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Abstract:
Dark-red air-stable crystals of Ir(8)Se(28)Br(14<bold>)2</bold>H2O were
obtained by reacting iridium in an excess of selenium and selenium
tetrabromide at 300 degrees C for one week. The crystal structure is
monoclinic (space group P2(1)/n) with a = 1274.66(3) pm, b = 1211.48(3)
pm, c = 1743.82(4) pm, and = 91.227(1)degrees at 296(1) K. The structure
consists of uncharged clusters Ir8Se28Br14 =
(Ir3+)(8)(Se-2(2-))(Se-4(2-))(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: (6)-(2) in the case of Se-2(2-)
and (4)-(3) for Se-4(2-). 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.
