Comparative reactivity of sterically crowded nf3 (C5Me5)3Nd and (C5Me5)3U complexes with CO:: Formation of a nonclassical carbonium ion versus an f element metal carbonyl complex

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.