The nature of uranium borohydride in its vapour and solid phases from infrared spectra

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A preliminary X-ray diffraction study of uranium borohydride, U(BH,),, has recently been reported’. It was concluded that the compound is polymeric in the solid with six BH4 groups associated with each uranium atom. Four of these are involved in U-(BH4)-U bridges and the rest are terminal [U-(BH,) type]. The authors deferred an assessment of the nature of the hydrogen bridging present until neutron diffraction studies were completed, but noted that the X-ray data were not inconsistent with double hydrogen bridges between all bonded uranium and boron atoms. An X-ray structure determination’ of solid (160 “C) zirconium borohydride, Zr(BH ) 4 4r and an electron diffraction study of Zr(BHq)q vapou? were both interpreted in terms of a monomeric structure in which tetrahedrally arranged boron atoms arc bonded through triple hydrogen bridges to the central metal atom. A recent vibrational study of zirconium borohydride and hafnium borohydride in the gas phase and in solution has strongly implied a triple bridge structure of Td molecular symmetry *. We wish to report the infrared spectrum of vapour phase uranium borohydride and compare it with the solid phase spectrum.