Electronic structures of experimentally verified on-top and incorporated configurations for the Ba/Ge(001) adsorption system is discussed. It is shown that the Ba-Ge interaction in both phases is predominantly ionic in nature. It is also demonstrated that this adatom-substrate interaction governs the formation of the extended on-top and the incorporated phases. In the latter the Ba adatoms interact with the second layer Ge atoms resulting in the formation of the unoccupied dangling bonds on the remaining surface Ge dimers.