Late Transition Metal Compounds Research Articles

Exploring Metastable Oxynitrides by Thin Film Growth Approach

Abstract Metal oxynitrides are expected to show unique chemical and physical properties suitable for various applications. However, compared with oxides, the chemical space of oxynitrides has not been well explored mainly due to their thermal instability and high reaction temperature required in conventional bulk synthesis methods. Highly nonequilibrium thin film growth is a promising route for obtaining those unexplored oxynitrides. In this account, we will overview recent studies on thin film synthesis of oxynitrides, especially focusing on exploration of metastable phases, such as mid- and late-transition metal compounds, metastable polymorphs, geometrical isomers with respect to anion coordination, and amorphous phases.

Electronic structure of negative charge transfer CaFeO3 across the metal-insulator transition

We investigated the metal-insulator transition for epitaxial thin films of the perovskite ${\mathrm{CaFeO}}_{3}$, a material with a significant oxygen ligand hole contribution to its electronic structure. We find that biaxial tensile and compressive strain suppress the metal-insulator transition temperature. By combining hard x-ray photoelectron spectroscopy, soft x-ray absorption spectroscopy, and density functional calculations, we resolve the element-specific changes to the electronic structure across the metal-insulator transition. We demonstrate that the Fe sites undergo no observable spectroscopic change between the metallic and insulating states, whereas the O electronic configuration undergoes significant changes. This strongly supports the bond-disproportionation model of the metal-insulator transition for ${\mathrm{CaFeO}}_{3}$ and highlights the importance of ligand holes in its electronic structure. By sensitively measuring the ligand hole density, however, we find that it increases by $\ensuremath{\sim}5--10%$ in the insulating state, which we ascribe to a further localization of electron charge on the Fe sites. These results provide detailed insight into the metal-insulator transition of negative charge transfer compounds and should prove instructive for understanding metal-insulator transitions in other late transition metal compounds such as the nickelates.

Ionic Model Approach to Battery Voltage of NaMO2

NaMO2 (M = Ti, V, Cr, Mn, Fe, Co, and Ni) with a layered rock-salt structure exhibits high battery voltage (V) and capacity (Q), and is considered to be a promising cathode material for sodium ion secondary batteries (SIBs). Among the battery parameters, V is directly related to the electronic structure of the compounds. Here, we apply the ionic model proposed by Torrance et al. [Physica C 182, 351 (1991) ]t o NaMO2 and estimated V using structural parameters. The ionic model reproduces the global feature of the M-dependence of V. This analysis suggests that an electron is removed from the O 2p state in the charge process in the late transition metal compounds (M = Mn–Ni).

Triple C–H Bond Activation of a Nickel-Bound Methyl Group: Synthesis and X-Ray Structure of a Carbide Cluster (NiCp)6(μ6-C)

A new hexanuclear cyclopentadienylnickel carbide cluster (NiCp)(6)(μ(6)-C) (1) was obtained through the thermolysis of the alkene complex [NiCp(CH(3))(η(2)-CH(2)═CHC(4)H(9))] (4). The X-ray molecular structure of 1 (monoclinic; P2(1)/c; Ni-C(carbide) = 1.767(4)-2.109(4) Å) reveals a highly deformed octahedral arrangement of nickel atoms with two octahedron edges opened (Ni-Ni bonding distances = 2.410(1)-2.623(1) Å, Ni···Ni nonbonding distances = 3.107(2) and 3.108(2) Å). Cluster 1 is the first example of a homoleptic, cyclopentadienylnickel carbide cluster. Moreover, (13)C-labeling studies proved that the carbido ligand in cluster 1 originated from the Ni-bound methyl group. This transformation requires a triple C-H bond activation in the methyl group, which has not been observed so far for late transition metal compounds.

