Chemical bonding in refractory transition metal compounds with 8, 9, and 10 valence electrons

The chemical bonding in the refractory transition metal compounds TiC, TiN, and VN is investigated by experimental and theoretical techniques. High-precision X-ray diffraction is used to determine the electron densities in these three compounds experimentally. The X-ray structure factors and the respective valence electron densities are used twice, once to understand the chemical bonding and once to relate the experimental charge densities to those obtained from band structure calculations. These calculations, which in general are in very good agreement with experimental data, utilize the linearized augmented plane wave (LAPW) method. Theory and experiment lead to a detailed analysis of the chemical bonding in these compounds with 8, 9, and 10 valence electrons. By decomposition of the theoretical charge density into contributions from different states (energy regions), it was possible to show the strong covalent nonmetal p-metal d interaction, which is otherwise apparent only in TiC, but not in TiN or VN. In the latter two compounds the additional electrons occupy mainly metal d states with t_2g symmetry, so that in the total valence charge densities the most important bonding feature is covered. In addition to covalent interactions all compounds have a metallic bonding contribution as well as a considerable charge transfer from the metal to the nonmetal site. This mixture in chemical bonding accounts for the unusual combination of properties such as ultrahardness, high melting points, and good conductivity.     

Partial LCAO densities of states for ScN,TiN, ZrN and ScP

Densities of states (DOS) and partial densities of states were calculated from self-consistent APW band structure calculations for four transition metal compounds (ScN, TiN, ZrN and ScP) using a recently published improved LCAO interpolation scheme. The total DOS and the LCAO non local partial metal s, p and d and non-metal s and p DOS of these compounds are presented and compared with non local LCAO partial DOS from earlier calculations as well as with local partial DOS obtained directly from the APW or LAPW wave functions. A LCAO charge analysis for all valence states and for the individual valence bands is also given.     

Resonance effects on the inner-valence levels of SF6 in the photon-energy range 52–72 eV

We report the photoionization partial cross section and asymmetry parameter in the 52–72 eV photon-energy range for the inner-valence orbitais (3t_1u, 2e_g, and 4a_1g) in gaseous SF₆. These results, combined with those for the (inner valence)/(outer valence) branching ratio, indicate resonant enhancement of the inner-valence levels at ≈ 59 eV photon energy which we associate with the 3t_1u → e_g shape resonance predicted by MSM Xα calculations.     

Electronic structure and disorder in NaxCoO2 and SrRh2O4

We discuss the electronic structure of Na_xCoO₂ from the point of view of first principles electronic structure calculations. The band structure contains low spin Co ions, with average charge 5 + x leading to a nearly full Co t_2g manifold. The bands corresponding to this manifold are narrow and separated from the O 2p bands and from the e_g bands, which are also narrow. There are two main sheets of Fermi surface, a large section derived from a_g symmetry states and small hole pockets. We find significant effects due to Na disorder on these small sections, with the result that they should be localized. This is discussed in relation to recent photoemission experiments. For comparison, we present a virtual crystal band structure of beta-SrRh₂O₄. Like Na_xCoO₂ it shows a large crystal field gap between narrow t_2g and e_g manifolds, but because of its stoichiometry is a semiconductor rather than a high carrier density metal.     

Theoretical investigation of half‐metallicity in Co/Ni substituted AlN

Results of Co and Ni substituted AlN in the zinc blende phase are presented. For spin up states, the hybridized N‐2p and Co/Ni‐3d states form the valance bands with a bandgap around the Fermi level for both materials, while in the case of the spin down states, the hybridized states cross the Fermi level and hence show metallic nature. It is found that, Al_0.75Co_0.25N and Al_0.75Ni_0.25N are ferromagnetic materials with magnetic moments of 4 μ_B and 3 μ_B, respectively. The integer magnetic moments and the full spin polarization at the Fermi level make these compounds half‐metallic semiconductors. Furthermore it is also found that the interaction with the N‐2p state splits the 5‐fold degenerate Co/Ni‐3d states into t_2g and e_g states. The t_2g states are located at higher energies than the e_g states caused by the tetrahedral symmetry of these compounds. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Charge transfer shake-up satellites in X-ray photoelectron spectra of solid compounds of 3d0 ions

Satellite structure has been observed at 8 eV and 12 eV below the main metal 2p peaks for compounds of Sc³⁺ and Ti⁴⁺. These features are attributed to charge transfer “shake-up”: transitions (t_2g → t_2g^* and e_g → e_g^* respectively). The 8 eV satellites are only found in sulphate complexes of Sc³⁺. This is the first observation of satellites which are probably due to t_2g → t_2g^* monopole charge transfer transitions. The probability for this transition is usually low, resulting in the observation of satellites due to the e_g → e_g transition.     

