic structures of trans-dioxometal complexes

We have employed computational methods based on density functional theory to elucidate the effects of equatorial ligands on the electronic structures of trans-dioxometal complexes. In complexes with amine (sigma-only) equatorial donors, the (1)A(1 g)(b(2 g))(2)-->(1)E(g)(b(2 g))(1)(e(g))(1) excitation energy increases with metal oxidation state: Mo(IV) < Tc(V) < Ru(vi) and W(IV) < Re(V) < Os(VI). Increasing transition energies are attributed to enhanced oxometal pi-donor interactions in the higher valent central metals. But in complexes with cyanide equatorial donors, the (1)A(1 g)(b(2 g))(2)-->(1)E(g)(b(2 g))(1)(e(g))(1) energy remains roughly independent of metal oxidation state, likely owing to the compensating increased pi-donation from the pi(CN) orbitals to the metal d(xy) orbitals as the oxidation state of the metal increases.     

Nucleophilic or electrophilic phosphinidene complexes ML(n)=PH; what makes the difference?

Density functional studies, based on the local density approximation including nonlocal corrections for correlation and exchange self-consistently, have been carried out for the equilibrium structures of the phosphinidene transition metal complexes ML(n)=PH, with M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ru, Os, Co, Rh, Ir and L = CO, PH(3), Cp. The chemical reactivity of the transition metal-stabilized phosphinidene P-R is influenced by its spectator ligands L. Ligands with strong sigma-donor capabilities on the metal increase the electron density on the phosphorus atom, raise the pi-orbital energy, and enhance its nucleophilicity. Spectator ligands with strong pi-acceptor capabilities lower the charge concentration on P and stabilize the pi-orbital, which results in a higher affinity for electron-rich species. The ML(n)=PH bond is investigated using a bond energy analysis in terms of electrostatic interaction, Pauli repulsion, and orbital interaction. A symmetry decomposition scheme affords a quantitative estimate of the sigma- and pi-bond strengths. It is shown that the investigated phosphinidenes are strong pi-acceptors and even stronger sigma-donors. The metal-phosphinidene interaction increases on going from the first to the second- and third-row transition metals.     

Energy decomposition analysis of metal-metal bonding in [M2X8]2- (X=Cl, br) complexes of 5f (U, Np, Pu), 5d (W, Re, Os), and 4d (Mo, Tc, Ru) elements

The electronic structures of a series of [M2X8]2- (X=Cl, Br) complexes involving 5f (U, Np, Pu), 5d (W, Re, Os), and 4d (Mo, Tc, Ru) elements have been calculated using density functional theory, and an energy decomposition approach has been used to carry out a detailed analysis of the metal-metal interactions. The energy decomposition analysis involves contributions from orbital interactions (mixing of occupied and unoccupied orbitals), electrostatic effects (Coulombic attraction and repulsion), and Pauli repulsion (associated with four-electron two-orbital interactions). As previously observed for Mo, W, and U M2X6 species, the general results suggest that the overall metal-metal interaction is considerably weaker or unfavorable in the actinide systems relative to the d-block analogues, as a consequence of a significantly more destabilizing contribution from the combined Pauli and electrostatic (prerelaxation) effects. Although the orbital-mixing (postrelaxation) contribution to the total bonding energy is predicted to be larger in the actinide complexes, this is not sufficiently strong to compensate for the comparatively greater destabilization originating from the Pauli-plus-electrostatic effects. A generally weak electrostatic contribution accounts for the large prerelaxation destabilization in the f-block systems, and ultimately for the weak or unfavorable nature of metal-metal bonding between the actinide elements. There is a greater variation in the energy decomposition results across the [M2Cl8]2- series for the actinide than for the d-block elements, both in the general behavior and in some particular properties.     

Coordination Numbers of Transition Metal Atoms in the Solid and Liquid Phases,According to Ultrasonic Data

Russian Journal of Physical Chemistry A - The coordination numbers of transition metal atoms (Hf, Nb, Cr, Mo, W, Re, Ru, Rh, Os, Ir, Pt) are calculated using acoustic data from 0 K to temperatures...     

