On the oxidation states of metal elements in MO3 - (M=V, Nb, Ta, Db, Pr, Gd, Pa) anions

Carbon Dioxide and Water Activation by Niobium Trioxide Anions in the Gas Phase

Transition metals are important in various industrial applications including catalysis. Due to the current concentration of CO₂ in the atmosphere, various ways for its capture and utilization are investigated. Here, we study the activation of CO₂ and H₂O at [NbO₃]^- in the gas phase using a combination of infrared multiple photon dissociation spectroscopy and density functional theory calculations. In the experiments, Fourier-transform ion cyclotron resonance mass spectrometry is combined with tunable IR laser light provided by the intracavity free-electron laser FELICE or optical parametric oscillator-based table-top laser systems. We present spectra of [NbO₃]^-, [NbO₂(OH)₂]^-, [NbO₂(OH)₂]^-(H₂O) and [NbO(OH)₂(CO₃)]^- in the 240-4000 cm^-1 range. The measured spectra and observed dissociation channels together with quantum chemical calculations confirm that upon interaction with a water molecule, [NbO₃]^- is transformed to [NbO₂(OH)₂]^- via a barrierless reaction. Reaction of this product with CO₂ leads to [NbO(OH)₂(CO₃)]^- with the formation of a [CO₃] moiety.

On the highest oxidation states of the actinoids in AnO4 molecules (An = Ac - Cm): A DMRG-CASSCF study

Actinoid tetroxide molecules AnO₄ (An = Ac - Cm) are investigated with the ab initio density matrix renormalization group (DMRG) approach. Natural orbital shapes are used to read out the oxidation state (OS) of the f-elements, and the atomic orbital energies and radii are used to explain the trends. The highest OSs reveal a "volcano"-type variation: For An = Ac - Np, the OSs are equal to the number of available valence electrons, that is, Ac^III , Th^IV , Pa^V , U^VI , and Np^VII . Starting with plutonium as the turning point, the highest OSs in the most stable AnO₄ isomers then decrease as Pu^V , Am^V , and Cm^III , indicating that the 5f-electrons are hard to be fully oxidized off from Pu onward. The variations are related to the actinoid contraction and to the 5f-covalency characteristics. Combined with previous work on OSs, we review their general trends throughout the periodic table, providing fundamental understanding of OS-relevant phenomena.

Spectroscopic and electrochemical characterization of a Pr4+ imidophosphorane complex and the redox chemistry of Nd3+ and Dy3+ complexes

The molecular tetravalent oxidation state for praseodymium is observed in solution via oxidation of the anionic trivalent precursor [K][Pr³⁺(NP(1,2-bis-^tBu-diamidoethane)(NEt₂))₄] (1-Pr(NP*)) with AgI at -35 °C. The Pr⁴⁺ complex is characterized in solution via cyclic voltammetry, UV-vis-NIR electronic absorption spectroscopy, and EPR spectroscopy. Electrochemical analyses of [K][Ln³⁺(NP(1,2-bis-^tBu-diamidoethane)(NEt₂))₄] (Ln = Nd and Dy) by cyclic voltammetry are reported and, in conjunction with theoretical modeling of electronic structure and oxidation potential, are indicative of principal ligand oxidations in contrast to the metal-centered oxidation observed for 1-Pr(NP*). The identification of a tetravalent praseodymium complex in in situ UV-vis and EPR experiments is further validated by theoretical modeling of the redox chemistry and the UV-vis spectrum. The latter study was performed by extended multistate pair-density functional theory (XMS-PDFT) and implicates a multiconfigurational ground state for the tetravalent praseodymium complex.

The smallest 4f-metalla-aromatic molecule of cyclo-PrB2− with Pr–B multiple bonds

The concept of metalla-aromaticity proposed by Thorn–Hoffmann (Nouv. J. Chim. 1979, 3, 39) has been expanded to organometallic molecules of transition metals that have more than one independent electron-delocalized system. Lanthanides, with highly contracted 4f atomic orbitals, are rarely found in multiply aromatic systems. Here we report the discovery of a doubly aromatic triatomic lanthanide-boron molecule PrB₂⁻ based on a joint photoelectron spectroscopy and quantum chemical investigation. Global minimum structural searches reveal that PrB₂⁻ has a C_2v triangular structure with a paramagnetic triplet ³B₂ electronic ground state, which can be viewed as featuring a trivalent Pr(III,f²) and B₂⁴⁻. Chemical bonding analyses show that this cyclo-PrB₂⁻ species is the smallest 4f-metalla-aromatic system exhibiting σ and π double aromaticity and multiple Pr–B bonding characters. It also sheds light on the formation of the rare B₂⁴⁻ tetraanion by the high-lying 5d orbitals of the 4f-elements, completing the isoelectronic B₂⁴⁻, C₂²⁻, N₂, and O₂²⁺ series.

