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The characterisation and X-ray crystal structure of pentabromodiselenium hexafluoroarsenate, Se2Br5AsF6; some thermodynamic considerations and the nonexistence of Se2I5AsF6
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| Zeitschriftentitel: | Canadian Journal of Chemistry |
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| Personen und Körperschaften: | , , |
| In: | Canadian Journal of Chemistry, 65, 1987, 7, S. 1584-1593 |
| Format: | E-Article |
| Sprache: | Englisch |
| veröffentlicht: |
Canadian Science Publishing
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| Schlagwörter: |
| Zusammenfassung: | <jats:p> The crystal structure of Se<jats:sub>2</jats:sub>Br<jats:sub>5</jats:sub>AsF<jats:sub>6</jats:sub> was determined by low-temperature X-ray diffraction methods. Single crystals of Se<jats:sub>2</jats:sub>Br<jats:sub>5</jats:sub>AsF<jats:sub>6</jats:sub> are rhombohedral, space group R3 with a = 13.367(4) Å, c = 19.000(6) Å, V = 2940 Å<jats:sup>3</jats:sup>, and Z = 9. The structure was refined to final agreement indices of R = 0.087, R<jats:sub>w</jats:sub> = 0.093 for 604 observed (I > 3σ(I)) reflections and 71 parameters. The structure consists of essentially discrete Se<jats:sub>2</jats:sub>Br<jats:sub>5</jats:sub><jats:sup>+</jats:sup> cations and AsF<jats:sub>6</jats:sub><jats:sup>−</jats:sup> anions with some cation–anion interaction. The Se<jats:sub>2</jats:sub>Br<jats:sub>5</jats:sub><jats:sup>+</jats:sup> cation, of essentially C<jats:sub>2h</jats:sub> symmetry contains two trans SeBr<jats:sub>2</jats:sub> units, linked by a bridging bromine atom. The two terminal selenium–bromine bond distances are 2.291(7) and 2.268(6) Å, of bond order 1, and the angle between them is 100.0(3)°. The bridging bromine atom lies at the inversion centre of the Se<jats:sub>2</jats:sub>Br<jats:sub>5</jats:sub><jats:sup>+</jats:sup> cation, with an Se—Br distance of 2.582(3) Å, 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<jats:sub>2</jats:sub>Br<jats:sub>5</jats:sub>AsF<jats:sub>6</jats:sub> decomposes slowly at room temperature and rapidly at 100 °C leading to the formation of SeBr<jats:sub>3</jats:sub>AsF<jats:sub>6</jats:sub>, SeBr<jats:sub>4</jats:sub>, and elemental selenium. It reacts with Br<jats:sub>2</jats:sub> to give SeBr<jats:sub>3</jats:sub>AsF<jats:sub>6</jats:sub> and SeBr<jats:sub>4</jats:sub>. Se<jats:sub>2</jats:sub>Br<jats:sub>5</jats:sub>AsF<jats:sub>6</jats:sub> is prepared by the reaction of Se<jats:sub>4</jats:sub>(AsF<jats:sub>6</jats:sub>)<jats:sub>2</jats:sub> and the appropriate quantity of Br<jats:sub>2</jats:sub>. The analogous reaction with I<jats:sub>2</jats:sub> leads to I<jats:sub>2</jats:sub>SeSeSeSeI<jats:sub>2</jats:sub>(AsF<jats:sub>6</jats:sub>)<jats:sub>2</jats:sub> and not Se<jats:sub>2</jats:sub>I<jats:sub>5</jats:sub>AsF<jats:sub>6</jats:sub>. These differences have been accounted for on the basis of estimates of the appropriate bond and crystal lattice energies. The <jats:sup>77</jats:sup>Se nmr of SeBr<jats:sub>3</jats:sub><jats:sup>+</jats:sup> and Se<jats:sub>2</jats:sub>Br<jats:sub>5</jats:sub><jats:sup>+</jats:sup> in SO<jats:sub>2</jats:sub> solution, and the Raman spectrum of SeBr<jats:sub>4</jats:sub> are reported. </jats:p> |
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| Umfang: | 1584-1593 |
| ISSN: |
0008-4042
1480-3291 |
| DOI: | 10.1139/v87-266 |