  To reveal the relative importance of charge doping and defect scattering in
substitutionally modified 122 iron pnictides, we perform a systematic first
principles study on selected bands at the Fermi level. Disorder effects are
induced by various substitutions using an orbital based coherent potential
approximation (CPA). Pronounced level shifts of individual bands suggest that
transition metal substitutions introduce mobile charge carriers into the
system. However, important deviations from such a rigid band scenario as well
as spectral broadenings due to impurity scattering correlate with the band
character. Finally a $T$-matrix analysis exhibits a larger intraband than
interband scattering consistent with an $s^{+-}$ pairing state. Comparing
different substitutions reveals an increase of pair-breaking along the
transition metal series.
