The yttrium hydride dimer (Cp-2*YH)(2) (2) reacted rapidly with 3,3-dimethyl-1,4-pentadiene in methyl-cyclohexane-d(14) at -78 degrees C to form the d(0) yttrium(III) pentenyl chelate complex Cp-2*Y[eta(1),eta(2)-CH2CH2C(CH3)(2)CH=CH2] (4). Low-temperature H-1, C-13 and H-1 NOESY NMR spectroscopy of 4 established bonding of the tethered alkene to the d(0) metal center. Quantitative analysis of the NOESY time course using the Conformer Population Analysis method demonstrates that the dominant conformers in solution are interconverting pairs of chelated complexes, one in a twist-boat conformation and the other a chair conformer. No significant contribution by a free alkene conformation is required to explain the spectroscopic data. Addition of THF to pentenyl chelate 4 at -78 degrees C displaced the alkene and formed the yttrium(III) pentenyl THF adduct Cp-2*Y[eta(1)-CH2CH2C(CH3)(2)CH=CH2](THF) (5), in which there is no interaction between the pendant alkene and the d(0) metal center. Yttrium hydride dimer 2 also reacted with either 1,4-pentadiene or methylenecyclobutane to form the same pentenyl chelate complex Cp-2*Y[eta(1),eta(2)-CH2CH2CH2CH=CH2] (6). The interconversion of deuterium-labeled chelate complexes 6-1,1,5,5-d(4) and 6-3,3,5,5-d(4) at -78 degrees C (Delta G(double dagger) = 14.4 +/- 0.2 kcal mol(-1)) establishes that intramolecular alkene insertion is occurring under the reaction conditions.