THERMOCHEMISTRY OF TETRACHALCOGEN-BIS-HEXAFLUOROARSENATE SALTS M(4)(ASF(6))(2) (M=SE OR TE) CONTAINING LATTICE-STABILIZED CYCLIC M(4)(2+) CATIONS

The massic energies of combustion of the tetrachalcogen-bis-hexafluoro arsenates of selenium and tellurium in fluorine: Se-4(AsF6)(2)(cr) + 1 1F(2)(g) = 4SeF(6)(g) + 2AsF(5)(g) Te-4(AsF6)(2)(cr) + 11F(2)(g) = 4Te F(6)(g) + 2AsF(5)(g) have been determined by combustion calorimetry, a nd from them the standard molar enthalpies of formation have been deri ved at T = 298.15K: Delta(f)H(m)(o){Se-4(AsF6)(2)} = -(3092.5 +/- 5.0) kJ . mol(-1); Delta(f)H(m)(o){Te-4(AsF6)(2)} = -(3353.6 +/- 10.6)kJ . mol(-1), where the uncertainties correspond to twice the standard devi ation of the mean. The enthalpy of the reaction: M(4)(AsF6)(2)(cr) = 2 M(2)AsF(6)(cr) is estimated to be positive for M = Te, which indicates that Te-4(2+) is lattice stabilized in the solid state. For the react ion with M = Se, the uncertainty is very much larger than the enthalpy of reaction, so the thermochemical information allows no firm conclus ion to be drawn about the stability of Se-4(2+). (C) 1996 Academic Pre ss Limited