The reaction of the alkyl complexes Cp*2LnCH(SiMe 3)2 (Ln = Y 1-Y, Ce 1-Ce, La 1-La; Cp* = η5-C5Me5) and Me2Si(η 5-C5Me4)2LnCH(SiMe3) 2 (Ln = Ce 5-Ce) with 1-methylalk-2-ynes CH3C≡CR (R = Me 3a, Et 3b, nPr 3c, tBu 3d, SiMe3 3e, Ph 3f, C6H4Me-2 3g, C6H3Me 2-2,6 3h, C6H3iPr2-2,6 3i, C6F5 3j) affords the corresponding η3-propargyl/allenyl complexes Cp*2LnCH 2CCR (4a-j-Ln) and Me2Si(η5-C 5Me4)2CeCH2CCR (6a-j-Ce) via propargylic metalation. The hydride complexes [Cp*2Ln(μ-H)] 2 (Ln = Y 2-Y, Ce 2-Ce, La 2-La) react rapidly with 3 to produce mixtures of insertion and propargylic metalation products, and the relative rate of these processes depends on the metal and alkyne substituent. Selected η3-propargy/allenyl complexes Cp*2YCH 2CCR (R = Me 4a-Y, Ph 4f-Y), Cp*2CeCH2CCR (R = Me 4a-Ce, Ph 4f-Ce), Cp*2CeCH(Me)CCEt (9b-Ce), Cp*2LaCH2CCR (R = Ph 4f-La, C6H 3Me2-2,6 4h-La) are obtained on a preparative scale and characterized by NMR spectroscopy, IR spectroscopy, and cryoscopy. Compounds 4f-Y and 4f-La are also characterized by single-crystal X-ray diffraction. The reactions of the η3-propargyl/allenyl complexes with Brønsted acids, such as alcohols and acetylenes, afford the corresponding substituted allenes (RCH=C=CH2) and 1-methylalk-2-ynes (CH 3C≡CR) as organic products. The reactions of 4f-Y and 4f-La with Lewis bases, such as pyridine and THF, yield die corresponding base adducts. The adduct 4f-La · py is characterized by single-crystal X-ray diffraction, revealing an η3-coordinated propargyl/allenyl ligand. © 2008 American Chemical Society.