The structure of the hexamolybdoperiodate anion in its potassium salt

Abstract

Ks[IMo6024]. 5H20, P i , a = 10.084 (1), b = 17.593(2), c = 7.692(1) A, a = 95.07(1), fl = 91.38 (1), y = 82.45 (1) °, U = 1347-5 (2) A 3, FW = 1372.10, Z = 2, D m = 3.37, D x = 3.42 Mg m -3, a = 4.375 mm -1 (for Mo Ka). The [IMo6024] 5anion has been found to be a typical Anderson-type heteropolyanion and has approximate 3m symmetry. The meta l oxygen bond lengths in this anion and in [TeMo6024] 6show the difference between the 17+-O and Te6+-O bond strengths. 0567-7408/80/030661-04501.00 Introduction. Many Keggin-type heteropolyanions have been studied; there is also another group of heteropolyanions called the Anderson type (Anderson, 1937). The latter type of polyanion consists of seven octahedra sharing edges with one another. Six Mo octahedra are arranged hexagonally around the octahedron containing the heteroatom. A number of examples of the preparations of Anderson-type polyanions have been reported (Gmelins Handbuch der Anorganischen Chemic, © 1980 International Union of Crystallography 662 THE HEXAMOLYBDOPERIODATE ANION IN ITS POTASSIUM SALT 1935), but fewer structural studies have been carried out (Evans, 1974; Perloff, 1970). The present structure was originally proposed by Anderson (1937) for this [IMo6024] 5anion. This paper reports its structure determination and the structure is compared with that of [TeMotO24 ]6-. Preparations of the periodatomolybdates have been described by Blomstrand (1892). The intensities of the reflections were measured with a Rigaku automatic four-circle diffractometer (graphite monochromator, Mo Ka radiation, w--20 scan technique) up to 20 = 65 ° . The intensities of 6627 observed independent reflections, with IFol > 3tr(IFol), were used for the structure determination. The dimensions of the crystal were 0.3 × 0-1 x 0.05 mm. The structure was solved by the heavy-atom method, except for the H atoms, and refined by the block-diagonal least-squares method. Calculations were performed with a local version of UNICS (Sakurai, 1967) on a HITAC 8800/8700 computer at the Computer Centre of the University of Tokyo. Anisotropic temperature factors were used for all the non-hydrogen atoms. Atomic scattering factors and corrections for anomalous scattering were taken from International Tables for X-ray Crystallography (1974). No absorption correction was made. The function minimized was ~ w(IFol IFcl) 2, where w = l/(a + IFol + clFo 12) with a = 43.03 and c = 0.00469 estimated experimentally according to Cruickshank (1970), for IFol > 21.0, and w = 0.122 for others. The final R values are Rw(F) = 0.050 and R(F) = 0.047, where R w ( F ) --~ wilFol -IFcll/~ WlFol and R ( F ) --X IIFolIF<ll/X iFoi. Atomic fractional coordinates and averaged bond distances and angles are listed in Tables 1 and 2.* Table 1. Fractional coordinates (x 104)