On the structural dichotomy of cationic, anionic, and neutral FeS2

Structural and thermochemical aspects of the FeS2+ cation are examined by different mass spectrometric methods and ab initio calculations using density functional theory. Accurate threshold measurements provide thermochemical data for FeS+, FeS2+, and FeCS+, i.e., D-0(Fe+-S) = 3.06 +/- 0.06 eV, D-0(SFe+-S) = 3.59 +/- 0.12 eV, D-0(Fe+-S-2) = 2.31 +/- 0.12 eV, and D-0(Fe+-CS) = 2.40 +/- 0.12 eV. Fortunate circumstances allow a refinement of the data for FeS+ by means of ion/molecule equilibria, and the resulting D-0(Fe+-S) = 3.08 +/- 0.04 eV is among the most precisely known binding energies of transition-metal compounds. The present results agree with previous experimental findings and also corroborate the computed data far FeS+ and FeS2+. Bb initio calculations predict a sextet ground state ((6)A(1)) for FeS2+ with a cyclic structure. The presence of S-S and Fe-S bonds accounts for the fact that not only reactions involving the disulfur unit but also sulfur-atom transfer can occur. In contrast, the FeS2- anion is an acyclic iron disulfide. In the gas phase, neutral FeS2 may adopt either acyclic or cyclic structures, which are rather close in energy according to the calculations.