The phenomenon of bond length alternation in the cyclopentadienyl (Cp) ligand of cyniantrene (eta(5)-C5H5)Mn(CO)(3) was investigated using high-resolution X-ray diffraction analysis (XRD) and quantum chemical calculations. It was shown that the trans-effect of strong field CO ligands causes the redistribution of electron density in the Cp ring, and it manifests in atomic charges and energy of its carbon atoms. The angle C(Cp)-Mn-CO is characteristic for this feature and can be used for the prediction of bond lengths and the charge distribution in the aromatic fragment in various piano-stool-type complexes.