  We use a quantitative convergent beam electron diffraction (CBED) based
method to image the valence electron density distribution in
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. We show a remarkable increase in both the
charge quadrupole of the Fe cations and the charge dipole of the arsenic anions
upon Co doping from $x=0$ ($T_c=0$ K) to $x=0.1$ ($T_c=22.5$ K). Our data
suggest that an unexpected electronic correlation effect, namely strong
coupling of Fe orbital fluctuation and anion electronic polarization, is
present in iron-based superconductors.