We report the smallest 4f-metalla-aromatic molecule of PrB₂⁻ exhibiting σ and π double aromaticity and multiple Pr–B bond characters.

Ligands enhanced the Ac[triple bond, length as m-dash]Ac triple bond

The multiple bonds between actinide atoms and their derivatives are computationally investigated extensively and compounds with an unsupported actinide-actinide bond, especially in low oxidation states, have attracted great attention. Herein, high level relativistic quantum chemical methods are used to probe the Ac-Ac bonding in compounds with a general formula LAcAcL (L = AsH3, PH3, NH3, H, CO, NO) at both scalar and spin-orbit coupling relativistic levels. H3AsAcAcAsH3, H3PAcAcPH3 and OCAcAcCO compounds show a type of zero valence Ac[triple bond, length as m-dash]Ac triple bond with a 1σ2g1π4u configuration, and H3AsAcAcAsH3 has been found to have the shortest Ac-Ac bond length of 3.012 Å reported so far. The Ac2 unit is very sensitive to the σ donor ligands and can form triple, double and even single bonds when suitable ligands are introduced, up to 3.652 Å with an Ac-Ac single bond in H3NAcAcNH3.

Density Functional Theoretical Study on the Electronic Structure of Rh2O7 + with Low Oxidation States

Rh₂O _n ⁺ (n = 2-10) species are prepared by the reaction of the laser-ablated rhodium atoms with oxygen; furthermore, they are characterized by employing time-of-flight mass spectroscopy. To reveal the stable electronic structure, in this study, we performed the density functional theory calculations for the possible isomers of Rh₂O₇ ⁺. A total of 29 geometries were obtained including cyclic Rh₂O₃, cyclic Rh₂O₂, and ring-opening structures with doublet, quartet, sextet, and octet states. It is noteworthy that no Rh-Rh bond was observed for all the optimized Rh₂O₇ ⁺ isomers including oxides, peroxides, superoxides, and oxygen groups. The optimized geometries were also confirmed to exhibit minimum structural energies by employing harmonic frequency analysis at the same energy level. Generally, two types of oxygen-bridged geometries were discovered with cyclic and pseudo-linear Rh₂O₇ ⁺, which contained one or more than one O₂ groups. It is concluded that the cyclic structure comprises a lower energy than that observed in pseudo-linear structures. In addition, Rh₂O₇ ⁺ tends to be unstable when the coordination groups change from O₂ to O₂ ^- unit. Finally, the localized orbital bonding analysis indicates that Rh has oxidation states of 1 or 2 in cyclic Rh₂O₇ ⁺ structures; this is true even in the presence of O^2-, O₂ ^-, and O₂ ^2- groups.

Molecular oxides of high-valent actinides

The past decade has been very productive in the field of actinide (An) oxides containing high-valent An. Novel gas-phase experimental and an impressive number of theoretical studies have been performed, mostly on pure oxides or oxides extended with other ligands. The review covers the structural properties of molecular An oxides with high (An≥V) oxidation states. The presented compounds include the actinide dioxide cations [AnO2]+ and [AnO2]2+, neutral and ionic AnOx (x = 3–6), oxides with more than one An atom like neutral dimers, trimers and dimers from cation–cation interactions, as well as large U-oxide clusters observed very recently in the gaseous phase.

The chemical and physical properties of tetravalent lanthanides: Pr, Nd, Tb, and Dy

The thermochemistry, descriptive chemistry, spectroscopy, and physical properties of the tetravalent lanthanides (Pr, Nd, Tb and Dy) in extended phases, gas phase, solution, and as isolable molecular complexes are presented.

The decisive role of 4f-covalency in the structural direction and oxidation state of XPrO compounds (X: group 13 to 17 elements)

