A twinned triclinic polymorph of dibromidotetra­kis(tetra­hydro­furan-[kappa]O)magnesium(II)

The title compound, [MgBr₂(C₄H₈O)₄], forms crystals which appear to be monoclinic but are actually twinned triclinic. The current form is a new triclinic polymorph, with Z'= 2, in addition to the already known tetra­gonal polymorph. Although the geometric parameters of the two polymorphs agree well, their packing patterns are completely different.

Setting ambiguity in C2/c with dibromidotetra­kis(1H-pyrazole-κN²)manganese(II) as an example

trans-Dibromidotetra­kis(1H-pyrazole-κN³)manganese(II), [MnBr₂(C₃H₄N₂)₄], crystallizes in the C2/c space group with the Mn atom located on a centre of inversion. As a result, there is just one half-mol­ecule in the asymmetric unit. Geometric parameters are in the usual ranges. The Mn centre is octa­hedrally coordinated by four pyrazole residues in the equatorial plane and by two bromide ligands in the axial positions. The mol­ecular conformation is stabilized by N—H...Br hydrogen bonds. The structure of the title compound had already been described [Lumme & Lindell (1987). J. Coord. Chem. 15, 383–392] in a different setting, with the Mn atoms located on inversion centres on Wyckoff position d (, , ; , , 0; , , ; , , 0). In the conventional setting, however, the Mn atoms are situated on Wyckoff position a (0, 0, 0; 0, 0, ; , , 0; , , ). In this special case, if the c axis has the same length as the short diagonal of the ac plane, the transformation from one setting into the other yields almost indistinguishable cell parameters, and the possibility of confusion arises. This setting ambiguity could be the reason why two structures in different settings might be taken as polymorphs even though they can easily be transformed. As a result of this, care should always be taken to use the conventional setting.

A new polymorph of tetra­phenyl­diboroxane

A new polymorph of tetra­phenyl­diboroxane [or oxybis(diphenyl­borane)], C₂₄H₂₀B₂O, (Ia), has been found. It is monoclinic, like the already known form, (Ib), and can be refined in the same space group, namely P2₁/c, or in the equivalent setting P2₁/n. The mol­ecular conformations of the two polymorphs differ in the rotations of two of the phenyl rings about the B-C bonds, leading to markedly different packing patterns and cell dimensions.