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14-Coordinate Uranium(1V). the Structure of Uranium Borohydride by Single-Crystal Neutron Diffraction

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14-Coordinate Uranium(1V). the Structure of Uranium Borohydride by Single-Crystal Neutron Diffraction 14-COORDINATE URANIUM (Iv) Inorganic Chemistry, Vol. 11, No. 12, 1972 3009 control of the geometry, and sulfur-sulfur bonding Science Foundation, GP-11701, is acknowledged. Also interactions may dominate the structure. we wish to thank the donors of the Petroleum Research Fund as administered by the American Chemical So- Acknowledgments.-The support of the National ciety for partial support of this work. CONTRIBUTION FROM THE CHEMISTRY DEPARTMENTSOF BROOKHAVENNATIONAL LABORATORY, UPTON, hTEW YORK 11973, COLUMBIAUNIVERSITY, NEW YORK,NEW YORK 10027, AND PRINCETONUNIVERSITY, PRINCETON, NEW JERSEY 08540 14-Coordinate Uranium(1V). The Structure of Uranium Borohydride by Single-Crystal Neutron Diffraction BY E. R. BERNSTEIN,Zs WALTER C. HAMILTON,*2bT. A. KEIDERLING,2a,3 SAM J. LA PLACA,2b S. J. LIPPARD,20 AND J. J. MAYERLE20 Received May 8, 1972 The structure of U(BH4)4 has been refined by a single-crystal neutron diffraction study. The X-ray structure (tetragonal, P4~2~2(P412~2), a = 7.49 (1) A, c = 13.24 (1) A, Z = 4, pcalcd = 2.66 g/cm*) has been confirmed and, in addition, all hydrogen atoms have been located to a precision of 0.04 A. Four of the six BHd-ions surrounding each uranium atom are attached to it by two hydrogen atoms, and use their remaining two hydrogen atoms to bridge neighboring uranium atoms in a helical polymeric structure. Two additional tetrahydroborate groups in a cis configuration are bonded to the uranium atom by three hydrogen atoms, resulting in an overall coordination number of 14. The mean U-H bond length is 2 38 (2) A. The BH4- ions are approximately tetrahedral with a mean B-H distance (corrected for thermal motion) of 1.29 (4) b. A capped hexagonal antiprism is considered to be a useful reference coordination polyhedron, and distortions from this idealized geometry are described. The difference between the solid-state structures of Zr(BH& and U(BH4)h are discussed in terms of sphere packing and molecular orbital considerations. The results have also been used in conjunction with diffraction data on other metal tetrahydroborate compounds and with tabulated ionic radii to develop a single and consistent picture in which the metal-boron distance is shown to correlate with the geometry of the metal borohydride attachment. Introduction The investigation of the crystal structure of uranium Knowledge of the molecular geometry of metal tetra- borohydride by X-rayll and neutron diffraction tech- hydroborate complexes is required for the interpreta- niques was motivated by two basic considerations. tion of their physical and chemical properties. Of First, knowledge of the geometry of the U-BHd at- special interest is the mode of attachment of the BH4- tachment was desired in conjunction with the analysis group to the central metal ion, in particular, whether of the spectroscopic properties of M(BH4)h compounds, there are one, two, or three hydrogen atoms in the M = Zr, Hf, U, Th.I2 The need for sound structural bridge bonds. To date, X-ray diffraction studies have data, in which the positions of all atoms are unambig- established a two-point attachment forbis(tetrahydrob0- uously located, is underscored by past difficulties with rato) beryllium (I I), tetrahydroboratobis (triphenylphos- vibrational analyses of M(BH4)4 molecule^.^^ For ex- phine)~opper(I),~tris(tetrahydroborato)trimethylaniine- ample, Zr(BH4)4 (12-coordinate, T, symmetry, four aluminum(II1) and tetrahydroboratobis(h5-cyclopenta- BHI- groups having a three-point attachment) 9,10 and dienyl) titanium(III),’ while electron diffraction re- U(BH4)4 (lPcoordinate, C2 symmetry, four BHd- sults imply a similar bonding mode for tris(tetrahydr0- groups having a two-point and two BH4- groups having borato) aluminum (I1I) .s Only tetrakis (tetrahydro- a three-point attachment, vide infra)” have been as- borato)zirconium(IV) has been reported as having sumed on the basis of infrared data to be isostruc- three hydrogen bridge bonds between the central tran- t~ral.