Abstract
The low temperature crystal structure of the complex, Cr(urea)4(H2O)2·3NO3 (1), displays intriguing pseudosymmetry that complicates the structure determination. 1 crystallises in the centrosymmetric monoclinic space group P21/c with Z = 8 and Z′ = 2; a = 18.3338(4) A, b = 16.5472(4) A, c = 13.9252(3) A and β = 106.3260(10)°, V = 4054.19(16) A3. The two symmetry independent [Cr(urea)4(H2O)2]3+ ions in the asymmetric unit both feature coordinated urea (O=C(NH2)2) and lie on general positions. A further six independent nitrate anions complete the structure. The unit cell is primitive but the two Cr3+ ions are related by a pseudo translation of ½, ½, 0. The deviation from a truly C-centred cell is clear from an examination of geometry of the urea ligands and the hydrogen bonds which they form. The identification of the space group in this case is described and methods for quantifying the degree of deviation from a C-centred cell discussed. Cr(urea)4(H2O)2·3NO3 adopts a primitive unit cell with Z′ = 2 and is pseudo C-centred; small differences in the orientations of the ligands are responsible for the removal of the C-centring.
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