Optical and magnetic properties of uranium borohydride and tetrakismethylborohydride

Abstract

The U(BD4)4/Hf(BD4)4 optical spectrum reported by Bernstein and Keiderling has been reanalyzed. All 19 allowed transitions have been identified. The crystal field is ∼2.5 times as strong as that of U4+/ThBr4, but the values of the Fk and ζ parameters are nearly the same. The magnetic susceptibility of the structurally related molecule U(BH3CH3)4 has been measured from 2–330 K. Using the eigenvectors from the optical analysis, the magnetic data can be fit with an orbital reduction factor k=0.85. For U(BD4)4/Hf(BD4)4 k=ζ/ζfree ion=0.91.

Complex Metal Borohydrides: From Laboratory Oddities to Prime Candidates in Energy Storage Applications

“…Yan et al have also tried ball-milling the reagents according to Equation (8) with the aim of obtaining pure Y(BH 4 ) 3 , then solvent metathesis of the grinded mixture; however, the obtained yttrium(III) borohydride was only ~85% pure (impurity: LiCl, 15 wt.%) [ 76 ]. Moreover, when the stoichiometric mixture YCl 3 : 3 LiBH 4 was employed for ball-milling at RT or cryo-ball-milling, the reaction mixture showed incomplete conversion to Y(BH 4 ) 3 , while YCl 3 : 4 LiBH 4 yielded almost completely the expected borohydride [ 67 , 77 , 78 , 79 , 80 ]. Usage of LiBH 4 was compulsory, as Y(BH 4 ) 3 was not obtained when replacing the borohydride source with NaBH 4 [ 74 ].…”

“…Zr(BH 4 ) 4 can be prepared by a series of solid-state reactions, for instance between NaZrF 5 + 2 Al(BH 4 ) 3 , ZrCl 4 + 2 Al(BH 4 ) 3 , or ZrCl 4 + 4 LiBH 4 [ 77 , 78 , 79 , 80 ].…”

Metal Borohydrides beyond Groups I and II: A Review

“…Actinide borohydrides, from Th to Pu, are volatile molecules near room temperature. Th(BH 4 ) 4 , Pa(BH 4 ) 4 , and U(BH 4 ) 4 are isomorphic, and their volatility increases with the increasing atomic number, whereas Np(BH 4 ) 4 and Pu(BH 4 ) 4 resemble the Zr-borohydride [ 223 ]. Additionally, Th, Pa, and U borohydrides have double-bridged [BH 4 ] −1 groups that link metal atoms in a low-symmetry polymeric structure in the crystalline phase [ 223 , 224 ].…”

“…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 …”

On the electronic structure of metal tetrahydroborates: quasi-relativistic X.alpha.-SW study of M(BH4)4 (M = zirconium, hafnium, thorium, uranium)

“…Crystalline monomeric 12-coordinate tetrakis-(methyltrihydroborato) compounds of Zr, Th, U, and Np have also been synthesized. 11 Of all these compounds a wide variety of physical, chemical, optical, and magnetic data have been recorded.6,11"14 It is therefore somewhat surprising that no systematic theoretical study of the electronic structure of the metal tetrakis(tetrahydroborates) has been undertaken so far.…”