Lanthanide oxo compounds are of vital importance in lanthanide chemistry, as well as in environmental and materials sciences. Praseodymium, as an exceptional element in lanthanides which can form a +V formal oxidation state (OSf) besides the dominant +III among the 4f-block element, displays the significant participation of the Pr 4f orbitals in bonding interactions which is commonly crucial in stabilizing the high oxidation state of Pr in PrO2+ and NPrO species. Here, we report a systematic theoretical study on the structures and stabilities of a series of XPrO (X: B, Al, C, Si, N, P, As, O, S, F, Cl) compounds along with [XPrO]+ cation (X: O, S) and [X3PrO] complexes (X: F and Cl). This work reveals that Pr is able to achieve the lowest and highest OSf and the OSf exhibits periodic variation from +I in BOPr and AlOPr to +II in SiOPr to +III in CPrO, FPrO, ClPrO and AsPrO to +IV in OPrO and SPrO and even to +V in NPrO, [OPrO]+, [SPrO]+, F3PrO and Cl3PrO. We found that the molecular structures are correlated to the Pr oxidation state due to the highly important 4f orbital in the chemical bonding of the high oxidation state compounds. Thus, not only the electronegativity of the ligand but also the quasi-degenerate Pr valence 4f orbitals, namely energetic covalency, control the oxidation state and play a fundamental role in affecting the electronic structural stability of Pr(v) compounds as well. This work demonstrates the structurally directing role of the f-orbital in the formation of the linear structure and is constructive for achieving the higher oxidation state of a given element by tuning the ligand.

Bonding trends across the series of tricarbonato-actinyl anions [(AnO2)(CO3)3]4- (An = U-Cm): the plutonium turn

Actinyl-tricarbonato anions [(AnO₂)(CO₃)₃]^4- (An = U-Cm) in various environments were investigated using theoretical approaches of quantum-mechanics, molecular-mechanics and cluster-models. Cations and solvent molecules in the 2^nd coordination sphere affect the equatorial An←O_eq bonds more than the axial An[triple bond, length as m-dash]O_ax bonds. Common actinide contraction is found for calculated and experimental axial bond lengths of ₉₂U to ₉₄Pu, though no longer for ₉₄Pu to ₉₆Cm. The tendency of U to Pu forming actinyl(vi) species dwindles away toward Cm, which already features the preferred An^III/Ln^III oxidation state of the later actinides and all lanthanides. The well known change from d-type to typical U-Pu-Cm type and then to f-type behavior is labeled as the plutonium turn, a phenomenon that is caused by f-orbital energy-decrease and f-orbital localization with increase of both nuclear charge and oxidation state, and a non-linear variation of effective f-electron population across the actinide series. Both orbital and configuration mixing and occupation of antibonding 5f type orbitals increase, weakening the AnO_ax bonds and reducing the highest possible oxidation states of the later actinides.

Diversity of Chemical Bonding and Oxidation States in MS4 Molecules of Group 8 Elements

The geometric and electronic ground-state structures of 30 isomers of six MS₄ molecules (M=Group 8 metals Fe, Ru, Os, Hs, Sm, and Pu) have been studied by using quantum-chemical density functional theory and correlated wavefunction approaches. The MS₄ species were compared to analogous MO₄ species recently investigated (W. Huang, W.-H. Xu, W. H. E. Schwarz, J. Li, Inorg. Chem. 2016, 55, 4616). A metal oxidation state (MOS) with a high value of eight appeared in the low-spin singlet T_d geometric species (Os,Hs)S₄ and (Ru,Os,Hs)O₄ , whereas a low MOS of two appeared in the high-spin septet D_2d species Fe(S₂ )₂ and (slightly excited) metastable Fe(O₂ )₂ . The ground states of all other molecules had intermediate MOS values, with S^2- , S₂^2- , S₂^1- (and O^2- , O^1- , O₂^2- , O₂^1- ) ligands bonded by ionic, covalent, and correlative contributions. The known tendencies toward lower MOS on going from oxides to sulfides, from Hs to Os to Ru, and from Pu to Sm, and the specific behavior of Fe, were found to arise from the different atomic orbital energies and radii of the (n-1)p core and (n-1)d and (n-2)f valence shells of the metal atoms in row n of the periodic table. The comparative results of the electronic and geometric structures of the MO₄ and MS₄ species provides insight into the periodicity of oxidation states and bonding.

Pentavalent lanthanide nitride-oxides: NPrO and NPrO- complexes with N≡Pr triple bonds

The neutral molecule NPrO and its anion NPrO^- are produced via co-condensation of laser-ablated praseodymium atoms with nitric oxide in a solid neon matrix. Combined infrared spectroscopy and state-of-the-art quantum chemical calculations confirm that both species are pentavalent praseodymium nitride-oxides with linear structures that contain Pr≡N triple bonds and Pr=O double bonds. Electronic structure studies show that the neutral NPrO molecule features a 4f⁰ electron configuration and a Pr(v) oxidation state similar to that of the isoelectronic PrO₂⁺ ion, while its NPrO^- anion possesses a 4f¹ electron configuration and a Pr(iv) oxidation state. The neutral NPrO molecule is thus a rare lanthanide nitride-oxide species with a Pr(v) oxidation state, which follows the recent identification of the first Pr(v) oxidation state in the PrO₂⁺ and PrO₄ complexes (Angew. Chem. Int. Ed., 2016, 55, 6896). This finding indicates that lanthanide compounds with oxidation states of higher than +IV are richer in chemistry than previously recognized.

