The characterisation and X-ray crystal structure of pentabromodiselenium hexafluoroarsenate, Se₂Br₅AsF₆; some thermodynamic considerations and the nonexistence of Se₂I₅AsF₆

Abstract

The crystal structure of Se2Br5AsF6 was determined by low-temperature X-ray-diffraction ~e t h o d s . Single crystals of Se2Br5AsF6 are rhombohedral, space group R3 with a = 13.367(4) A, c = 19.000(6) A, V = 2940 A~, and Z = 9. The structure was refined to final agreement indices of R = 0.087, R,, = 0.093 for 604 observed ( I > 3 4 1 ) ) reflections and 71 parameters.The structure consists of essentially discrete S e 2~r S + cations and AsF6-anions with some cation-anion interaction. The Se2BrsC cation, of essentially Czl, symmetry contains two trans SeBr, units, linked by a bridging bromine atom. The two terminal selenium-bromine bond distances are 2.291(7) and 2.268(6) A, of bond order 1, and the angle between them is 100.0(3)". The bridging bromine atom lies at the inversion centre of the SezBrS+ cation, with an Se-Br distance of 2.582(3) A, corresponding to a bond order of about 0.5. The two angles between the bridging Se-Br and terminal Se-Br bonds are 97.4(1)" and 98.9(2)". Se,BrSAsF6 decomposes slowly at room temperature and rapidly at 100°C leading to the formation of SeBr3AsF6, SeBr,, and elemental selenium. It reacts with Br2 to give SeBr3AsF6 and SeBr4. Se2BrSAsF6 is prepared by the reaction of Se4(AsF6)2 and the appropriate quantity of Br,. The analogous reaction with I2 leads to 12SeSeSeSeIz(AsF6)z and not Se215AsF6. These differences have been accounted for on the basis of estimates of the appropriate bond and crystal lattice energies. The 77Se nmr of SeBr3+ and Se2Br5 (4) and a-and P-SeBr ( 5 , 6) have been determined by X-ray crystallography, while SeBrz has been identified by Raman and uv-visible spectroscopy (7). The selenium-bromine anion, SeBr62-, has been shown to have an octahedral structure by infrared and Raman spectroscopy (8), and X-ray crystal structures of the novel anions S e~r~~-and Se2Br6'-(with [P(C6H5)4+]2 counterions) have been determined (9). The only binary selenium-bromine cation to be characterised fully is SeBr3+, prepared as the salt of the weakly basic AsF6-, SbF6-(lo), anions. The Raman spectra of SeBr3MF6 (M = As, Sb) (10) showed the SeBr3+ cation to have essentially C3" symmetry, with stretching frequencies and force constants somewhat higher in energy than those of the isoelectronic AsBr3 (12). More recently, the structure of

Quantum chemical modeling of the thermochemical characteristics and acidity of polynitroazole salts

“…the sum of anionic and cationic volumes). For MX salts (1:1), Bartlett's relationship is widely applicable to calculate enthalpies of formation [5][6][7][8][9][10][11][12]. The dependence of the lattice energy U POT of the M q X p salt from the molecular volume (V m ) can be expressed by the equation:…”

Preparation, structural and spectroscopic characterisation of the salts M3X3AF6(A = As or Sb, M = S or Se, X = Cl or Br) containing the novel sulfur– and selenium–halogen cations (X2MMMX)+

“…Attempts to synthesise the mixed-chalcogen cations (Br,Se-SSRr) and (Br,SeSSeBr)+ according to equations ( 5) and ( 6) were unsuccessful, but SBr,SbF, and selenium possibly reacted as shown in equation (7). * Assuming that these mixed chalco-2SBr,SbF, + 4Se soL(I1 Se,Br,SbF, + SeBr,SbF, + i s 8 (7) gen cations would adopt similar structures to those of the known M , X 3 + cations (M = S or Se, X = C1 or Br), the energetics of these and similar reactions leading to the possible cations (Rr,SeSSBr)+, (Br,SSeSeBr)+ and (Br,SeSSeBr)+ were investigaled, based on bond and atomic ionisation energies and ignoring lattice energies and entropy changes. The results of these estimates implied that the formation of (Br,SeSSBr) according to equation ( 5) is thermodynamically unfavourable by ca.…”

Synthese, Struktur und Eigenschaften der neuen Bromoselenate(II) [Se3Br8]2−, [Se4Br14]2− und [Se5Br12]2− Kristallstrukturen von [Cu(i‐PropCN)4]2[Se3Br8], [EtPh3P]2[Se4Br14] und [n‐Prop4N]2[Se5Br12]

“…8 und Tab. 9), liegt aber in der gleichen Gr6Denordnung wie im [Se,Br,]+-Kation des [Se,Br,] [A#,], das eine lineare Se-Br-Se-Bruckenbindung aufweist (2,582 A) [43]. Die Asymmetrie und die Abweichung des Bindungswinkels von 180" sind in ahnlichen Gr6J3enordnungen auch in den vergleichbaren Te-1-Te-Bindungen im isostrukturellen [Te4I,J2- [29, 441 und in den Y-X-Y' -Bindungen (X, Y Halogenatome) in Trihalogenid-Ionen zu beobachten (Tab.…”