The structure of tricesium octabromodimolybdate: an example of a space-group ambiguity

Abstract

When tetraacetatodimolybdenum is dissolved in aqueous hydrobromic acid and a solution of CsBr is added, a precipitate of microcrystalline CssMozBr8 is formed. By a special technique, crystallographically useful single crystals were obtained and the substance has been structurally characterized. The structure consists of Cs + cations and MozBr 3-anions, the latter having the shape of an MzX9 confacial bioctahedron but with crystallographically disordered vacancies in the bridging positions. The dimensions of the Mo2Br~-ion indicate that there is bonding interaction of medium strength between the molybdenum atoms. The crystals belong to the hexagonal system, with space groups P~2c, P63/mmc and P63mc admissible on the basis of systematic absences. P6smc was eliminated convincingly on the basis of unsatisfactory refinement, but no clear choice between the other two space groups was possible using diffraction data. The unit-cellj~parameters are a= 7.757 (3), c= 17.804 (7)A, Z= 2; dcale =4"40; doh~=4"33 g cm -3. The Mo-Mo distance is 2.439 (7); Mo-Br(terminal)=2.554 (3); Mo-Br(bridge)= 2.672 (5) A.

Structural studies of some chromium and molybdenum(III) coordination compounds with 4‐methylpyridine

“…Numbering scheme of MoBr,N, octahedron and in the 4-methylpyridine ringsMo-Br bond lengths in MoBr,Pic, are comparable with the literature data. Mo-Br terminal bond lengths in Cs,Mo,Br, and Cs3M02Br,H are both 2,544 (3)A[13,14]. Bonding distancesinMoBr,arefrom 2.53to 2.57.& [15].…”

Preparation of the new heteronuclear hydride-bridged anion MoWCl8H3- and determination of the metal-hydrogen bond distance in Mo2X8H3- (X = chlorine, bromine) by infrared spectroscopy

“…Subsequent work, however, elucidated the true formulation of these salts as M3Mo2Cl8H containing the ¿i-hydrido-di-jU-chloro-hexachlorodimolybdate(III) anions,3 and the corresponding salt Cs3Mo2Br8H was also reported. 3,4 In the later study the reactions of both the hydride and deuteride derivatives with aqueous acid, the infrared spectra and the magnetic susceptibilites all were found to be consistent with a structure of the anions having a H atom as one of the bridging ligands in the confacial bioctahedral arrangement.…”

“…Comparison of the x-raypowder diffraction photographs of Cs3Mo2Cl8H and Cs3MoWClgH clearly shows that the compounds are isomorphous. From x-raypowder data (Guinier camera, Cu Ka, radiation, 1.54050 Á, and film correction with the lines of elemental Si) the following d spacings (Á) were observed (relative intensities in parentheses): 8.60 (4), 6.46 (5), 6.03 (7), 5.06 (3), 4.27 (6), 3.70 (10), 3.56 (3), 3.44 (3), 3.22 (3), 3.02 (7), 2.854 (8), 2.798 (3), 2.570 (6), 2.400 (2), 2.343 (3), 2.284 (2), 2.232 (4), 2.142 (5), 2.028 (3), 1.981 (4), 1.943 (3), 1.850 (7), 1.780 (4), 1.718 (3), 1.595 (2), 1.510 (2), 1.369 (1). From these d spacings the hexagonal lattice parameters a = 7.416 (3) and c = 17.106 (6) Á were computed.…”

Preparative and structural studies of halodimolybdates(II). I. Preparation and crystal structure of (picH)2Mo2Br6(H2O)2

“…21], Cs,Mo,Br, 2.544(3) 8[22] and MoBr, 2.53-2.57 8[23] are shorter. Mo-Br2 bond lengths is shorter for steric reasons.…”