title: Computational Gas-Phase Formation Enthalpy and Electron Affinity for Platinum Hexafluoride: Is Gaseous PtF6Diamagnetic because of a Relativistic Effect?
abstract: In the present work, a computational thermochemistry study (semiempirical, PM6 method) for platinum hexafluoride (PtF6) is performed. The gas-phase formation enthalpy (ΔfH°) and electron affinity are calculated and the results compared with the experimental data. A calculated ΔfH°(g) value of -669.47 kJ mol-1, in very good agreement with the experimental data from the literature, was obtained by Knudsen cell mass spectrometry. However, such a value was obtained when a PtF6molecule with no unpaired electrons (diamagnetic) was modeled. Such a fact is in contrast with the four-unpaired-electron configuration (t2g3eg1) generally accepted for gaseous (and solid) PtF6but is in agreement with the fact (based on quantum relativistic calculations) that the triplet state t2gis split in the tetragonal field into a singlet and a lower-lying doublet, with four 5d4electrons occupying the 5dxzand 5dyzatomic orbitals in the spin paired state. The modeled compound exhibits a distorted structure and a dipole moment of 0.30 D. The calculated electron affinity is 7.6 eV, in very good agreement with the experimental and calculated data. The computed zero-point energy, G°, H°, and S° values for such a structure are 34.71 kJ mol-1, -725.59 kJ mol-1, -612.11 kJ mol-1, and 380.31 J K-1mol-1. The positive S° and negative G° and H° values show that, from a thermodynamic point of view, the structure it stable. A working hypothesis is proposed in which the coordination number in PtF6is 8 and the fluoride has a polymeric nature.
authors.affiliation: Universidade Federal do Rio Grande do Norte . Cx. Postal 1664, 59078-970 Natal-Rio Grande do Norte, Brazil.
authors.forename: Robson
authors.initials: R
authors.lastname: Fernandes De Farias