Conformational analysis of 18-azacrown-6 and its bonding with late first transition series divalent metals: insight from DFT combined with NPA and QTAIM analyses

Density functional theory calculations, together with quantum theory of atoms in molecules (QTAIM) analyses, have been performed to investigate 18-azacrown-6 complexes of the high-spin late first transition series divalent metal ions in the gas phase and, in some cases, in aqueous solution simulated by a polarizable continuum model. Six intramolecular H-H bonding interactions in the meso-complexes are found to arise from folding of the ligand upon its electrostatic interaction with the metal ions, which are largely absent in the lowest-energy C(2h) conformer of the free ligand. The ligand-to-metal charge transfer obtained from QTAIM analysis, among other things, is found to be an important factor that controls the stability of these complexes. The inter-relationship between the ligand preorganization energy, the zero-point corrected formation energy of the metal complexes, and the H-H bonding pair distances, as well as the dependence of the electron density and the total energy density at the H-H bond critical points on the H-H bonding pair distances, provides a physical basis for understanding and explaining the stabilizing nature of these closed-shell interactions, which are often viewed as steric clashes that lead to complex destabilization.     

Solubilizing single‐walled carbon nanotubes with pyrene‐functionalized block copolymers

The effect of pyrene distribution within pyrene‐functionalized random and block copolymers on noncovalent polymer/single‐walled carbon nanotube (SWNT) interactions was investigated. The block copolymers served as superior solubilizing agents in comparison with the random copolymers. Also, increasing the pyrene content within a polymer, while a constant molecular weight was maintained, improved SWNT solubility and therefore had to result in stronger polymer–nanotube interactions. However, increasing the length of the pyrene‐containing block diminished nanotube solubility, likely because of a lower number of polymer chains that were capable of binding to the nanotube surface. Atomic force microscopy and transmission electron microscopy indicated that the polymer–SWNT interactions were capable of partially debundling the nanotubes into individual solvated structures. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1941–1951, 2006     

Adsorbate-adsorbate interactions and chemisorption at different coverages studied by accurate ab initio calculations: CO on transition metal surfaces

We use density functional theory (DFT) with the generalized gradient approximation (GGA) and our first-principles extrapolation method for accurate chemisorption energies (Mason et al. Phys. Rev. B 2004, 69, 161401R) to calculate the chemisorption energy for CO on a variety of transition metal surfaces for various adsorbate densities and patterns. We identify adsorbate through-space repulsion, bonding competition, and substrate-mediated electron delocalization as key factors determining the preferred chemisorption patterns for different metal surfaces and adsorbate coverages. We discuss how the balance of these interactions, along with the inherent adsorption site preference on each metal surface, can explain the observed CO adsorbate patterns at different coverages.     

Ligand dependence of metal-metal bonding in the d(3)d(3) dimers M(2)X(9)(n-) (M(III) = Cr, Mo, W; M(IV) = Mn, Tc, Re; X = F, Cl, Br, I)

The ligand dependence of metal-metal bonding in the d(3)d(3) face-shared M(2)X(9)(n-) (M(III) = Cr, Mo, W; M(IV) = Mn, Tc, Re; X = F, Cl, Br, I) dimers has been investigated using density functional theory. In general, significant differences in metal-metal bonding are observed between the fluoride and chloride complexes involving the same metal ion, whereas less dramatic changes occur between the bromide and iodide complexes and minimal differences between the chloride and bromide complexes. For M = Mo, Tc, and Re, change in the halide from F to I results in weaker metal-metal bonding corresponding to a shift from either the triple metal-metal bonded to single bonded case or from the latter to a nonbonded structure. A fragment analysis performed on M(2)X(9)(3-) (M = Mo, W) allowed determination of the metal-metal and metal-bridge contributions to the total bonding energy in the dimer. As the halide changes from F to I, there is a systematic reduction in the total interaction energy of the fragments which can be traced to a progressive destabilization of the metal-bridge interaction because of weaker M-X(bridge) bonding as fluoride is replaced by its heavier congeners. In contrast, the metal-metal interaction remains essentially constant with change in the halide.     

