  Unconventional superconductivity in iron pnictides and chalcogenides has been
suggested to be controlled by the interplay of low-energy antiferromagnetic
spin fluctuations and the particular topology of the Fermi surface in these
materials. Based on this premise, one would also expect the large class of
isostructural and isoelectronic iron germanide compounds to be good
superconductors. As a matter of fact, they, however, superconduct at very low
temperatures or not at all. In this work we establish that superconductivity in
iron germanides is suppressed by strong ferromagnetic tendencies, which
surprisingly do not originate from changes in bond-angles or -distances with
respect to iron pnictides and chalcogenides, but are due to changes in the
electronic structure in a wide range of energies happening upon substitution of
atom species (As by Ge and the corresponding spacer cations). Our results
indicate that superconductivity in iron-based materials may not always be fully
understood based on $d$ or $dp$ model Hamiltonians only.
