The title compound, C5H5Cl5N5P3, crystallizes as a zwitterion in which the pyridine N atom is protonated. An S(6) ring motif is formed via an intra­molecular C—H⋯N hydrogen bond. The triaza­triphosphinine ring adopts an envelope conformation, with one N atom displaced by 0.145 (1) Å from the other atoms. In the crystal, N—H⋯N and C—H⋯N hydrogen bonds link the mol­ecules into centrosymmetric dimers containing one R 2 2(7) ring motif and two R 2 2(8) ring motifs.

In the title salt, C19H15N4 +·Br−, the central imidazole ring makes dihedral angles of 83.1 (2) and 87.6 (2)° with the terminal benzene rings. The dihedral angle between the terminal benzene rings is 6.77 (19)°; the cyanide substituents have an anti orientation. In the crystal, the cations and anions are linked via C—H⋯N and C—H⋯Br hydrogen bonds, forming sheets lying parallel to the ac plane.