Octacarbonyl Anion Complexes of Group Three Transition Metals [TM(CO)8]− (TM=Sc,Y, La) and the 18‐Electron Rule

We report the gas‐phase synthesis of stable 20‐electron carbonyl anion complexes of group 3 transition metals, TM(CO)₈^? (TM=Sc, Y, La), which are studied by mass‐selected infrared (IR) photodissociation spectroscopy. The experimentally observed species, which are the first octacarbonyl anionic complexes of a TM, are identified by comparison of the measured and calculated IR spectra. Quantum chemical calculations show that the molecules have a cubic (O_h) equilibrium geometry and a singlet (¹A_1g) electronic ground state. The 20‐electron systems TM(CO)₈^? are energetically stable toward loss of one CO ligand, yielding the 18‐electron complexes TM(CO)₇^? in the ¹A₁ electronic ground state; these exhibit a capped octahedral structure with C_3v symmetry. Analysis of the electronic structure of TM(CO)₈^? reveals that there is one occupied valence molecular orbital with a_2u symmetry, which is formed only by ligand orbitals without a contribution from the metal atomic orbitals. The adducts of TM(CO)₈^? fulfill the 18‐electron rule when only those valence electrons that occupy metal–ligand bonding orbitals are considered.     

Octacarbonyl Anion Complexes of Group Three Transition Metals [TM(CO)8]− (TM=Sc,Y, La) and the 18‐Electron Rule

We report the gas‐phase synthesis of stable 20‐electron carbonyl anion complexes of group 3 transition metals, TM(CO)₈⁻ (TM=Sc, Y, La), which are studied by mass‐selected infrared (IR) photodissociation spectroscopy. The experimentally observed species, which are the first octacarbonyl anionic complexes of a TM, are identified by comparison of the measured and calculated IR spectra. Quantum chemical calculations show that the molecules have a cubic (O_h) equilibrium geometry and a singlet (¹A_1g) electronic ground state. The 20‐electron systems TM(CO)₈⁻ are energetically stable toward loss of one CO ligand, yielding the 18‐electron complexes TM(CO)₇⁻ in the ¹A₁ electronic ground state; these exhibit a capped octahedral structure with C_3v symmetry. Analysis of the electronic structure of TM(CO)₈⁻ reveals that there is one occupied valence molecular orbital with a_2u symmetry, which is formed only by ligand orbitals without a contribution from the metal atomic orbitals. The adducts of TM(CO)₈⁻ fulfill the 18‐electron rule when only those valence electrons that occupy metal–ligand bonding orbitals are considered.     

A valence bond study of the oxygen molecule

Ab initio valence bond calculations are performed for the three lowest states of the oxygen molecule (³Σ⁻_g, ¹Δ_g, and ¹Σ⁺_g). One objective of the present study was to make a contribution to previous valence bond discussions about the oxygen “double” bond. Further, we study the origin of a small barrier in the potential energy surface of the ground state. Two compact models are employed to maintain the clear picture that can be offered by the valence bond method. The first model has only the Rumer structures that are essential for bonding and a proper dissociation. The second model, in addition, has structures which represent excited atoms. These prove to be important for the dissociation energies. For both models, the orbitals are fully optimized. The spectroscopic data obtained are significantly better than are the (few) valence bond results on O₂ that have been published and have the quality of multiconfiguration self-consistent field calculations in which the same valence space is used. The “hump” in the potential energy surface of the ground state is shown to arise from a spin recoupling. The free atoms correspond to a spin coupling that is incapable of describing the formation of bonds. Only at short distances, an alternative spin coupling provides bonding and the repulsive curve is converted into an attractive one. Our results on this subject support a valence bond explanation previously given by McWeeny [R. McWeeny, Int. J. Quantum Chem. Symp. 24 , 733 (1990)]. © 1996 John Wiley & Sons, Inc.     

Broken symmetry in valence molecular region within Hartree-Fock calculations

Symmetry restricted and unrestricted Hartree-Fock calculations at theab initio LCAO-MO-SCF level have been carried out on the ground, core and valence hole states of N₂ at various N-N distances. A one-particle criterion for symmetry breaking is discussed. Strong broken-symmetry effects in the inner valence molecular region of N₂ have been found at larger N-N distances. This breaking of symmetry accompanying the symmetry unrestricted Hartree-Fock calculations of the inner valence hole states at large internuclear separations can be considered to be a common phenomenon with all highly symmetric molecules. The outer valence broken-symmetry effects with N₂ have showed some deviations as compared with these effects in the inner valence and core molecular regions.     