Modeling doped and defective oxides in catalysis with density functional theory methods: Room for improvements

Due to the well-known problem of the self-interaction, standard density functional theory (DFT) methods tend to produce delocalized holes and electrons in defective oxide materials even when there is ample experimental evidence of a strong localization. For late transition metal compounds or rare earth oxides, this results in the incorrect description of the electronic structure of the system (e.g., magnetic insulators are predicted to be metallic). Practical ways to correct this deficiency are based on the use of hybrid functionals or of the DFT+U approach. In this way, most of the limitations related to the self-interaction are removed, and the electronic structure is properly described. What is less clear is to what extent hybrid functionals, DFT+U approaches, or standard DFT functionals can properly describe the strength of the chemical bonds at the surface of an oxide. This is a crucial question if one is interested in the catalytic properties of oxide surfaces. Oxidation reactions often involve oxygen detachment from the surface and incorporation into an organic substrate. Oxides are doped with heteroatoms to create defects and facilitate oxygen removal from the surface, with formation of oxygen vacancies. Do standard DFT calculations provide a good binding energy of the missing oxygen despite the failure in giving the right electronic structure? Can hybrid functionals or the DFT+U approach provide a simple yet reliable way to get accurate reaction enthalpies and energy barriers? In this essay, we discuss these problems by analyzing some case histories and the relatively scarce data existing in the literature. The conclusion is that while modern electronic structure methods accurately reproduce and predict a wide range of electronic, optical, and magnetic properties of oxides, the description of the strength of chemical bonds still needs considerable improvements.

Cu63andAu197nuclear quadrupole moments from four-component relativistic density-functional calculations using correct long-range exchange

The electric field gradient in late transition metal compounds is incorrectly determined by most density functionals. We show that the coupling of short-range density functional based with long-range wave function based methods using a reparametrization of the Coulomb-attenuated Becke three-parameter Lee-Yang-Parr approximation gives reliable results for the electric field gradients of copper and gold for a series of compounds. This results in nuclear quadrupole moments of $\ensuremath{-}0.208\phantom{\rule{0.3em}{0ex}}\mathrm{b}$ for $^{63}\mathrm{Cu}$ and $+0.526\phantom{\rule{0.3em}{0ex}}\mathrm{b}$ for $^{197}\mathrm{Au}$ in good agreement with experimental values of $\ensuremath{-}0.220(15)$ and $+0.547(16)\mathrm{b}$, respectively.

Catalytic Cyclopropanation of Alkenes via (2‐Furyl)carbene Complexes from 1‐Benzoyl‐cis‐1‐buten‐3‐yne with Transition Metal Compounds.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Catalytic Cyclopropanation of Alkenes via (2-Furyl)carbene Complexes from 1-Benzoyl-cis-1-buten-3-yne with Transition Metal Compounds

The reaction of alkenes with conjugated ene-yne-ketones, such as 1-benzoyl-2-ethynylcycloalkenes, with a catalytic amount of Cr(CO)(5)(THF) gave 5-phenyl-2-furylcyclopropane derivatives in good yields. The key intermediate of this cyclopropanation is a (2-furyl)carbene complex generated by a nucleophilic attack of carbonyl oxygen to an internal alkyne carbon in pi-alkyne complex or sigma-vinyl cationic complex. A wide range of late transition metal compounds, such as [RuCl(2)(CO)(3)](2), [RhCl(cod)](2), [Rh(OAc)(2)](2), PdCl(2), and PtCl(2), also catalyzes the cyclopropanation of alkenes with ene-yne-ketones effectively. When the reactions were carried out with dienes as a carbene acceptor, the more substituted or more electron-rich alkene moiety was selectively cyclopropanated with the (2-furyl)carbenoid intermediate.

The accuracy of density functionals for electric field gradients. Test calculations for ScX, CuX and GaX (X=F, Cl, Br, I, H and Li)

In a previous paper [J. Chem. Phys. 111, 3357 (1999)] we showed that the electric field gradient at the copper nucleus in CuCl is incorrectly described by most of the density functionals currently in use, including gradient corrected and hybrid versions of DFT. Here we analyze whether this error is systematic or not by comparing DFT electric field gradients for a number of diatomic compounds MX. The molecules chosen include representatives from early transition metal compounds, ScX, from late transition metal compounds, CuX, and from main group compounds, GaX, where X=F, Cl, Br, I, H and Li. From experimental nuclear quadrupole coupling data and electric field gradient calculations for each of the three sets (ScX, CuX and GaX) the nuclear quadrupole moment at the metal can be deduced at a specific DFT level. It is demonstrated that density functionals work well for main group compounds (GaX), but contain large systematic errors for transition metals such as copper. This leads to unreasonable copper nuclear quadrupole moments obtained for the CuX set. The error mainly originates from the incorrect description of the Cu(3d) core being polarized by electronegative ligands such as fluorine. For copper this can be remedied in an ad-hoc way by adjusting the Hartree–Fock contribution in the exchange part of the hybrid functional (modified B3LYP). Using this modified B3LYP functional we obtain reasonable results for the copper electric field gradient in CuF3, a case where the Cu(3d) core is strongly polarized by the fluorine ligands.

