M-F interatomic distances and effective volumes of second and third transition series MF₆^- and MF₆^2- anions

Synchrotron X-ray powder diffraction data (SPDD) for representative LiMF₆ and Li₂MF₆ salts of the second and third transition series have provided unit-cell parameters and, from Rietveld analysis, M-F interatomic distances. M-F distances have also been obtained from X-ray single-crystal structural analyses of LiOsF₆, Li₂PtF₆, and KRhF₆. The LiMF₆ all have the LiSbF₆ structure type (space group R3). For M = Ta to Au the primitive unit cell volume decreases with increasing nuclear charge (Z), the volumes (σ = 0.01 A³) being as follows: Ta, 111.26; Os, 102.42; Ir, 100.77; Pt, 99.62; and Au, 99.12 A³. A similar contraction, with increase in Z, occurs from Nb to Rh, the primitive cell volume (σ = 0.01 A³) being: Nb, 110.92; Ru, 100.51; and Rh, 98.64 A³. For the TaF₆^- to AuF₆^- the M-F distances are not significantly different across the series, at ~1.87(1) A; also, Nb-F, Ru-F, and Rh-F = 1.86(1) A. In each series, the a and c values of the hexagonal-cell representation for the LiMF₆ structure (separate layers of MF₆^- and Li⁺ stacked along c) change smoothly. As Z increases, a decreases and c increases. The variation in a, like the volume change, indicates that the size of MF₆^- is decreasing with Z. The variation in c suggests that the charge on the F-ligand is decreasing with Z. In the trirutile Li₂MF₆ series, M = Mo to Pd, the formula-unit volume decreases with Z (Mo, 100.92(6); Ru, 98.21(1); Rh, 97.43(1); Pd, 96.83(1) A³) and a shortening in M-F occurs (Mo-F = 1.936(4); Ru-F = 1.921(7); Rh-F = 1.910(7); Pd-F = 1.899(4) A). The less abundant data for MF₆^2- salts of the third transition series indicate similar trends. For both series, M-F ditances of MF₆^2- are longer by 0.03-0.09 A than in MF₆^-.