Studies on peroxomolybdates XX. Crystal structure of ammonium dodecaoxodiperoxotetramolybdate(VI)-water(1/2), (NH4)4[Mo4O12(O2)2] · 2H2O1

The crystal structure of ammonium dodecaoxodiperoxotetramolybdate(VI)-water (1/2), (NH 4 ) 4 [Mo 4 O 12 (O 2 ) 2 ].2 H 2 O, has been determined from single-crystal X-ray diffraction data. The yellow compound crystallizes in the monoclinic space group P2 1 /m with a = 8.401(1) A, b = 8.819(1) A, c = 12.802(1) A, β = 100.01(1)°, V = 934.1(2) A 3 and Z = 2. Full matrix least-squares refinement of 179 structural parameters gave R = 0.031 (R w = 0.048) for 2161 observed [I > 3a(I)], independent reflections. The anions are disordered in the crystals but the disorder could be resolved. In the [Mo 4 O 12 (O 2 ) 2 ] 4- anion the four molybdenum atoms form almost a square and are six-coordinated in a distorted octahedral manner by oxygen atoms. Each peroxo group is coordinated to all four molybdenum atoms and each molybdenum atom is coordinated to two bridging oxy oxygen atoms, two bridging peroxo oxygen atoms and two terminal oxygen atoms. The anion has approximately D 2d symmetry and has a structure unusual for peroxometallates. Selected bond distances: Mo-O peroxo 2.233(3) A-2.247(3) A, Mo = O terminal 1.694(4) A1.740(4) A, Mo-O bridging 1.918(3) A-1.945(2) A and (O-O) peroxo 1.457(6) A-1.462(5) A. Mo... Mo is 3.2727(5) A, 3.2850(6) A and 3.2870(6) A.