Structure of the tetrahydrogentetramolybdotetraarsenate(V)(4–) polyanion

Abstract

Na4[H4As4Mo4026].6H20, triclinic, P i , a ---10 .136(6) , b = 10 .697(8) , c = 7. 744 (5) /~, a = 111.68 (6), fl = 85.06 (6), y = 118.77 (5) ° , U = 679.6 ( 6 ) A a, F W 1303.5, Z = 1, D, , = 3.18, D x = 3.19 Mg m -3, /~ = 7.065 mm -1 (for Mo Ka); R = 0 .051, R~ = 0 .062 for 3402 reflections. The structure contains the discrete heteropoiyanion [H4As4Mo4026] 4which consists of four M o O 6 octahedra forming a pair of Mo2010 groups and four AsO4 te t rahedra bridging the groups with corner sharing. Protonat ion at the four terminal O a toms of AsO4 is observed. Introduction. A preparat ive description of heteropoly Mo(1) complexes with the ratio A s : M o = : was reported Mo(2) as early as 1892 (Friedheim & Mach, 1892). Later , As(l) As(2) Contan t (1973) proposed the existence of the 1 Na(l) heteropolyanion only in a strongly acidified solution Na(2) containing a large excess of H A s O 2-, but its structure o(1) has been entirely unknown. 0(2) 0(3) The sodium salt, Na4[n4As4Mo4026] . 6H20 , crystal0(4) lized as colourless triclinic parallelepipeds from an 0(5) aqueous solution (pH = 2) of the stoichiometric 0(6) quantities of Na2MoO4 . 2 H 2 0 and Na2HAsO4 . 7 H 2 0 0(7) on the addition of NaC1. o(8) o(9) All the Xray diffraction intensities were measured 0(10) on a Philips automat ic four-circle diffractometer in the O(11) t~-20 scan mode using graphi te -monochromated Mo O(12) Ka radiation. The intensities were corrected for the O(13) Aq(1) absorption effect and 3402 independent reflections [6 ° Aq(2) 3cr(IFol)] were used. The Aq(3) dimensions of the crystal were 0 .28 x 0-20 × 0 .10 mm. The structure was solved by the usual heavy-a tom method. Atomic scattering factors and corrections for anomalous scattering were taken from International Tables for X-ray Crystallography (1974). Anisotropic temperature factors were applied to all the nonhydrogen atoms. Refinements were made by the block-diagonal least-squares method. The function minimized was ~. w(IF o I I Fcl) 2, where w = 02 for IFol < 15 and w = 1/[cr(IFol) 2 + ClFo 121 otherwise. The parameter c was estimated to be 0 .02 from the * Present address: Sumitomo Chemicals, Kitahama, Osaka 541, Japan. 0567-7408/82/010242-03501.00 fluctuation of the intensities of the s tandard reflections during the da ta collection and t~(IFol) was from counting statistics. The final R and R~, values were 0.051 and 0 .062, respectively, where R w = ~ wlIFo I IFcII/~ WIFol. All the calculations were performed on Table 1. Fractional coordinates (× 104) and equivalent isotropic thermalparameters (/~2 x 103)