Electron density distribution in a single crystal of (formula presented) (formula presented)

The electron density distribution in (formula presented) has been obtained by the precise x-ray structure analysis on high-quality single crystals. A slight deficiency (formula presented) of Mg has been observed. The obtained sectional Fourier map in the B plane shows the triangle-shaped electron distribution extended along the B-B bonds, which indicates the typical (formula presented) orbitals of the boron atom. Further, the difference Fourier maps clearly reveal the (formula presented) covalent bonds within the B plane and (formula presented) orbitals perpendicular to the B plane. Both σ and π bonds construct the two-dimensional honeycomb network of the B atoms along the (001) plane. In contrast, there is no covalency between the B and Mg atoms, indicating the ionic bonds between B and Mg layers.