Infrared spectra of UF₆, WF₆, MoF₆, and SF₆ complexes with hydrogen fluoride in solid argon

UF₆, WF₆, MoF₆, and SF₆ have been codeposited with hydrogen fluoride in excess argon at 12 K. The FTIR spectra of UF₆ and HF revealed a strong UF₆-HF complex absorption at 3848 cm^-1 along with a weak, broad band at 3903 cm^-1 due to the UF₆-FH complex. Only one 1:2 complex with a UF₆-H_aF-H_bF arrangement was detected at higher HF concentrations and sample annealings. Similarly, the HF interaction with tungsten hexafluoride formed two 1:1 complexes. However, the 3884-cm^-1 band due to the anti-hydrogen-bonded complex WF₆-FH was considerably stronger than the WF₆-HF absorption at 3911 cm^-1. Increasing the HF concentration produced a single 1:2 complex with a WF₆-FH_b-FH_a structure. The band positions and relative intensities for the MoF₆ complexes with HF and DF were very comparable to their WF₆ counterparts. This change in the dominant binding interaction for these HF-metal hexafluoride complexes was anticipated due to the significant difference in the fluoride affinities of UF₆ and WF₆ (46 and 69 kcal/mol, respectively). In sharp contrast to the metal hexafluorides, SF₆ and HF produced a weak triplet at 3905, 3903, and 3901 cm^-1 and a strong doublet at 3819 and 3818 cm^-1, which are tentatively assigned to the SF₆-HF and SF₆-H_aF-H_bF structures, respectively. Finally, 1:3 and 1:4 complexes were observed with each hexafluoride, and the structures of these HF complexes are determined by the cyclic nature of the HF polymers.