Selenium-Rich Octanuclear Iridium Clusters with Mixed-Valent Selenium Ligands in the Densest Known Packing of Ellipsoids

Dark‐red air‐stable crystals of Ir8Se28Br14·2H2O were obtained by reacting iridium in an excess of selenium and selenium tetrabromide at 300 °C for one week. The crystal structure is monoclinic (space group P21/n) with a = 1274.66(3) pm, b = 1211.48(3) pm, c = 1743.82(4) pm, and β = 91.227(1)° at 296(1) K. The structure consists of uncharged clusters Ir8Se28Br14 = (Ir3+)8(Se22–)(Se42–)6(SeBr2)2(Br–)10 with two water molecules attached to each of them. Selenium adopts three different oxidation states, –I in the diselenide dumbbell, –I and ±0 in the tetraselenide chains, and +II in the SeBr2 units. The iridium(III) cations are coordinated octahedrally by selenium atoms and bromide ions. The octahedra share common vertices, while the oligoselenide anions provide high intra‐cluster connectivity: μ6‐η2 in the case of Se22– and μ4‐η3 for Se42–. In parts the cluster resembles a cut‐out of the rhombohedral Ir3Se8 framework structure. The ellipsoidal clusters form an excellent approximation of the mathematically predicted densest known packing of ellipsoids, featuring the coordination number of 14.