TIME-OF-FLIGHT NEUTRON POWDER DIFFRACTION STUDY ON THE 3RD-ROW TRANSITION-METAL HEXAFLUORIDES WF6, OSF6, AND PTF6

A neutron diffraction study on the third-row transition metal hexafluo rides MF(6) (M=W, Os, Pt) has been performed using the high resolution neutron powder diffractometer (HRPD) at the spallation source ISIS, E ngland. The previously unknown structures of the low-temperature phase s of OsF6 and PtF6 are reported. WF6, OsF6, and PtF6, which exhibit a (5dt(2g))(0), (5dt(2g))2, and (5dt(2g)>(4) electronic configuration, r espectively, are found to be isostructural and crystallize in the UF6 structure, space group Pmnb, (No. 62). The geometry of the MF(6) molec ules is to good approximation octahedral for each compound, the mean M -F bond length increasing only slightly from 182.5 (W) to 185.0 (Pt). For WF6 deviations from ideal octahedral geometry are only marginally significant [181.8(2) to 183.2(2) pm] and may be interpreted on the ba sis of packing effects. Deviations for the d(2) complex OsF6, are some what larger [181.5(2) to 184.4(3) pm] and may be assumed to be caused by packing effects essentially the same as for WF6, in addition to a f irst-order Jahn-Teller effect arising from the (5dr(2g))(2) electronic configuration. While eliminating the effects of packing by a comparis on of individual M-F bond lengths for WF6 and OsF6, the OsF6 molecule shows to have D-4h symmetry with two apical M-F bonds about 1.8 pm lon ger than the four equatorial bonds as a result of the Jahn-Teller dist ortion. Only small deviations from ideal octahedral geometry [184.4(3) to 185.8(3) pm] are found for the d(4) complex PtF6. Within the serie s W to Pt a substantial shortening of the F ... F van der Waals contac t distances is observed. This shortening more than compensates for the increase in the M-F bond lengths and leads to unit cell volumes and c ell parameters decreasing continuously from W to Pt. The variation of F ... F contact distances and M-F bond lengths may be rationalized in terms of polarization of the F-ligands in the field of the highly char ged nuclei of the central atoms which are only incompletely shielded b y the 5d electrons. (C) 1996 American Institute of Physics.