Аннотация:A new magnesium boridometallide Mg1-xRhB has been prepared by reaction of the elements in an argon-filled sealed Ta container at 1323 K. A narrow homogeneity range {0.05 ≤ x ≤ 0.08) was established by X-ray powder diffraction data and EDXS investigations. The crystal structure of Mg1-xRhB (x = 0.05) was solved using single-crystal X-ray diffraction data and confirmed by electron diffraction and high-resolution electron microscopy (space group P6(2)22, a = 5.2394(1), c = 9.4251(3) &ANGS;, Z = 6, 228 reflections, 19 variables, R(F) = 0.025). The structure can be topologically described as a derivative of the CeCo3B2 structure type where the positions in the cobalt layers are occupied by magnesium and rhodium atoms in an ordered manner. From the bonding analysis using the electron localization function, Mg1-xRhB is described as a two-dimensional [RhB] polyanion with embedded magnesium cations. The material is weakly diamagnetic and reveals metallic electrical conductivity.