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Selenium Fluorides

Two fluorides of selenium have been described, the hexafluoride, SeF6, and the tetrafluoride, SeF4. The former was obtained as a stable gas by the action of fluorine on selenium at 78° C. Its boiling-point, melting-point and critical temperature are -34.5° C. (in a sealed tube), -39° C. and +72° C., respectively. The vapour density, 97.23, agrees with the formula SeF6. The gas does not attack glass.

Using a copper containing-vessel for the selenium, Lebeau obtained a colourless liquid which he analysed and concluded to be selenium tetrafluoride. By the same method, however, Lebeau later obtained a gaseous product the composition of which approximated to SeF6 but which contained selenium in slight excess of the theoretical amount. Doubt was therefore expressed as to the exact nature of these products, and Prideaux has shown that the liquid product described by Lebeau somewhat resembles in properties the oxyfluoride SeOF2, or the mixture SeO2 + 2HF. The existence of the hexafluoride was subsequently acknowledged by Lebeau, and that of the tetrafluoride has recently been established by Prideaux, who has prepared it by the interaction of selenium tetrachloride and silver fluoride. A mixture of these compounds (1 part SeCl4 to 4.5 parts AgF) was heated in a platinum bottle to 50° to 60° C. The reaction proceeded vigorously and selenium tetrafluoride distilled over as a colourless fuming liquid of density 2.77 and boiling-point 93° C. It could be readily condensed to a solid, which melted at -13.2° C. The liquid readily attacked glass, was hydrolysed by water with formation of selenious and hydrofluoric acids, heat being evolved, and reacted with red phosphorus, gaseous fluorides of phosphorus, selenium and selenium dioxide being produced. It should be observed that the samples of tetrafluoride examined by Prideaux contained some oxyfluoride and the latter reacts in a similar manner towards the reagents mentioned, as also to some extent does the mixture of selenium dioxide and hydrofluoric acid, SeO2 + 4HF. Further investigation is therefore desirable.

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