## Abstract

The three polyhalide complexes of trimesic acid (benzene-1,3,5-tricarboxylic acid; TMA) with compositions TMA$\cdot$0.7H$_2$O$\cdot$0.09 HI$_5$(I), TMA$\cdot$0.7H$_2$O$\cdot$0.103 HBr$_5$(II), TMA$\cdot$0.7H$_2$O$\cdot$0.167 HIBr$_2$(III) are isomorphous (for example, for I, a = 21.945(7), b = 17.917(6), c = 16.711(6) A, the space group is I222, Z = 24; II and III have very similar cell dimensions). Crystal structure analysis (four-circle diffractometer, graphite-monochromated MoKa: I, 2273 reflexions, R = 12.2%; III, 1747 reflexions, R = 12.4%) shows that all three have the same TMA framework, which consists of two mutually-perpendicular infinite hexagonal planar networks of hydrogen-bonded TMA molecules. The two networks are parallel to (110) and (1$\overline{1}$0) respectively and are mutually triply catenated. The arrangement is such as to leave channels of square cross section along the z-direction, which contain the linear polyhalide ions. I and II have pentahalide ions incommensurable with the TMA framework; the diffuse scattering pattern from I allows the structure of the I$^-_5$ ion to be determined as consisting of strongly interacting $\cdots$I$_2$---I$^-$---I$_2\cdots$ units and it is inferred that Br$^-_5$ has a similar structure. Compound III has IBr$^-_2$ ions commensurable with the TMA framework, but some difficulties remain in the interpretration of these results. The TMA framework has additional voids in the region about $\frac{1}{2}$, 0, $\frac{1}{2}$ and these are occupied by the water molecules (ca. sixteen per unit cell) in disordered fashion. The protons required to balance the charges of the polyhalide ions are presumably attached to some of these water molecules. The materials are characterized as crystalline acid-molecule complexes.