M-F interatomic distances and effective volumes of second and third transition series MF6- and MF62- anions

Synchrotron X-ray powder diffraction data (SPDD) for representative LiMF6 a nd Li2MF6 salts of the second and third transition series have provided uni t-cell parameters and, from Rietveld analysis, M-F interatomic distances. M -F distances have also been obtained from X-ray single-crystal structural a nalyses of LiOsF6, Li2PtF6, and KRhF6. The LiMF6 all have the LiSbF6 struct ure type (space group R (3) over bar). For M = Ta to Au the primitive unit cell volume decreases with increasing nuclear charge (Z), the volumes (sigm a = 0.01 Angstrom(3)) being as follows: Ta, 111.26; Os, 102.42; Ir, 100.77; Pt, 99.62; and Au, 99.12 Angstrom(3). A similar contraction, with increase in Z, occurs from Nh to Rh, the primitive cell volume (sigma = 0.01 Angstr om(3)) being: Nb, 110.92; Ru, 100.51; and Rh, 98.63 Angstrom(3). For the Ta F6- to AuF6- the M-F distances are not significantly different across the s eries, at similar to 1.87(1) Angstrom; also, Nb-F, Ru-F, and Rh-F = 1.86(1) Angstrom. In each series, the a and c values of the hexagonal-cell represe ntation for the LiMF6 structure (separate layers of MF6- and Li+ stacked al ong c) change smoothly. As Z increases, a decreases and c increases. The va riation in a, like the volume change, indicates that the size of MF6- is de creasing with Z. The variation in c suggests that the charge on the F-ligan d is decreasing with Z. In the trirutile Li2MF6 series, M = Mo to Pd, the f ormula-unit volume decreases with Z (Mo, 100.92(6); Ru, 98.21(1); Ph, 97.43 (1); Pd, 96.83(1) Angstrom(3)) 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) Angstrom). The less abundant data for MF62- salts of the third transition series indicate simil ar trends. For both series, M-F distances of MF62- are longer by 0.03-0.09 Angstrom than in MF6-.