Electronic Structure of M(BH₄)₄, M = Zr, Hf, and U, by Variable Photon-Energy Photoelectron Spectroscopy and Density Functional Calculations

Abstract

Photoelectron (PE) spectra have been obtained for the M(BH(4))(4) (M = Zr, Hf and U) molecules in the 20-60 eV photon-energy range, and for M = U, also in the 90-120 eV region. Derived branching ratios (BR) and relative partial-photoionization cross sections (RPPICS) of the valence bands are used to confirm band assignment and demonstrate d-orbital covalency for all three compounds and f-orbital covalency for U(BH(4))(4). Core ionizations are identified and used to confirm resonance features in the RPPICS. The absorption spectrum of Zr(BH(4))(4) between 20 and 60 eV shows 4p absorption at 35.5 eV, coincident with the 4p-4d resonance in the RPPICS of the 1e and 2t(2) ionization bands of Zr(BH(4))(4). Less intense absorption bands at 32.5 and 33.8 eV correspond with shape resonance features in the 1a(1) and 1t(2) PE bands. The RPPICS of the f band of U(BH(4))(4) shows two strong resonant features between 95 and 120 eV. Direct photoemission of the 5f electrons from U(BH(4))(4) results only in the observation of the (2)F(5/2) ion state of [U(BH(4))(4)](+), but in the 5d-5f resonant region, a weak band corresponding to the (2)F(7/2) ion state is also observed. The splitting of the 1t(2) band of U(BH(4))(4) is attributed to a small contribution of U 6p semi-core electrons to this MO. Density functional calculations give a good estimate of the pattern of ionization energies, although the calculated absolute values are lower than the experimental values, the first IE by 0.5 eV for Zr(BH(4))(4) and Hf(BH(4))(4) and 1.0 eV for U(BH(4))(4). The MO compositions are in very good agreement with the deductions made from the BR and RPPICS analyses.

Metal Borohydrides beyond Groups I and II: A Review

“…The sublimation heat is between 55.9 and 56.9 kJ/mol [ 86 ]. Zr(BH 4 ) 4 displays d-orbital covalency [ 87 ].…”

“…Hf(BH 4 ) 4 is also a volatile compound at room temperature, and its gas-phase structure is monomeric η 3 , with tetrahedral arrangement and high vapor pressure (~2 kPa at RT) [ 11 , 72 , 81 , 92 ]. The crystal structure of Hf(BH 4 ) 4 was determined at 110 K: cubic space group P-43m in the solid-state ( Table 6 ); [BH 4 ] −1 coordination to Hf is η 3 , with d-orbital covalency [ 87 , 93 ]. As in the case of Zr, Hf can form a complex ion, [Hf(BH 4 ) 5 ] −1 [ 88 ]: which is unstable at low temperature and decomposes (~253 K) to produce the precursor materials [ 88 ].…”

“…U(BH 4 ) 4 possesses high symmetry ( T d ), efficient screening of the metal atom, and the limited bridging ability of the [BH 4 ] −1 ion [ 81 ]. The optical spectrum of U(BH 4 ) 4 can be revised at [ 223 , 227 ] and, relatively recently, the f-orbital covalency in U(BH 4 ) 4 was demonstrated [ 87 ]. U(BH 4 ) 4 is fairly stable at room temperature [ 229 ], but can decompose upon the action of ultraviolet photons of 253.5 nm (4.8 eV) at 295 K, with B 2 H 6 being a decomposition product [ 228 ]: 2U(BH 4 ) 4 → 2U(BH 4 ) 3 + B 2 H 6 + H 2 or U(BH 4 ) 4 → U(BH 4 ) 2 + B 2 H 6 + H 2 …”

Spectroscopy and Kinetics of U02 Fuel Oxidation Hydrolysis and Radiolysis: Applications to Radioactive Waste Management

“…Despite this lack of sensitivity, x-ray photoelectron studies have proven useful in probing the electronic structure of inner valence and core-like electrons in uranium oxide compounds [60] [132] [44] [133]. Electronic transition energies to and from the uranium 5d, 6p, and 6s levels are typically probed in these studies, yielding insight into the nature of these higher energy transitions.…”

A New 5,5′‐Bitetrazole Thorium(IV) Compound: Synthesis, Crystal Structure and Quantum Chemical Investigation

“…[5] Till this day the discussion is vivid whether Th IV has to be considered as an actinide element or as a large member of the group IV elements, with recent contributions on the chemistry of the complexes [(Cp*) 2 M(Me) 2 ] (M = Zr, Hf, Th), [6] on quantum chemical calculations on the Cp derivatives [(Cp) 2 M(Me) 2 ] (M = Zr, Th, U), [7] or on spectroscopic similarities and differences. [8] Recently, our group reported on the 5,5Ј-bitetrazole coordination chemistry of a number of trivalent 4f element ions. [9] Generally, the 5,5Ј-bitetrazolate dianion BT 2- [10][11][12] and related tetrazolyl systems are interesting candidates for rich coordination chemistry: For example, BT 2-contains eight N atoms, being able to act as mono-, bi-, tri-or tetradentate ligand (cf.…”