Electronic structures of 3d transition metal (Ti–Cu) oxides probed by a core hole

Electronic structures of various 3d transition metal oxides are calculated using the spin-unrestricted discrete-variational (DV) Hartree–Fock–Slater (Xα) method for the ground state and 1s −1 core hole state. Model clusters MO n are used in place of calculating infinite size solids, where M is the transition metal elements from Ti ( Z=22) to Cu ( Z=29) and n the coordination number of oxygen; n=6 for O h symmetry and n=4 for T d or D 4h symmetry. The calculated results indicate that the number of unpaired 3d electrons reduces in the creation of the core hole for late transition metal compounds, i.e. do not represent the ground-state spin state for these compounds, but conserves for early transition metal compounds. These results provide theoretical basis on the validity of spin selective X-ray absorption spectroscopy for early transition metal compounds by using the high-resolution X-ray fluorescence yield method, and theoretical basis on the validity or nonvalidity of spin state analysis using core-level spectroscopy, such as X-ray emission, X-ray absorption, and photoelectron spectroscopies. It is also found that the spin of a class of compounds sometimes flips in the creation of the core hole.

Electron Counting Scheme Relevant for Late Transition Metal Compounds with Weakly Electronegative Ligands. Electronic Band Structure Study of Phosphosilicides PtSi3P2 and NiSi2P3

The conventional electron counting scheme often leads to erroneous predictions for late transition metal compounds with weakly electronegative main group ligand atoms. The cause for this deficiency...

On the non-local screening mechanisms in the 2 p photoelectron spectra of NiO and La 2NiO 4

An unresolved question in solid state physics concerns the nature of charge transfer mechanisms in the late transition metal compounds. This paper shows that a structure similar to the doubly peaked feature detected in the Ni 2 p X-ray photoelectron spectra of NiO and attributed to non-local screening is observed also in the Ni 3 s spectrum. However, in this case the spectrum is fully accounted for on the basis of charge transfer, near degeneracy, and exchange mechanisms and neglecting non-local screening. In addition, it is demonstrated that non-local screening mechanisms are not observed in La 2NiO 4 where such an effect, on the basis of the NiO octahedra network, is expected. Finally, the significant differences among the Ni 2 p XPS spectra in NiO, Ni 0.1Mg 0.9O and La 2NiO 4 suggest that core line spectra in nickel oxide based compounds cannot be evaluated simply on the basis of the NiO cluster network but it is necessary to consider the long range crystal structure effects.

High-resolution X-ray emission spectra of transition-metal compounds

A theoretical interpretation of the X-ray emission spectra of transition-metal compounds is described. While the late transition-metal compounds are interpreted by the charge-transfer effect, the early transition-metal compounds are interpreted by the exchange splittings due to the unpaired 3d electrons. However, we have a problem on the line shape of K α 2 which has not yet been solved.

Interatomic and Intra-Atomic Configuration Interactions in Core-Level X-Ray Photoemission Spectra of Late Transition-Metal Compounds

We theoretically investigate X-ray photoemission spectra (XPS) from transition metal 2 p and 3 s core levels in late transition metal compounds in terms of an MX 6 cluster model (M=Mn∼Ni; X=halogen or oxygen), paying particular attention to the interplay between the intra-atomic multiplet coupling and covalency mixing effects. In the analysis of 3 s -XPS, we also take into account the intra-atomic configuration interaction between 3 s 1 3 p 6 3 d n and 3 s 2 3 p 4 3 d n +1 electron configurations. The systematic change of 2 p -XPS and 3 s -XPS on going from Ni to Mn compounds or from fluorides to bromides is mainly ascribed to that of the charge transfer energy from X to M ion. In the analysis, we show that the final state effects are very important in the 3 s -XPS as well as in the 2 p -XPS.