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 (sigma = 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 (sigma = 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 approximately 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 distances of MF6(2-) are longer by 0.03-0.09 A than in MF6-.