Metformindiium dibromide

The crystal structure of the title salt, C₄H₁₃N₅ ²⁺·2Br^-, is reported in space group P2₁/c, with Z = 4. Unexpectedly, this crystal is not isomorphous to the dichloride analogue, which has been reported in space group P2₁ (Z = 2). This originates from the shift of one halide anion in the asymmetric unit, which modifies the network of N-H⋯halide hydrogen bonds. For the here-reported structure, centrosymmetric R ₄ ²(8) ring motifs are formed, affording a one-dimensional structure along [100], and the network is further expanded to a three-dimensional structure including larger ring motifs. This report is an example of a routine refinement carried out using now well-established techniques of quantum crystallography.