Space groups P1 and Cc: how are they doing?

A survey of recent entries in the Cambridge Structural Database suggests that the percentage of structures described in space groups of inappropriately low symmetry has decreased from about 10% in the early 2000s to less than 5% today for space group Cc, but that for space group P1 it remains close to 8%

1,6-Diazabicyclo[4.4.4]tetradecane and its lnside-protonated lon: Changes in Space Groups

The crystal structures of 1,6-diazabicyclo[4.4.4]tetradecane (1) and of its inside-protonated ion (2), which were originally described in space group C2/c, should be described instead in trigonal space groups, (1) in R3c and (2) in P31c; both moieties have crystallographically imposed point symmetry D_3 rather than C_2

A revised structure for α-dicalcium silicate hydrate

The structure of α-dicalcium silicate hydrate, Ca_2(SiO_3OH)(OH), has recently been described and refined [Y ano, Urabe, Ikawa, Teraushi, Ishizawa & Udagawa (1993). Acta Cryst. C49, 1555-1559] in space group P2_12_12_1 (orthorhombic; a = 9.487 (4), b = 9.179 (4), c = 10.666 (7) Å, Z = 8). It is better described in Pbca. Revised coordinates are given

On the structure of 9-amino-2,7-di-tert-butylacridine

The crystal structure of 9-amino-2,7-di-tert-butylacridine hydrochloride monohydrate, C_(21)H_(27)N_2^+.Cl^-.H_2O, has been described in space group P3_1 [trigonal, a = 11.086 (2), c = 14.367 (4) Å, Z = 3; McKenna, Agbandje & Neidle (1992). Acta Cryst. C48, 188-190]. It is properly described in space group P3_121. Further refinement in P3_121, based on the original F values, leads to an improved R (0.049 vs 0.067) and to a revised position for the Cl^- ion

On the structure of 'potassium nitroprusside 0·8 hydrate'

The crystal structure of the compound reported as K_2[Fe(CN)_5NO].0·8H_2O, potassium pentacyanonitrosylferrate hydrate (1/0·8), recently described and refined in the non-centrosymmetric space group Pna2_1 [orthorhombic; a = 30·000 (8), b = 11·272 (3), c = 16·053 (4) Å, Z = 20; Castellano, Rivero, Piro & Amalvy (1989). Acta Cryst. C45, 1207-1210], is properly described as centrosymmetric, space group Pnma. Refinement in Pnma shows that one of the K atoms is, instead, an Na atom and that three of the sites of the water molecules are partially occupied; the proper formulation of the compound is K_9Na[Fe(CN)_5NO]_5.nH_2O (n = 4·8)

The importance of weak reflections in resolving the centrosymmetric-noncentrosymmetric ambiguity: a cautionary tale

The weak diffraction intensities are crucially important in determining whether a crystal structure has a real or only an approximate, center of symmetry; if these intensities are deleted from the data set, an intelligent conclusion may be impossible. In addition, statistical distribution tests, if based only on the stronger intensities, may be strongly biased toward a noncentrosymmetric indication. In one recently published structure determination [Cotton & Fanwick (1980), Acta Cryst. B36, 457-459] a distribution test led to the assignment of the noncentrosymmetric space group Cc to a structure which can be better described and refined in the centrosymmetric space group C2/c

Is There Evidence for Non-Linear Thiocyanate or Selenocyanate Ions?

The structure of a mixed-valence copper complex with 1,10-phenanthroline, [Cu^(II)(phen)_2]-NC-[Cu^I(phen)((Se,S)CN)]- [(Se,S)CN].0.5(C_2H_5OH), µ-cyano-1κN:2κC-(seleno,thio)cyanato-2κN-tris(1,10-phenanthroline)-1κ^4N,N';2κ^2N,N'-dicopper(I,II) (seleno,thio)cyanate semiethanolate, has been revised from space group Cc to C2/c. Refinement in C2/c has shown that, contrary to the Cc results, the (Se,S)CN anions can be described as approximately linear, as expected

The centrosymmetric–noncentrosymmetric ambiguity: some more examples

Four more examples are provided to emphasize the extreme difficulty in deciding, by diffraction methods, whether a crystal structure is centrosymmetric or only approximately so. In these examples, earlier workers described and refined structures in noncentrosymmetric space groups; refinements in the corresponding centrosymmetric space groups, based on the original data, lead to improved results. In one case, apparent violations of systematic absences seem to preclude the centrosymmetric description; however, other evidence - in particular, improved agreement for the very weak reflections (which are the most sensitive to the centrosymmetric non-centrosymmetric ambiguity) - suggest that the space-group violations might be spurious. In any event, the moral is clear: extreme caution is needed when attempting to derive a noncentrosymmetric description of a closely centrosymmetric structure

On the crystal structure of bis(dipicolinato)ferrate(III) dihydrate

The structure of [H_5O_2]+[Fe(C_7H_3NO_4)_2]^- was recently described and refined [Cousson, Nectoux & Rizkalla (1992). Acta Cryst. C48, 1354-1357] in space group Pnn2 [orthorhombic, a= 8.860 (8), b = 11.007 (2), c = 16.316 (4) Å, Z = 4]; it is better described in Pnna. Revised coordinates are given