Structure of octakis(methylammonium) decamolybdate(8-) dihydrate

[NH 3 (CH 3 )] 8 [Mo 10 O 34 ].2H 2 O, M r = 1795·9, monoclinic, P2 1 /n, a= 12·596(2), b=17·175(2), c = 10·653(2) A, β=9l·04(2)°, V=2304·2(6)A 3 Z = 2, D m = 2·56 gcm -3 (flotation). D x = 2·58 g cm -3 , Mo Kα, λ = 0·71069 A, μ= 11·38cm -1 , F(000) = 1728, T= 293 K, R = 0·041 for 4838 observed data. The discrete |Mo l0 O 34 | 8- anion has a center of symmetry and is built up of one Mo 8 0 28 , unit and two Mo0 4 tetrahedra joined to the unit at the corners. The Mo 8 0 28 unit is composed of right MoO 6 octahedra joined together by edges. The Mo-0 distances in Mo0 4 can be separated into three classes: 1·695 (4)- 1·768(4), 1·869(3)-2·011 (4) and 2·142(4)-2·433 (3) A, which is also observed in the related isopolymolybdate anions.