Aiming at half-sandwich complexes of the type (FluR)­Ln­(AlMe4)2, homoleptic tetramethylaluminates Ln­(AlMe4)3 (Ln = Y, La, Nd, and Lu) were treated with equimolar amounts of R-substituted potassium fluorenyls. The salt metathesis reaction of La­(AlMe4)3 with K­(Flu) (Flu = fluorenyl = C13H9) and K­(Flu tBu) (Flu tBu = di­(tert-butyl) fluorenyl) selectively gave the desired half-sandwich complexes (FluR)­La­(AlMe4)2. The corresponding reactions of Y­(AlMe4)3 with K­(Flu)/K­(Flu tBu) and Lu­(AlMe4)3 with K­(Flu tBu) gave mixtures of half-sandwich and sandwich complexes, while treatment of Lu­(AlMe4)3 with K­(Flu) produced only the lutetocene complex (Flu)2Lu­(AlMe4). Sterically more demanding 1-trimethylsilyl fluorenyl (FluSi) allowed for the isolation of half-sandwich complexes for the entire Ln­(III) size range (Ln = La, Nd, Y, and Lu), in crystalline yields up to 94%. Upon activation with routinely employed borate or borane activators [Ph3C]­[B­(C6F5)4], [PhNMe2H]­[B­(C6F5)4], and B­(C6F5)3, highly active initiators for isoprene polymerization were obtained. The catalyst activity as well as molecular weight (distribution) and stereoregularity of the obtained polyisoprenes are governed by the rare-earth metal size, fluorenyl ligand, and cocatalyst: highest activity for La/FluSi/[Ph3C]­[B­(C6F5)4], lowest M w/M n = 1.11 for La/Flu/[PhNMe2H]­[B­(C6F5)4], maximum trans-1,4 selectivity = 85% for La/Flu tBu/[PhNMe2H]­[B­(C6F5)4], maximum cis-1,4 selectivity = 78% for Lu/FluSi/[Ph3C]­[B­(C6F5)4]. The formations of the active species were investigated by NMR spectroscopy revealing not only established cationization pathways but also fluorenyl abstraction in lanthanum complexes (Flu)­La­(AlMe4)2 and (Flu tBu)­La­(AlMe4)2 by trityl borate [Ph3C]­[B­(C6F5)4]. The reaction of half-sandwich complexes (FluR)­Ln­(AlMe4)2 with equimolar amounts of Me2AlCl did not give access to catalytically active species. Crystallization of binary mixtures (FluSi)­Y­(AlMe4)2/Me2AlCl in distinct molar ratios of 1:1 and 1:1.7 yielded complexes [(FluSi)­Y­(AlMe4)­(μ-Cl)]2 and (FluSi)6Y6Cl12, respectively.

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