In order to understand the nature of the putative cationic 12-electron species [M(eta(5)eta(1):TlI-C5R4SiMe2NR')R"](+) of titanium catalysts supported by a linked amido-cyclopentadienyl ligand, several derivatives with different cyclopentadienyl C5R4 and amido substituents R' were studied systematically. The use of tridentate variants (C5R4SiMe2NCH2CH2X)(2-)(C5R4 = C5R4, C5ME4, C5H4 C5H3 Bu-t; X = OMe, SMe, NMe2) allowed the NMR spectroscopic observation of the titanium benzyl cations [Ti(eta(5):eta(1)-C5Me4SiMe2NCH2CH2X)(CH2Ph)](+). Isoelectronic neutral rare earth metal complexes [Ln(eta(5):eta(1)-C5R4SiMe2NR')R"] can be expected to be active for polymerization. To arrive at neutral 12-electron hydride and alkyl species of the rare earth metals, we employed a lanthanide tris(alkyl) complex [Ln(CH2SiMe3)(3)(THF)(2)] (Ln = Y, Lu, Yb, Er, Tb), which allows the facile synthesis of the linked amido-cyclopentadienyl complex [Ln(eta(5):eta(1) -C5Me4SiMe2NCMe3) (CH2SiMe3)(THF)]. Hydrogenolysis of the linked amido-cyclopentadienyl alkyl complex leads to the dimeric hydrido complex [Ln(eta(5):eta(1)-C5Me4SiMe2NCMe3)(THF)(mu-H)](2). These complexes are single-site, single-component catalysts for the polymerization of ethylene and a variety of polar monomers such as acrylates and acrylonitrile. Nonpolar monomers such as (alpha-olefins and styrene, in contrast, give isolable mono-insertion products which allow detailed studies of the initiation process. (C) 2002 Elsevier Science B.V. All rights reserved.