  A highly intriguing aspect in iron-pnictide superconductors is the
composition-dependent electronic structure, in particular the question if and
how charge carriers are introduced to the system upon substitution of Ba by
alkali metals or of Fe by other transition metals, TM. We report on a
systematic study of spatial structure and electronic states by x-ray
diffraction and x-ray absorption on a large number of compositions in the
(Ba,K)(Fe,TM)2As2 family. The coherent combination of detailed structural
information with an in-depth analysis of the electronic structure allows us to
sensitively disentangle (charge-carrier) "doping" effects from "substitutional"
effects. Results include a doping character that is site-decoupled, as well as
TM 3d energy-level schemes that exhibit non-standard level sequences and even
t2-e level crossings. Our study indicates that doping per se seems to play a
lesser role than expected for pnictide superconductivity and magnetism.
