Investigation of anhydrous metal(II) nitrates. I. Syntheses and crystal structures of Mg(NO3)2, Co(NO3)2 and Ni(NO3)2, with a stereochemical discussion

Mg(NO 3 ) 2 , Co(NO 3 ) 2 and Ni(NO 3 ) 2 were obtained as tiny crystals by dehydration of melts of the respective hydrates in the range 410-450 K. Structure determinations and refinements are based on single crystal X-ray data (CCD area detector) measured at 120-370 K. Crystal parameters at 120K: Mg(NO 3 ) 2 : R3, a = 10.474(1), c = 38.782(4) A, Z = 36, R 1 = 0.028; Co(NO 3 ) 2 : R3, a=10.445(1), c = 12.769(2) A, Z= 12, R 1 = 0.019; Ni(NO 3 ) 2 : R3, a = 10.332(1), c = 12.658(2) A, Z= 12, R 1 = 0.025. Co(NO 3 ) 2 and Ni(NO 3 ) 2 are isotypic; the structure of Mg(NO 3 ) 2 is closely related, although it is not simply a stacking variant - it forms a new structure type. In all three compounds the MO 6 octahedra share all comers with the NO 3 groups to form framework structures; any edge sharing is thereby avoided. The geometries of the structures are also discussed in relation to a simple theoretical cubic structure (space group Pa3) with the same connection pattern of the MO 6 and NO 3 building units. Neither X-ray single crystal and powder work nor differential thermoanalytical investigations indicated polymorphism in the temperature range investigated.