Tetramethylaluminato/halogenido(X) ligand exchange reactions in half‐sandwich complexes [CpRLa(AlMe4)2] are feasible in non‐coordinating solvents and provide access to large coordination clusters of the type [CpRLaX2]x. Incomplete exchange reactions generate the hexalanthanum clusters [CpR6La6X8(AlMe4)4] (CpR=Cp*=C5Me5, X=I; CpR=Cp′=C5H4SiMe3, X=Br, I). Treatment of [Cp*La(AlMe4)2] with two equivalents Me3SiI gave the nonalanthanum cluster [Cp*LaI2]9, while the exhaustive reaction of [Cp′La(AlMe4)2] with the halogenido transfer reagents Me3GeX and Me3SiX (X=I, Br, Cl) produced a series of monocyclopentadienyl rare‐earth‐metal clusters with distinct nuclearity. Depending on the halogenido ion size the homometallic clusters [Cp′LaCl2]10 and [Cp′LaX2]12 (X=Br, I) could be isolated, whereas different crystallization techniques led to the aggregation of clusters of distinct structural motifs, including the desilylated cyclopentadienyl‐bridged cluster [(μ‐Cp)2Cp′8La8I14] and the heteroaluminato derivative [Cp′10La10Br18(AlBr2Me2)2]. The use of the Cp′ ancillary ligand facilitates cluster characterization by means of NMR spectroscopy.

A series of solvent‐free heteroleptic terminal rare‐earth‐metal alkyl complexes stabilized by a superbulky tris(pyrazolyl)borato ligand with the general formula [TptBu,MeLnMeR] have been synthesized and fully characterized. Treatment of the heterobimetallic mixed methyl/tetramethylaluminate compounds [TptBu,MeLnMe(AlMe4)] (Ln=Y, Lu) with two equivalents of the mild halogenido transfer reagents SiMe3X (X=Cl, I) gave [TptBu,MeLnX2] in high yields. The addition of only one equivalent of SiMe3Cl to [TptBu,MeLuMe(AlMe4)] selectively afforded the desired mixed methyl/chloride complex [TptBu,MeLuMeCl]. Further reactivity studies of [TptBu,MeLuMeCl] with LiR or KR (R=CH2Ph, CH2SiMe3) through salt metathesis led to the monomeric mixed‐alkyl derivatives [TptBu,MeLuMe(CH2SiMe3)] and [TptBu,MeLuMe(CH2Ph)], respectively, in good yields. The SiMe4 elimination protocols were also applicable when using SiMe3X featuring more weakly coordinating moieties (here X=OTf, NTf2). X‐ray structure analyses of this diverse set of new [TptBu,MeLnMeR/X] compounds were performed to reveal any electronic and steric effects of the varying monoanionic ligands R and X, including exact cone‐angle calculations of the tridentate tris(pyrazolyl)borato ligand. Deeper insights into the reactivity of these potential precursors for terminal alkylidene rare‐earth‐metal complexes were gained through NMR spectroscopic studies.

Treatment of the half-sandwich complexes Cp R Ln-(AlMe 4 ) 2 (Cp R = C 5 Me 5 , C 5 Me 4 SiMe 3 ; Ln = Y, La, Lu) with the mild halogenido transfer reagents SiMe 3 X (X = Cl, Br, I) resulted in efficient and selective halogenido/tetramethylaluminato exchange. Depending on the size of the rare-earth metal, dimeric [Cp R Ln-(AlMe 4 )(μ-X)] 2 (Ln = Y, Lu) and decametallic [Cp R 3 La 3 (AlMe 4 ) 2 (μ-X) 4 ] 2 could be obtained. Donor (THF)-induced tetramethylaluminato cleavage gave access to "methyl-free" mixed methylidene/halogenido complexes Cp R 3 Ln 3 (μ-X) 3 (μ 3 -X)(μ 3 -CH 2 )(THF) 3 for yttrium (X = Cl, Br) and lanthanum (X = Cl, Br, I) in good yields. Additionally, mixed methylidene/halogenido Y(III) complexes could be obtained via methyl/halogenido exchange employing (C 5 Me 5 ) 3 Y 3 (μ-Me) 3 (μ 3 -Me)(μ 3 -CH 2 )(THF) 2 and SiMe 3 X via tetramethylsilane elimination. All methylidene complexes were probed in olefination reactions and found to act as efficient Schrock-type nucleophilic carbenes converting ketones and aldehydes into the respective terminal alkenes. Such reactivity is as high as that of the prominent Tebbe reagent but is less tolerant toward sterically demanding and functionalized substrates (such as esters). In contrast to the Tebbe reagent, complexes Cp R 3 Ln 3 (μ-X) 3 (μ 3 -X)(μ 3 -CH 2 )(THF) 3 polymerize δ-valerolactone in an efficient manner, generating polylactones with molecular weight distributions M w /M n as low as 1.13. Moreover, the bromido variant of the Tebbe reagent, Cp 2 Ti(μ-CH 2 )(μ-Br)AlMe 2 , is described, underlying similar synthesis limitations: that is, the coformation (and hence cocrystallization) of the trivalent species Cp 2 Ti(μ-Br) 2 AlMe 2 .

Various mixed methyl aryloxide complexes Tp tBu,Me LnMe(OAr) (Ln = Y, Lu) were obtained in moderate to high yields according to distinct synthesis protocols dependent on the metal size and sterics of the phenolic proligand. The reaction of Tp tBu,Me LuMe 2 and Tp tBu,Me YMe(AlMe 4 ) via protonolysis with 1 or 2 equiv HOC 6 H 2 tBu 2 -2,6-Me-4 in n-hexane gave the desired complexes Tp tBu,Me LnMe(OAr). Corresponding treatment of Tp tBu,Me LuMe 2 with the sterically less demanding HOC 6 H 3 Me 2 -2,6, HOC 6 H 3 iPr 2 -2,6 and HOC 6 H 3 (CF 3 ) 2 -3,5 led to the formation of the bis(aryloxy) lutetium complexes Tp tBu,Me Lu(OAr) 2 . Application of a salt-metathesis protocol employing Tp tBu,Me LnMe(AlMe 4 ) and the potassium aryloxides KOAr made complexes Tp tBu,Me LnMe(OAr) accessible for the smaller aryloxy ligands as well. All complexes were analyzed by X-ray crystallography to compare the terminal Ln−Me bond lengths and to evaluate the implication of the methyl/aryloxy coordination for the exact cone angles Θ°of the [Tp tBu,Me ] ancillary ligand. Treatment of Tp tBu,Me LnMe(AlMe 4 ) (Ln = Lu, Y) with HOC 6 H 2 tBu 2 -2,6-Me-4 in the presence of 4-(dimethylamino)pyridine (dmap) produced ion-separated complexes [Tp tBu,Me LnMe(dmap) 2 ]-[Me 3 AlOC 6 H 2 tBu 2 -2,6-Me-4)]. The thermal instability of Tp tBu,Me LuMe(OC 6 H 2 tBu 2 -2,6-Me-4) was revealed by the formation of (Tp (tBu-H)/(tBu)2,Me )Lu(OC 6 H 2 tBu 2 -2,6-Me-4) via intramolecular C−H-bond activation.

Thermal treatment of Cp*2YMe(thf) (Cp* = C5Me5), obtained from Cp*2Y(AlMe4) via donor-induced AlMe3 cleavage, in THF resulted in the concomitant formation of vinyloxide Cp*2Y(OC2H3)(thf) and 2 ethylene-tetrahydrofuranyl complex Cp*2Y(2-C2H4-OC4H7) via...