~~,~~Thus, when dealing with such systems as sition metal atom and boron. This geometry has been M(BH4)4, knowledge of the molecular geometry is suggested both by single-crystal X-ray diffractiong and especially useful, perhaps even necessary, for the inter- gas-phase electron diffractionlo studies. pretation of vibrational data. Second, optical and electron paramagnetic resonance spectroscopic results (1) Work done in part under the auspices of the U. S. Atomic Energy Commission. We also wish to acknowledge grants from the National on U(BH4)4 are now availableI5 and their analysis de- Science Foundation (Grant number GP-29118) and U. S. Army-Durham pends in part on exact structural details. (Contract DAH-CO4-71-CO027) to E. R. B. and from the National Science A report of the single-crystal X-ray diffraction study Foundation (Grant number GP-27239X) to S. J. L. (2) (a) Princeton University. (b) Brookhaven National Laboratory. of U(BH4)4 has already appeared.11116 The crystal (c) Columbia University. (3) NSF Predoctoral Fellow 1969-1972. (11) E. R. Bernstein, T. A. Keiderling, S. J. Lippard, and J. J Mayerle, (4) D. S. Marynick and W. N. Lipscomb, J. Amer. Chem. Soc., 98, 2322 J. Amev. Chem. Soc., 94, 2552 (1972). (1971); Inorg. Chem., 11, 820 (1972). (12) In this regard the molecular and crystal structure of Hf(BH4)r is (5) S. J Lippard and K. M. Melmed, zbzd., 6,2223 (1967); J. Amev. Chem. being studied by neutron diffraction at Brookhaven Soc., 89, 3929 (1967). (13) A review of infrared and Raman data for metal tetrahydroborate (6) N. A. Bailey, P. H. Bird, and M. G. H. Wallbridge, Inovg. Chem., complexes can be found in B D James and M. G. H. Wallbridge, PYO~V. 7, 1575 (1968) Inoug. Chem., 11, 99 (1970), and more recent work appears and is cited in (7) K. M. Melmed, D Coucouvanis, and S. J. Lippard, Inovg. Chem , in B. E Smith and B. D. James, Inorg. Nucl. Chem. Let6 , 7, 857 (1971) press. (14) V. V. Volkov, K. G. Myakishev, and 2. A. Grankina, Russ. J. Inorg. (8) A Almenningen, G. Gundersen, and A. Haaland, Acta Chem. Scand, Chem., 15, 1490 (1970). 22, 328 (1968). (15) E. R. Bernstein and T. A. Keiderling, unpublished results (9) P. H. Bird and M. R. Churchill, Chem. Commun., 403 (1967) (16) Full details may be found in the Ph D dissertation of J. J Mayerle, (10) V. Plato and K. Hedberg, Inovg. Chem., 10, 590 (1971). Columbia University, 1972. 3010 Inorganic Chemistry, VoZ. 11, No. 12, 1972 HAMILTON,et al. structure was determined to be, tetragonal, space group neutron wavelength was 1.014 (1) b monochromatized by reflec- P4&2 (p41212), a = 7.49 (1) A, c = 13.24 (1) A, = tion from the (220) plane of a germanium single crystal. The 2 neutron flux at the U(BH4)Icrystal was approximately 5 x 106 4, Pcalcd = 2.66 g/cm3, uranium site symmetry 2. A cm-* sec-'. A total of 543 scans of Bragg reflections were made surprising feature of the structure revealed by the X-ray out to a scattering angle of 28 = 90" ((sin O)/h = 0.7). Reflections diffraction study is that each uranium atom is sur- were not scanned if a preliminary check indicated that the net rounded by six boron atoms, two of which (in a cis con- intensity at the peak position was less than the estimated stan- dard deviation based on counting statistics for a count of 20 sec. figuration) are 2.53 A from a single uranium atom and Background counting rates were about 400 cpm. four of which are approximately equidistant from tlyo All observable reflections with h, k, and 1 all positive, as well as uranium atoms with an average U-B distance of 2.87 A. a small number with h negative, were scanned. In the absence of These latter bridging tetrahydroborate groups serve to anomalous scattering, IF(hk1)l = lF(khZ)l for point group 422; link the uranium atoms in a polymeric structure, with but due to the anomalous scattering of boron, this condition does not obtain for this compound. The 543 reflections included 19 individual members of the interlocking helical chains measurements of (402) and (322) made at regular intervals. related by a fourfold screw axis. While no hydrogen These measurements showed there was no systematic intensity atoms could be located, it was guessed that the bridging change with time. The agreement among replicated measure- BH4- were symmetrically bonded to two uranium ments was consistent with Poisson counting statistics. The data were corrected for absorption using a Gaussian grid integration atoms by two-point attachments, as in Be(BH4)2,4and procedure and a linear absorption coefficient of 12.54 cm-1 that the terminal groups were also bonded through two (based on a cross section for pure absorption of 428 x 10-24 cm2 hydrogen atoms. As described in the present report, a for natural boron20 and 40 X cm2 for incoherent scattering neutron diffraction investigation confirms the geom- of hydrogen). Transmission coefficients ranged from 0.21 to 0.43. etry proposed for the bridging tetrahydroborate Equivalent reflections were averaged to produce 403 independent reflections, 80 of which had been observed more than once. The ligands, but shows the terminal BH4- groups to be co- agreement among these reflections was such that X/F12- FZ21/ ordinated by three-point attachment. ~'/Z(FI~f Fa2) = 0.153 and [2w(Fi2 - F22)2/(i/2)Xw(F~2+ F22)2]1/2= 0.081 with weights w based on counting statistics. Experimental Section These values are larger than typical for high-quality neutron Uranium borohydride was prepared according to the method of diffraction work but are indicative of the low intensities for this Schlesinger and Brown17 from uranium tetrafluoride (Research crystal.
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