The asymmetric unit of [C2H5N4]4Bi2Cl10 is formed by two 5-amino-1-H-1,2,4-triazol-4-ium cations and one half of a decachlorodibismuthate(III) dimer. Each Bi FTI 277 HCl is coordinated to six chlorine atoms and BiCl63− coordination entities are united in pairs by two bridging Cl atoms giving rise to binuclear [Bi2Cl10]4− anions, whose final geometry is that of two octahedra sharing one edge (Fig. 1). Each [Bi2Cl10]4− anion is located on a crystallographic C2 symmetric axis, with Bi?Bi (1.5−x, 0.5−y, 1−z) distance of 4.268 Å. The BiCl distances involving the bridging atoms are the longest ( Table 2) and are consistent with those found in other [Bi2Cl10]4− anions for which the mean value is 2.91(1) Å (44 entries in CSD). As for the other BiCl distances, listed in Table 2, they fall within the wide range of 2.50–3.05 Å calculated from the literature data. The different bond lengths, together with the bond angles distribution, give rise to a slight distortion of the [Bi2Cl10]4− octahedra ; such a distortion, actually, is correlated not only to the primary deformations resulting from the stereochemical activity of Bi lone electron pair  and , but also to secondary deformations resulting from hydrogen bond interactions . The involvement of any particular chlorine atom in hydrogen bonding results in a shift of the lone electron pair of the Bi atom in the direction of the H atom, which generally leads to an increase of the BiCl distance. In the present case such a behavior is observed for BiCl2, BiCl1 and BiCl3 bond lengths which are long with respect to the others. The inspection of endo- and exocyclic CN bond lengths of the organic cations, reported in Table 2, shows that in the ring these values are between those of a single bond and a double bond while C1AN4A and C1BN4B are approximately equal to the CN double bond length, that means that the N4A and N4B atoms are sp2 hybridized . These features indicate a great contribution of the resonance to the overall cations’ geometry that indeed is planar. On the other hand, the comparison between the values of C1AN1AN2A, C2AN2AN1A and C1BN1BN2B, C2BN2BN1B angles indicate that the protonated atoms are N1A and N1B.