BIrMg, hexagonal, P6222 (no. 180), a = 5.2752(7) A, c = 9.439(2) A, V = 227.5 A, Z = 6, Rgt(F) = 0.017, wRref(F) = 0.019, T = 295 K. Source of material The MgIrB compound was prepared using powders of Mg (99.8 %), Ir (99.9%) and crystalline B (99.999%) as startingmaterials. The elements mixture was ground, pressed into pellets, embedded in Ta ampoules and sealed under argon. The annealing was performed at 1293 K for 14 days and at 1333 K for 15 days with intermediate re-grinding. Experimental details The single phase samplewas obtained from the samplewith nominal composition Mg1.1IrB1.1. The X-ray powder diffraction patternwas indexedwith a hexagonal unit cell and lattice parameters a = 5.2736(3)A and c = 9.4358(7)A.When changing initial mixture compostition to Mg0.96IrB0.94 a slight lattice parameters reduction to a = 5.2702(2) A, c = 9.429(1) A was observed. The largest lattice parameters (cf. Abstract) were found for theMgIrB single crystal obtained from the sample with nominal composition Mg1.38IrB1.46 by annealing at 1273 K for 18 days. Analogously to the Mg RhB 1.x phase [1] we assume a small homogeneity range for Mg IrB 1.x with the minimum x close to 0 observed in the single crystal used for the structure determination. The Flack parameter value of 0.0 reveals correctness of the absolute structure. Discussion The synthesis of numerous ternary magnesium rhodium borides [1-4] initiated us to study the Mg-Ir-B system with iridium as the electron analogue of rhodiumwith the aim to investigate the platinummetal nature influence on the crystal structure and chemical bonding of ternary borides. The crystal structure of MgIrB is similar to Mg RhB 1.x [1]. It can be described as a framework of the distorted trigonal prisms [BIr4(Mg2)2], sharing the bases and side edges. The distorted hexagonal channels of the framework directed along c axis are occupied by Mg1 atoms. Each boron atom forms a short distance of 1.87(1) A to the next boron atom and, in addition, short distances of 2.203(3) and 2.210(7)A to the next iridium atoms. On this way a 2D [IrB] network is formed. In Mg RhB 1.x [1] the 2D [RhB] network was shown to be anionic. In analogy, we suppose the same character for 2D [IrB]. For theMg RhB 1.x compound the vacancies inMg1 site were experimentally observed and then interpreted using the results of chemical bonding analysis. The single crystal diffraction data for theMgIrB show the full occupation of theMg1 positions (99.8(2) %) and yield the composition MgIrB (x = 0). It agrees with the maximum lattice parameters found for the single crystal data. Z. Kristallogr. NCS 224 (2009) 17-18 / DOI 10.1524/ncrs.2009.0010 17 © by Oldenbourg Wissenschaftsverlag, Munchen Crystal: black prism, size 0.050 × 0.020 × 0.020 mm Wavelength: Mo K0 radiation (0.7107 A) %: 930.5 cm Diffractometer, scan mode: Rigaku AFC7, )/2 2max: 63.01° N(hkl)measured, N(hkl)unique: 2830, 244 Criterion for Iobs, N(hkl)gt: Iobs > 2 !(Iobs), 227 N(param)refined: 15 Programs: WinCSD [5], ATOMS [6], SHELXS-97 [7] Table 1. Data collection and handling. B 6j 0.602(2) 2x$1 1⁄2 0.006(2) Table 2. Atomic coordinates and displacement parameters (in A). Atom Site x y z Uiso