Although (C5Me5)(3)m is an extremely sterically crowded molecule, it displays high reactivity with a variety of substrates including CO, THF, ethylene, hydrogen, nitriles, isonitriles, isocyanates, 1,3,5,7-cyclooctatetraene, azobenzene, and Ph3P=E (E = O, S, Se). The reactions include polymerization, insertion, ring-opening, and reduction. Depending on the substrate, (C5Me5)(3)Sm can react (1) as if it were a bulky alkyl complex of formula (C5Me5)(2)SmR in which R is an eta(1)-C5Me5 group or (2) as if it were the zwitterion [(C5Me5)(2)Sm](+)[C5Me5](-) in which the [C5Me5](-) component is a one-electron reductant. In the former mode, this compound (a) reacts with CO to form (C5Me5)(2)Sm(O2C7Me5), which has a Ligand containing a nonclassical carbocationic center, (b) undergoes hydrogenolysis with H-2 to form [(C5Me5)(2)Sm(mu-H)](2), (c) ring-opens THF to form (C5Me5)(2)Sm[O(CH2)(4)(C5Me5)](THF), (d) inserts PhCN to form (C5Me5)(2)Sm[NC(Ph)(C5Me5)](NCPh), 2, and (e) reacts with PhNCO to form (C5Me5)(2)SmOC(C5Me5)N(Ph)C(NPh)O, 3, a product which can be rationalized by a C-N coupling between a coordinated PhNCO and a PhNCO unit inserted into a Sm(eta(1)-C-5-Me-5) bond. On the other hand, (C5Me5)(3)Sm reduces (a) Ph3P=O to form PPh3, (C5Me5)(2), and [(C5Me5)(2)-Sm](2)(mu-O), Co) Ph3P=E (E = S, Se) to form PPh3, (C5Me5)(2), and a complex which adds TMF to form [(C5Me5)(2)Sm(THF)](2)(mu-E), (b) cyclooctatetraene to form (C5Me5)Sm(C8H8) and (C5Me5)(2), (c) azobenzene to form (C5Me5)(2)Sm(N2Ph2) and (C5Me5)(2), and (d) Me3CNC to form [(C5Me5)(2)Sm(mu-CN)(CNCMe3)](3), 4. (C5-Me5)(3)Sm also initiates the polymerization of ethylene. This reaction chemistry is described here as well as structural data on 2-4, each of which has a formal eight-coordinate bent metallocene geometry around samarium.