Direct spectroscopic evidence for the high-spin state of dioxidomanganese(V).

The spin state of metal centers in many catalytic reactions has been demonstrated to be a rate limiting factor when high-valent metal centers such as manganese are involved. Although numerous manganese(V) complexes, including a few manganese(V) oxo complexes, have been identified, thus far only one of these, [MnVH3 buea(O)], has been directly confirmed to exist in a high spin state. Such a high-spin manganese(V) center may play a crucial role in the dioxygen formation process in the elusive S4 state of the Kok cycle in photosystem II. In this study, we provide direct experimental evidence, using X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS), of gas phase [OMnO]+ as the second known high-spin manganese(V) oxo complex. We conclusively assign the ground state as 3B1 (C2v). Additionally, we provide fingerprint spectra not only for [OMnV O]+, but also for the high-spin hydroxidooxidomanganese(IV) ion [OMnIV OH]+ in its 4A'' (Cs) ground state that is expected to exhibit similar XAS and XMCD spectral signatures to neutral dioxidomanganese(IV).

Exposing the Oxygen-Centered Radical Character of the Tetraoxido Ruthenium(VIII) Cation [RuO4 ].

The tetraoxido ruthenium(VIII) radical cation, [RuO4 ]+ , should be a strong oxidizing agent, but has been difficult to produce and investigate so far. In our X-ray absorption spectroscopy study, in combination with quantum-chemical calculations, we show that [RuO4 ]+ , produced via oxidation of ruthenium cations by ozone in the gas phase, forms the oxygen-centered radical ground state. The oxygen-centered radical character of [RuO4 ]+ is identified by the chemical shift at the ruthenium M3 edge, indicative of ruthenium(VIII), and by the presence of a characteristic low-energy transition at the oxygen K edge, involving an oxygen-centered singly-occupied molecular orbital, which is suppressed when the oxygen-centered radical is quenched by hydrogenation of [RuO4 ]+ to the closed-shell [RuO4 H]+ ion. Hydrogen-atom abstraction from methane is calculated to be only slightly less exothermic for [RuO4 ]+ than for [OsO4 ]+ .

Electronic Structure of the Complete Series of Gas-Phase Manganese Acetylacetonates by X-ray Absorption Spectroscopy.

Metal centers in transition metal-ligand complexes occur in a variety of oxidation states causing their redox activity and therefore making them relevant for applications in physics and chemistry. The electronic state of these complexes can be studied by X-ray absorption spectroscopy, which is, however, due to the complex spectral signature not always straightforward. Here, we study the electronic structure of gas-phase cationic manganese acetylacetonate complexes Mn(acac)1-3+ using X-ray absorption spectroscopy at the metal center and ligand constituents. The spectra are well reproduced by multiconfigurational wave function theory, time-dependent density functional theory as well as parameterized crystal field and charge transfer multiplet simulations. This enables us to get detailed insights into the electronic structure of ground-state Mn(acac)1-3+ and extract empirical parameters such as crystal field strength and exchange coupling from X-ray excitation at both the metal and ligand sites. By comparison to X-ray absorption spectra of neutral, solvated Mn(acac)2,3 complexes, we also show that the effect of coordination on the L3 excitation energy, routinely used to identify oxidation states, can contribute about 40-50% to the observed shift, which for the current study is 1.9 eV per oxidation state.

Iron L3-edge energy shifts for the full range of possible 3d occupations within the same oxidation state of iron halides.

Oxidation states are integer in number but dn configurations of transition metal centers vary continuously in polar bonds. We quantify the shifts of the iron L3 excitation energy, within the same formal oxidation state, in a systematic L-edge X-ray absorption spectroscopy study of diatomic gas-phase iron(II) halide cations, [FeX]+,where X = F, Cl, Br, I. These shifts correlate with the electronegativity of the halogen, and are attributed exclusively to a fractional increase in population of 3d-derived orbitals along the series as supported by charge transfer multiplet simulations and density functional theory calculations. We extract an excitation energy shift of 420 meV ± 60 meV spanning the full range of possible 3d occupations between the most ionic bond in [FeF]+ and covalently bonded [FeI]+.

The Highest Oxidation State of Rhodium: Rhodium(VII) in [RhO3 ].

Although the highest possible oxidation states of all transition elements are rare, they are not only of fundamental interest but also relevant as potentially strong oxidizing agents. In general, the highest oxidation states are found in the electron-rich late transition elements of groups 7-9 of the periodic table. Rhodium is the first element of the 4d transition metal series for which the highest known oxidation state does not equal its group number of 9, but reaches only a significantly lower value of +6 in exceptional cases. Higher oxidation states of rhodium have remained elusive so far. In a combined mass spectrometry, X-ray absorption spectroscopy, and quantum-chemical study of gas-phase R h O n + (n=1-4), we identify R h O 3 + as the 1 A 1 ' trioxidorhodium(VII) cation, the first chemical species to contain rhodium in the +7 oxidation state, which is the third-highest oxidation state experimentally verified among all elements in the periodic table.