Bis(2-amino-1H-benzimidazol-3-ium) tetrakis(μ-but-2-enoato)-κ4O:O';κ3O,O':O;κ3O:O,O'-bis[bis(but-2-enoato-κ2O,O')holmium(III)].

The title ionic compound, (C(7)H(8)N(3))(2)[Ho(2)(C(4)H(5)O(2))(8)], is constructed from two almost identical independent centrosymmetric anionic dimers balanced by two independent 2-amino-1H-benzimidazol-3-ium (Habim(+)) cations. The asymmetric part of each dimer is made up of one Ho(III) cation and four crotonate (crot or but-2-enoate) anions, two of them acting in a simple η(2)-chelating mode and the remaining two acting in two different μ(2):η(2) fashions, viz. purely bridging and bridging-chelating. Symmetry-related Ho(III) cations are linked by two Ho-O-Ho and two Ho-O-C-O-Ho bridges which lead to rather short intracationic Ho···Ho distances [3.8418 (3) and 3.8246 (3) Å]. In addition to the obvious Coulombic interactions linking the cations and anions, the isolated Ho(2)(crot)(8) and Habim(+) ions are linked by a number of N-H···O hydrogen bonds, in which all N-H groups of the cation are involved as donors and all (simple chelating) crot O atoms are involved as acceptors. These interactions result in compact two-dimensional structures parallel to (110), which are linked to each other by weaker π-π contacts between Habim(+) benzene groups.