Theoretical studies on cerium nickel aluminides: polar intermetallics with heavy fermion behavior

The electronic structures of Ce₄Ni₆Al₂₃, CeNiAl₄, CeNi₂Al₅, CeNiAl and CeNi₄Al have been calculated using the TB-LMTO-ASA (tight-binding, linear muffin-tin orbital, atomic-spheres approximation) approach to probe relationships between chemical bonding and physical properties in this series of intermetallic compounds. Analysis from crystal orbital Hamilton populations (COHP) reveal that the Al-rich compounds may be considered as “polar intermetallic” because the Fermi level coincides to the separation of bonding and antibonding states of the Ni-Al framework. On the other hand, although the densities of states (DOS) of CeNiAl suggest “polar intermetallic” behavior, the bonding is more complex. Finally, the Ni-rich example, CeNi₄Al, has significant Ni-3d character at the Fermi level. The results of these calculations are also discussed in connection with heavy fermion or possible valence fluctuation behavior observed for some of these intermetallic compounds: those showing exceptional properties also exhibit significant “lattice covalency” between Ce and the Ni-Al nets.     

Semiempirical study of electronic and bonding properties of iron silicide clusters

Summary Molecular orbital calculations of iron, silicon, and iron silicide clusters have been carried out using the UHF-MINDO/SR method. The nature of the bonding in these compounds has been investigated by analyzing the importance of bonding indexes and diatomic components of the total energy. It has been found that in iron silicide the strongest bond is formed between Fe-Si and that it arises mainly as the result ofsp-sp type orbital interactions. Althoughd orbitals show very little overlap withs-p orbitals, they do contribute significantly to bonding through electrostatic type diatomic interactions. By means of a detailed analysis ofsp, andd orbitals and total density of states (DOS) of Fe₇Si₇, Si₇Fe₇, Fe₁₅, and Si₁₇ clusters, the present calculations have permitted us to explain the origin of the iron silicide UPS experimental peaks.     

A Heteroleptic Push–Pull Substituted Iron(II) Bis(tridentate) Complex with Low‐Energy Charge‐Transfer States

A heteroleptic iron(II) complex [Fe(dcpp)(ddpd)]²⁺ with a strongly electron‐withdrawing ligand (dcpp, 2,6‐bis(2‐carboxypyridyl)pyridine) and a strongly electron‐donating tridentate tripyridine ligand (ddpd, N,N′‐dimethyl‐N,N′‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine) is reported. Both ligands form six‐membered chelate rings with the iron center, inducing a strong ligand field. This results in a high‐energy, high‐spin state (⁵T₂, (t_2g)⁴(e_g*)²) and a low‐spin ground state (¹A₁, (t_2g)⁶(e_g*)⁰). The intermediate triplet spin state (³T₁, (t_2g)⁵(e_g*)¹) is suggested to be between these states on the basis of the rapid dynamics after photoexcitation. The low‐energy π^* orbitals of dcpp allow low‐energy MLCT absorption plus additional low‐energy LL′CT absorptions from ddpd to dcpp. The directional charge‐transfer character is probed by electrochemical and optical analyses, Mößbauer spectroscopy, and EPR spectroscopy of the adjacent redox states [Fe(dcpp)(ddpd)]³⁺ and [Fe(dcpp)(ddpd)]⁺, augmented by density functional calculations. The combined effect of push–pull substitution and the strong ligand field paves the way for long‐lived charge‐transfer states in iron(II) complexes.     

Multiconfigurational SCF approach for high-symmetry molecules and its applications to fullerene trianion

A symmetry-adapted multiconfiguration self-consistent field (MC SCF) approach aimed at calculations of high-symmetry molecules is proposed. The self-consistency procedure applicable to the molecular terms of any symmetry and multiplicity is developed. It holds the symmetry transformation properties of varied molecular orbitals, thus taking advantage of the relationships within the set of two-electron integrals through molecular invariants. For orbital optimization, a unified coupling operator is constructed on the basis of the pseudosecular method providing for efficient convergence to energy minimum. Based on the group-theory technique, computer codes have been developed for straightforward determination of the invariant expansions for two-electron integrals and configuration interaction (CI) matrix elements. Calculated in this way, the expansion coefficients are presented for the three-electron states that originate from joint t_1u and t_1g shells of an icosahedral fullerene C₆₀, the case important for the calculations of anion C₆₀³⁻ representing the charge state of the fullerene molecule in the superconducting ionic solids K₃C₆₀ or Rb₃C₆₀. The results of MC SCF calculations for lowest quasi-π-electronic states of C₆₀³⁻ are discussed. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 293–304, 1998     

Valence-shell photoabsorption by CO2 and its connections with electron-CO2 scattering

Photoabsorption cross sections and photoelectron asymmetry parameters, calculated with the multiple-scattering model (MSM), are reported for the 4σ_g, 3σ_u, 1π_u and 1π_g valence levels of CO₂. The results are discussed in the context of photoabsorption and electron energy-loss measurements and other theoretical calculations. Further comparisons are made with previously reported MSM calculations of elastic electron-CO₂ scattering. The close connection between the sets of shape resonances in the electron-scattering and photoabsorption by CO₂ is emphasized with plots of continuum eigenchannel wavefunctions for shape-resonant and non-resonant eigenchannels of σ_u symmetry.     

