On the basis of the high-resolution single-crystal X-ray diffraction data and theoretical B3LYP/6-311+G(d) and B3LYP/cc-pVDZ calculations, the electron density distribution in boratrane and 1-methylsilatrane has been investigated. The B-N bond in the boratrane molecule was found to correspond to the "shared" (covalent) interatomic interaction in terms of Bader's "atoms in molecules" (AIM) theory. On the contrary, the Si...N bond in 1-methylsilatrane and B-N bond in a series of some acyclic donor-acceptor boron complexes and other boratrane derivatives correspond to interaction of an "intermediate" type. Such character of the B-N bond is caused by an influence of the atrane "cage effect". The influence of this effect results in deformation of the boron atom and displacement of the bond critical point (3,-1) to the region of the electron lone pair of the nitrogen atom. This changes the type of the interatomic interaction in comparison to other compounds investigated and explains high hydrolytic stability of boratrane as well as silatranes and germatranes.