In this contribution, using first principles calculations within a density functional theory framework, we report, for the first time, evidence for the formation of a fourfold magnesium organometallic compound upon metal deposition on perylene-3,4,9, 10-tetracarboxyl dianhydride (PTCDA) organic semiconductor. Current investigation clearly indicates that in the bulk of the organic crystallographic structure the magnesium atom mainly interacts with three PTCDA molecules. The reactive metal is bound both to carboxylic oxygen atoms of the 3 anhydride-end moieties and to a perylene carbon atom which changes its hybridization state, from sp(2) to sp(3), in the presence of metal impurities. In turn, the analysis of the electronic structure of the reacted system prevalently reveals the formation Of four covalent bonds, as a consequence of a weak charge transfer toward the organic material. Such a result confirms the capability of the PTCDA thin films to host metal atoms providing, inside their structural empty channels, a rather accessible and soft chemical environment. Interestingly, in the light of these findings and of previous works, a relationship between first ionization potential of the doping metal and the character of the newly formed chemical bonds is confirmed.