Tebbe reagent [Cp 2 Ti{(m-CH 2 )(m-Cl)Al(CH 3 ) 2 }] (A; Cp= cyclopentadienyl) belongs to the most enigmatic organometallic compounds.[1] Its successful synthesis, resulting from the careful investigation of the reaction of [Cp 2 TiCl 2 ] with two equivalents Al(CH 3 ) 3 , was triggered by important discoveries in two fundamentally different areas of homogeneous catalysis. Indeed, the initial studies of methane (and methylidene) formation from [Cp 2 TiCl 2 ]/Al(CH 3 ) 3 mixtures were conducted in the context of Ziegler-Natta polymerization catalysis, [2] but the methylene unit was structurally characterized by X-ray crystallography for the first time in tantalum alkylidene complexes, such as [Cp 2 Ta(CH 2 )(CH 3 )], [3] and tungsten methylene compounds, related to proposed catalysts for olefin metathesis.[4] Although catalytically active in olefin metathesis, [5] the Tebbe reagent is currently used for efficient carbonyl methylenation reactions.[ [8] have been reported, there are no X-ray structures of the Tebbe reagent nor of discrete metallacycles of the type [M(m-CH 2 )(m-R)Al(CH 3 ) 2 ] (R = Cl, CH 3 ). [9,10] Previous studies from our laboratories on rare-earthmetal(III) tetramethylaluminate complexes [L x Ln{Al-(CH 3 ) 4 } y ] (y = 1, 2, 3; x + y = 3, L = monovalent ancillary ligand, Ln = lanthanides and Sc, Y, La) as polymerization catalysts [11,12] led to the isolation of Ln III clusters with methylene, [13,14] methine, [15] and carbide functionalities. [16] We also found that complex [Cp* 3 Y 3 (m-Cl) 3 (m 3 -Cl)(m 3 -CH 2 )-(thf) 3 ] (Cp* = C 5 (CH 3 ) 5 ) displayed Tebbe-like reactivity.[13]

Non‐cyclopentadienyl lanthanide dihydrides [{(TpR,R′)LnH2}n] (R,R′=Me, n=4; R,R′=H, n=6) and [{(Tp Me 2)YH2}3(thf)2–3] were isolated and characterized. Their polynuclear frameworks are maintained in solution, and their nuclearity depends on the ligand and the solvent used in their preparation. The structure of [{(Tp Me 2)YH2}4] is shown (B red, H white, N blue, Y orange). TpR,R′=tris(3‐R‐5‐R′‐pyrazolyl)borate.

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