The displacements of the methyl substituents away from the metal and out of the cyclopentadienyl ring plane are compared in sterically crowded (C5Me5)(3)M Complexes vs sterically normal f-element complexes in an attempt to evaluate the utility of this parameter in predicting unusual (C5Me5)(1-) ring reactivity. The out-of-plane displacements of 16 sterically crowded tris(cyclopentadienyl) complexes of general formula, (C5Me5)(3)M (C5Me4R)(3)M (R = Et, Pr, Bu-t, SiMe3) (C5Me5)(3)MX (X = anion), and (C5Me5)(3)ML (L = neutral ligand) are compared with [(C5Me5)(2)U](2)(C6H6), (C5Me5)(2)Sm((PC4H2Bu2)-Bu-t), and 33 representative examples of f-element bis(cyclopentadienyl) complexes with normal cyclopentadienyl behavior and coordination numbers ranging from 6 to 10. In general, the methyl displacement values of sterically crowded complexes overlap with those in the other complexes, which demonstrates that the basis of the structural distortions is complex. However, if the most extreme out-of-plane displacement in each of the sterically crowded complexes is examined vs the analogous maximum out-of-plane displacement in less crowded systems, there appears to be a basis for predicting cyclopentadienyl reactivity.