Alkene Dissociation from a d0 Yttrocene−Alkyl−Alkene Complex Generates a Pyramidal Alkyl Yttrium Complex That Inverts Slowly

The d 0 yttrium(III)−alkyl−alkene chelate Cp* 2 Y[η 1:η 2 -CH 2 CH 2 CH(CH 3 )CH CH 2 ] ( 5 ) was prepared by reaction of 3-methyl-1,4-pentadiene with (Cp* 2 YH) 2 . Variable-temperature 13 C NMR spectroscopy of 5 showed that the environments of the two diastereotopic Cp* ligands were averaged by a fluxional process which requires both reversible alkene dissociation and inversion at the yttrium center; the energy barrier for alkene dissociation plus yttrium inversion is 9.6 ± 0.3 kcal mol - 1 .