Pentavalent Lanthanide Compounds: Formation and Characterization of Praseodymium(V) Oxides

The chemistry of lanthanides (Ln=La-Lu) is dominated by the low-valent +3 or +2 oxidation state because of the chemical inertness of the valence 4f electrons. The highest known oxidation state of the whole lanthanide series is +4 for Ce, Pr, Nd, Tb, and Dy. We report the formation of the lanthanide oxide species PrO4 and PrO2 (+) complexes in the gas phase and in a solid noble-gas matrix. Combined infrared spectroscopic and advanced quantum chemistry studies show that these species have the unprecedented Pr(V) oxidation state, thus demonstrating that the pentavalent state is viable for lanthanide elements in a suitable coordination environment.

On the Highest Oxidation States of Metal Elements in MO4 Molecules (M = Fe, Ru, Os, Hs, Sm, and Pu)

Metal tetraoxygen molecules (MO4, M = Fe, Ru, Os, Hs, Sm, Pu) of all metal atoms M with eight valence electrons are theoretically studied using density functional and correlated wave function approaches. The heavier d-block elements Ru, Os, Hs are confirmed to form stable tetraoxides of Td symmetry in (1)A1 electronic states with empty metal d(0) valence shell and closed-shell O(2-) ligands, while the 3d-, 4f-, and 5f-elements Fe, Sm, and Pu prefer partial occupation of their valence shells and peroxide or superoxide ligands at lower symmetry structures with various spin couplings. The different geometric and electronic structures and chemical bonding types of the six iso-stoichiometric species are explained in terms of atomic orbital energies and orbital radii. The variations found here contribute to our general understanding of the periodic trends of oxidation states across the periodic table.

How Much Can Density Functional Approximations (DFA) Fail? The Extreme Case of the FeO4 Species

A thorough theoretical study of the relative energies of various molecular Fe·4O isomers with different oxidation states of both Fe and O atoms is presented, comparing simple Hartree-Fock through many Kohn-Sham approximations up to extended coupled cluster and DMRG multiconfiguration benchmark methods. The ground state of Fe·4O is a singlet, hexavalent iron(VI) complex (1)C2v-[Fe(VI)O2](2+)(O2)(2-), with isomers of oxidation states Fe(II), Fe(III), Fe(IV), Fe(V), and Fe(VIII) all lying slightly higher within the range of 1 eV. The disputed existence of oxidation state Fe(VIII) is discussed for isolated FeO4 molecules. Density functional theory (DFT) at various DF approximation (DFA) levels of local and gradient approaches, Hartree-Fock exchange and meta hybrids, range dependent, DFT-D and DFT+U models do not perform better for the relative stabilities of the geometric and electronic Fe·4O isomers than within 1-5 eV. The Fe·4O isomeric species are an excellent testing and validation ground for the development of density functional and wave function methods for strongly correlated multireference states, which do not seem to always follow chemical intuition.

Experimental and theoretical identification of the Fe(vii) oxidation state in FeO4−

Two isomers of iron tetraoxygen anion, dioxoiron peroxide [(η²-O₂)FeO₂]⁻ and tetroxide FeO₄⁻ were characterized by experiment and theoretical calculations, with heptavalent Fe(vii) oxidation state identified in the later.

Is Octavalent Pu(VIII) Possible? Mapping the Plutonium Oxyfluoride Series PuO(n)F(8-2n) (n = 0-4)

While the oxidation state Pu(VIII) is shown to be less stable than Pu(V) in the PuO4 molecule, it is not clear if the more electronegative fluorine can help to stabilize Pu(VIII). Our calculations on PuO(n)F(8-2n) (n = 0-4) molecules notably confirm that PuO2F4 has both (1)D(4h) and (5)C(2v) minima with the oxidation states Pu(VIII) and Pu(V), respectively, with the latter having lower energy. The hybrid-DFT, CCSD(T), and CASSCF methods all give the same result. The results conform to a superoxide ligand when n ≥ 2. PuF8 in a (1)O(h) state can decompose to PuF6 and F2, and PuOF6 in a (1)C(2v) state also can break down to PuF6 and 1/2 O2. The Pu(VIII) anion PuO2F5(-) does have a D(5h) minimum, which also lies above a (5)C(2v) Pu(V) peroxide structure. However, the energy differences between the different minima are not large, indicating that metastable species with oxidation states higher than Pu(V) cannot be completely excluded.