Unraveling σ and π Effects on Magnetic Anisotropy in cis‐NiA4B2 Complexes: Magnetization,HF‐HFEPR Studies,First‐Principles Calculations,and Orbital Modeling

By using complementary experimental techniques and first‐principles theoretical calculations, magnetic anisotropy in a series of five hexacoordinated nickel(II) complexes possessing a symmetry close to C_2v, has been investigated. Four complexes have the general formula [Ni(bpy)X₂]ⁿ⁺ (bpy=2,2′‐bipyridine; X₂=bpy ( 1 ), (NCS^?)₂ ( 2 ), C₂O₄^2? ( 3 ), NO₃^? ( 4 )). In the fifth complex, [Ni(HIM₂‐py)₂(NO₃)]⁺ ( 5 ; HIM₂‐py=2‐(2‐pyridyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazolyl‐1‐hydroxy), which was reported previously, the two bpy bidentate ligands were replaced by HIM₂‐py. Analysis of the high‐field, high‐frequency electronic paramagnetic resonance (HF‐HFEPR) spectra and magnetization data leads to the determination of the spin Hamiltonian parameters. The D parameter, corresponding to the axial magnetic anisotropy, was negative (Ising type) for the five compounds and ranged from ?1 to ?10 cm^?1. First‐principles SO‐CASPT2 calculations have been performed to estimate these parameters and rationalize the experimental values. From calculations, the easy axis of magnetization is in two different directions for complexes 2 and 3 , on one hand, and 4 and 5 , on the other hand. A new method is proposed to calculate the g tensor for systems with S=1. The spin Hamiltonian parameters (D (axial), E (rhombic), and g_i) are rationalized in terms of ordering of the 3 d orbitals. According to this orbital model, it can be shown that 1) the large magnetic anisotropy of 4 and 5 arises from splitting of the e_g‐like orbitals and is due to the difference in the σ‐donor strength of NO₃^? and bpy or HIM₂‐py, whereas the difference in anisotropy between the two compounds is due to splitting of the t_2g‐like orbitals; and 2) the anisotropy of complexes 1 – 3 arises from the small splitting of the t_2g‐like orbitals. The direction of the anisotropy axis can be rationalized by the proposed orbital model.     

Investigation of structural and electronic properties by pnictogen substitution in the layered oxypnictides (LaO)ZnPn (Pn = P,As, Sb)

We study the structural and electronic properties of p-type layered oxypnictides (LaO)ZnPn (Pn = P, As, Sb), calculated by first principles. Pn substitution from P to Sb increases D_2d-type local symmetry distortions at ZnPn₄ and OLa₄ tetrahedra. (LaO)ZnP and (LaO)ZnAs exhibit direct band gaps (Γ → Γ) of 0.621 eV and 0.528 eV, respectively, while (LaO)ZnSb exhibits an indirect band gap (Γ → 0.2Λ) of 0.029 eV. The band gaps come from valence Pn p _x/p _y and conduction Zn 4s states. Moreover, the substitution increases split-off energy at Z and Γ points. We find localized valence degeneracy-lifted Zn 3d states because of the possible second-order Jahn-Teller effect, which induces the local symmetry distortions. The localized Zn 3d states are followed by minor bonding s-p hybridization of Zn and Pn. Above them, we show major bonding s-p hybridization; O 2p states in electron-blocking [LaO]⁺ layers, which are essential for thermoelectricity; and nonbonding Pn p states near Fermi level. In the conduction band, antibonding s-p hybridization is found. Our result shows new insights and findings of structural and electronic properties, which explain previous experimental results, as the focus of this study is related to inorganic chemistry. This study is important for future functional device applications.     

Pseudopotential calculations for methyl compounds of zinc and magnesium

Pseudopotentials and valence basis sets to be used in calculations for organometallic compounds of zinc and magnesium have been tested in calculations for the M(CH₃)_n (M = Zn, Mg; n = 1,2) molecules. Valence correlation effects are treated at the SDCI and CEPA levels. The capability of a polarization potential on zinc to account for the valence shell contracting effect of core valence correlation is studied. Properties considered are geometries, force constants, Mulliken populations, ionization potentials, atomization, and binding energies. Differences in bonding between the two dimethyl compounds are discussed.