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Trending: Metal Oxo Bonds Print
Wednesday, 29 May 2013 00:00

Metal oxides are important for scientific and technical applications in a variety of disciplines, including materials science, chemistry, and biology. Highly covalent metal–oxygen multiple bonds (metal oxos) are the building blocks of metal oxides and have a bearing on the oxide’s desirable chemical, magnetic, electronic, and thermal properties. The lack of a more sophisticated grasp of bonding in metal oxides constitutes a roadblock to innovation in a wide variety of important emergent technologies, including industrial catalysis, biomimetic transformations, and artificial photosynthesis. To address this problem, a research team from four national laboratories, three Department of Energy synchrotron user facilities, and the University of Washington has applied spectroscopic and computational analyses to a number of metal oxides, quantifying trends in metal oxo bonding for groups of metals across the periodic table.

Chasing Electrons
in Metal Oxides

Covalent bonds are the result of adjacent atoms "sharing" their outer valence shell electrons, while an ionic bond consists of an electrostatic attraction between positively and negatively charged ions. Although these descriptions of the chemical bond are fundamental to chemistry, they are not simple concepts. These electrons play a key role in determining a material's physical properties, including geometric structure, chemical reactivity, thermal capacity, magnetic behavior, and conductivity. A better understanding of their behavior would help open up new avenues for innovation in a variety of scientific and technical areas.

In this research, Minasian et al. describe a method to directly and quantitatively probe covalent bonds in molecular metal oxides using x-ray absorption spectroscopy. Metal oxides were chosen because they have highly covalent metal–oxygen multiple bonds and are desirable for several industrial processes, are used by nature in enzymatic transformations, and are widespread reagents for synthesis in the laboratory. The results show a surprising variability for different metals in the periodic table, illustrating the challenges associated with using traditional models to evaluate multiple bonding in metal oxides.

Tetrahedral metal oxide molecules used in this study.

Metal oxides are key components in both in technological and biological processes that are often governed by careful control over the physical and chemical properties of metal–oxygen bonds. For example, knowledge of the exact nature of highly covalent metal oxo bonds involving heavy metals such as uranium, neptunium, and plutonium is important to efficient remediation of radioactively contaminated sites. Drug metabolism and cholesterol synthesis (by the P450 family of enzymes) and the "splitting" of water (by photosystem II, a light-dependent protein complex) are examples of metal oxide functions found in nature. Techniques that can improve our models of electronic structure for metal–oxygen interactions are important to the development of artificial systems that can mimic the selectivity and efficiency of such systems.

The orbital interactions for a tetrahedron as visualized by superimposing in a cube. Oxygen atoms are located at opposite vertices, and metal atoms are located at the cube center.

Among approaches explored previously, ligand K-edge x-ray absorption spectroscopy (XAS) has emerged as an effective method for quantitatively probing electronic structure and orbital mixing in molecules and materials. The presence of covalent mixing is observed as a pre-edge feature in ligand K-edge XAS; it appears only if the vacant metal d orbital contains a significant component of ligand p orbital character. Similar studies of metal–oxygen systems, however, are complicated by experimental barriers at the low energy of the oxygen K-edge (about 530 eV), which magnifies issues associated with surface contamination and self-absorption effects.

These challenges were overcome by applying state-of-the-art soft x-ray techniques in conjunction with hybrid density functional theory (DFT) calculations. To ensure that accurate pre-edge peak intensities were obtained, oxygen K-edge XAS was performed in transmission mode with a scanning transmission x-ray microscope (STXM) and with nonresonant inelastic x-ray scattering (NIXS). The spectroscopic work was performed at the Molecular Environmental Sciences Beamline 11.0.2 (STXM) at the ALS, Beamline 6.2 at the Stanford Synchrotron Radiation Lightsource (NIXS), and the LERIX facility at the Advanced Photon Source (NIXS). The samples included highly symmetric metal–oxygen anions (MO4n-) based on metals from Group 6 (M = Cr, Mo, W; n = 2) and Group 7 (M = Mn, Tc, Re; n = 1) of the perodic table; they were chosen because historically they were foundational in the development of valence bond theory. In addition, the radioactive pertechnetate anion (TcO41-) is important to nuclear waste remediation and in radiopharmaceuticals.

Oxygen K-edge XAS (black) and time-dependent DFT (red) show two large pre-edge features. The e* antibonding peak increases as one moves down the periodic table, from Cr to W and from Mn to Re, while the t2* peak remains relatively constant, suggesting two different trends in orbital mixing.

Despite being isoelectronic, the anions exhibited unexpected differences in orbital mixing. Moving from Group 6 to 7 or down the triads increases M–O mixing in the e* orbitals, more than doubling from CrO42- to ReO41-. Mixing in the t2* orbitals remains relatively constant within a Group but increases on moving from Group 6 to 7. This research shows that models of metal oxo electronic structure should not rely solely on periodic changes in either d orbital energy or radial extension. On the contrary, orbital composition is influenced by a complex interplay between both factors, leading to changes in the energy and composition of the frontier orbitals even for formally isoelectronic metal oxos.

 


 

Research conducted by: S.G. Minasian (Los Alamos National Laboratory and Berkeley Lab); J.M. Keith, E.R. Batista, K.S. Boland, S.R. Daly, S.A. Kozimor, R.L. Martin, and G.L. Wagner (Los Alamos National Laboratory); J.A. Bradley (Lawrence Livermore National Laboratory); W.W. Lukens and D.K. Shuh (Berkeley Lab); D. Nordlund, D. Sokaras, and T.-C. Weng (SLAC National Accelerator Laboratory); G.T. Seidler (University of Washington); T. Tyliszczak (ALS); and P. Yang (Pacific Northwest National Laboratory).

Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). Operation of ALS, SSRL, and APS is supported by DOE BES.

Publication about this research: S.G. Minasian, J.M. Keith, E.R. Batista, K.S. Boland, J.A. Bradley, S.R. Daly, S.A. Kozimor, W.W. Lukens, R.L. Martin, D. Nordlund, G.T. Seidler, D.K. Shuh, D. Sokaras, T. Tyliszczak, G.L. Wagner, T.-C. Weng, and P. Yang, "Covalency in metal-oxygen multiple bonds evaluated using oxygen K-edge spectroscopy and electronic structure theory," J. Am. Chem. Soc. 135, 1864 (2013).

ALS Science Highlight #270

 

ALSNews Vol. 342