  The substitutional dependence of valence and spin-state configurations of
Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($x =$ 0, 0.05, 0.11, 0.17, and 0.38) is
investigated with near-edge x-ray absorption fine structure at the $L_{2,3}$
edges of Fe, Co, and As. The present data provide direct spectroscopic evidence
for an effectively isovalent substitution of Fe$^{2+}$ by Co$^{2+}$, which is
in contrast to the widely assumed Co-induced electron-doping effect. Moreover,
the data reveal that not only does the Fe valency remain completely unaffected
across the entire doping range, but so do the Co and As valencies as well. The
data underline a prominent role of the hybridization between (Fe,Co) 3$d_{xy}$,
$d_{xz}$, $d_{yz}$ orbitals and As $4s/4p$ states for the band structure in
$A$(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and suggest that the covalency of the
(Fe,Co)-As bond is a key parameter for the interplay between magnetism and
superconductivity.