Origin of trans-bent geometries in maximally bonded transition metal and main group molecules

Recent crystallographic data unambiguously demonstrate that neither Ar'GeGeAr' nor Ar'CrCrAr' molecules adopt the expected linear (VSEPR-like) geometries. Does the adoption of trans-bent geometries indicate that Ar'MMAr' molecules are not "maximally bonded" (i.e., bond order of three for M = Ge and five for M = Cr)? We employ theoretical hybrid density functional (B3LYP/6-311++G) computations and natural bond orbital-based analysis to quantify molecular bond orders and to elucidate the electronic origin of such unintuitive structures. Resonance structures based on quintuple M-M bonding dominate for the transition metal compounds, especially for molybdenum and tungsten. For the main group, M-M bonding consists of three shared electron pairs, except for M = Pb. For both d- and p-block compounds, the M-M bond orders are reflected in torsional barriers, bond-antibond splittings, and heats of hydrogenation in a qualitatively intuitive way. Trans-bent structures arise primarily from hybridization tendencies that yield the strongest sigma-bonds. For transition metals, the strong tendency toward sd-hybridization in making covalent bonds naturally results in bent ligand arrangements about the metal. In the p-block, hybridization tendencies favor high p-character, with increasing avidity as one moves down the Group 14 column, and nonlinear structures result. In both the p-block and the d-block, bonding schemes have easily identifiable Lewis-like character but adopt somewhat unconventional orbital interactions. For more common metal-metal multiply bonded compounds such as [Re2Cl8]2-, the core Lewis-like fragment [Re2Cl4]2+ is modified by four hypervalent three-center/four-electron additions.     

Evolution of the electronic properties of metallic single-walled carbon nanotubes with the degree of CCl2 covalent functionalization

The changes in energetic, structural, and electronic properties of the metallic (5,5) single-walled carbon nanotube (SWNT) with the degree of sidewall covalent functionalization of CCl(2) are investigated extensively by using density functional theory calculations. The saturation concentration of CCl(2) covalent functionalization is predicted to be 33.3%. The cycloadducts always adopt an open structure. A band gap opens as the functionalization concentration reaches 11% and then basically increases with increasing functionalization concentration. These results are in agreement with available experiments and can be applied to accurately predict the band gap of metallic SWNTs produced by the HiPco method at a given CCl(2) functionalization concentration.     

Principles Determining the Structure of Transition Metals

For the better part of a century researchers across disciplines have sought to explain the crystallography of the elemental transition metals: hexagonal close packed, body centered cubic, and face centered cubic in a form similar to that used to rationalize the structure of organic molecules and inorganic complexes. Pauling himself tried with limited success to address the origins of transition metal stability. These early investigators were handicapped, however, by incomplete knowledge regarding the structure of metallic electron density. Here, we exploit modern approaches to electron density analysis to first comprehensively describe transition metal electron density. Then, we use topological partitioning and quantum mechanically rigorous treatments of kinetic energy to account for the structure of the density as arising from the interactions between metallic polyhedra. We argue that the crystallography of the early transition metals results from charge transfer from the so called “octahedral” to “tetrahedral cages” while the face centered cubic structure of the late transition metals is a consequence of anti-bonding interactions that increase octahedral hole kinetic energy.     

Homoleptic tetranuclear osmium carbonyls: from the rhombus via the butterfly to the tetrahedron

The structures of the tetranuclear osmium carbonyl derivatives Os4(CO)n (n = 16, 15, 14, 13, 12) have been investigated using the density functional theory method MPW1PW91 with the SDD effective core potential basis set, found to be effective in previous work for the study of Os3(CO)12. The Os4 clusters in the lowest energy structures for Os4(CO)16, Os4(CO)15, and Os4(CO)14 are found to be rhombi, butterflies, and tetrahedra with four, five, and six Os-Os bonds, respectively, in accord with structures determined by X-ray diffraction as well as the 18-electron rule. The fluxionality of tetrahedral Os4(CO)14, suggested by experimental work of Johnston, Einstein, and Pomeroy, is confirmed by our DFT studies, which find four Os4(CO)14 structures within 1.5 kcal mol-1 of each other with similar tetrahedral Os4 frameworks but with different arrangements of bridging and semibridging carbonyl groups. The lowest energy structures for the more unsaturated Os4(CO)13 and Os4(CO)12 are also based on Os4 tetrahedra but with shorter Os-Os edge lengths than in Os4(CO)14 suggesting delocalized multiple bonding in the more highly unsaturated systems. Thus the global minimum for Os4(CO)12 is predicted to have tetrahedral symmetry, with all terminal carbonyl groups analogous to the experimentally known structure of (mu3-H)4Re3(CO)12, but without the face-bridging hydrogen atoms.