Preparation, X-ray crystal structure determination, lattice potential energy, and energetics of formation of the salt S-4(AsF6)(2)center dot ASF(3) containing the lattice-stabilized tetrasulfur [2+] cation. Implications for the understanding of the stability of M-4(2+) and M-2(+) (M = S, Se, and Te) crystalline salts

S-4(AsF6)(2). AsF3 was prepared by the reaction of sulfur with arsenic pent afluoride in liquid AsF3 (quantitatively) and in anhydrous HF in the presen ce of trace amounts of bromine. A single-crystal X-ray structure of the com pound has been determined: monoclinic, space group P2(1)/c, Z = 4, a = 7.88 6(1) Angstrom, b = 9.261(2) Angstrom, c = 19.191(3) Angstrom, beta = 92.82( 1)degrees, V = 1399.9(4) Angstrom(3), T = 293 K, R-1 = 0.052 for 1563 refle ctions (I > 2 sigma(I) 1580 total and 235 parameters). We report a term-by- term calculation of the lattice potential energy of this salt and also use our generalized equation, which estimates lattice energies to assist in pro bing the homopolyatomic cation thermochemistry in the solid and the gaseous states. We find S-4(ASF(6))(2). ASF(3) to be more stable (Delta(f)H degree s[S-4(AsF6)(2). AsF3,c] approximate to -4050 +/- 105 kJ/mol) than either th e unsolvated S-4(AsF6)(2) (Delta(f)H degrees[S-4(ASF(6))(2),c] approximate to -3104 +/- 117 kJ/ mel) by 144 kJ/mol or two moles of S2AsF6 (c) and AsF3 (1) by 362 kJ/mol. The greater stability of the S-4(2+) Salt arises becaus e of the greater lattice potential energy of the 1:2 solvated salt (1734 kJ /mol) relative to twice that of the 1:1 salt (2 x 541 = 1082 kJ/mol). The r elative lattice stabilization enthalpies of M-4(2+) ions relative to two M- 2(+) ions (i.e., in M-4(AsF6)(2) (c) With respect to two M2AsF6 (c) (M = S, Se, and Te)) are found to be 218, 289, and 365 kJ/mol, respectively. Evalu ation of the thermodynamic data implies that appropriate presently availabl e anions are unlikely to stabilize M-2(+) in the solid phase. A revised val ue for Delta(f)H degrees[Se-4(AsF6)(2),c] = -3182 +/- 106 kJ/mol is propose d based on estimates of the lattice energy of Se-4(AsF6)(2) (c) and a previ ously calculated gasphase dimerization energy of 2Se(2)(+) to Se-4(2+).