PNNL

Abstract

High level electronic structure calculations were used to evaluate reliable, self-consistent thermochemical data sets for the third row transitionmetal hexafluorides. The electron affinities, heats of formation, first (MF6 ? MF5 + F) and average M-F bond dissociation energies, and fluoride affinities of MF6 (MF6 + F?? MF7 ?) and MF5 (MF5 + F?? MF6 ?) were calculated. The electron affinities which are a direct measure for the oxidizer strength increase monotonically from WF6 to AuF6, with PtF6 and AuF6 being extremely powerful oxidizers. The inclusion of spin orbit corrections is necessary to obtain the correct qualitative order for the electron affinities. The calculated electron affinities increase with increasing atomic number, are in good agreement with the available experimental values, and are as follows: WF6 (3.15 eV), ReF6 (4.58 eV), OsF6 (5.92 eV), IrF6 (5.99 eV), PtF6 (7.09 eV), and AuF6 (8.20 eV). A wide range of density functional theory exchange-correlation functionals were also evaluated, and only three gave satisfactory results. The corresponding pentafluorides are extremely strong Lewis acids, with OsF5, IrF5, PtF5, and AuF5 significantly exceeding the acidity of SbF5. The optimized geometries of the corresponding MF7? anions for W through Ir are classical MF7? anions with M-F bonds; however, for PtF7? and AuF7? non-classical anions were found with a very weak external F-F bond between an MF6? fragment and a fluorine atom. These two anions are text book examples for “superhalogens” and can serve as F atom sources under very mild conditions, explaining the ability of PtF6 to convert NF3 to NF4?, ClF5 to ClF6?, and Xe to XeF? and why Bartlett failed to observe XePtF6 as the reaction product of the PtF6/Xe reaction.