Computational Gas-Phase Formation Enthalpy and Electron Affinity for Platinum Hexafluoride: Is Gaseous PtF₆...

In the present work, a computational thermochemistry study (semiempirical, PM6 method) for platinum hexafluoride (PtF₆) is performed. The gas-phase formation enthalpy (Δ_fH°) and electron affinity are calculated and the results compared with the experimental data. A calculated Δ_fH°(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 PtF₆ molecule with no unpaired electrons (diamagnetic) was modeled. Such a fact is in contrast with the four-unpaired-electron configuration (t_2g³e_g¹) generally accepted for gaseous (and solid) PtF₆ but is in agreement with the fact (based on quantum relativistic calculations) that the triplet state t_2g is split in the tetragonal field into a singlet and a lower-lying doublet, with four 5d⁴ electrons occupying the 5d_xz and 5d_yz atomic 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^-1 mol^-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 PtF₆ is 8 and the fluoride has a polymeric nature.