Sterically crowded isoelectronic nf(3) (C5Me5)(3)M complexes of neodymium and uranium, compounds which have unconventionally long metal ligand distances, are found to react very differently with CO as a substrate. The 4f(3) complex (C5Me5)(3)Nd reacts with CO to form a nonclassical carbonium ion complex, (C5Me5)(2)Nd(O2C7Me5), which contains a three-coordinate planar carbon. (C5Me5)(3)U reacts with CO to form an even more crowded CO adduct through a reaction type never observed before for (C5Me5)(3)M compounds. The rare uranium carbonyl complex, (C5Me5)(3)U(CO), has upsilon(CO) = 1922 cm(-1) and a U-C(CO) distance of 2.485(9) Angstrom.