The explanation of the MgBX5 (X = F, Cl) systems existence and stability is provided on the basis of theoretical considerations supported by ab initio calculations. The equilibrium structures and most probable fragmentation processes of 26 (including isomers) MgBX5 molecules and six OBX5 compounds (for comparison) were examined and discussed. The competition between the ionization potential (IP) of the MgX subunit and the electron binding energy (VDE) of the BX4 fragment was found to be the key factor determining the ability to form a stable compound of MgBX5 type. (C) 2010 Elsevier B.V. All rights reserved.