Pentaneopentyltantalum, Ta(CH2But)(5) (1), was directly observed earlier in the formation of the archetypical alkylidene complex ((BuCH2)-C-t)(3)Ta=CHBut (2) from the reaction of either ((BuCH2)-C-t)(3)TaCl2 (3) with 2 equiv of (BuCH2Li)-C-t or ((BuCH2)-C-t)(4)TaCl (4) with 1 equiv of (BuCH2Li)-C-t. Ta(CH2But)(5) (1) was, however, short-lived, and its H-1 NMR resonances were mixed with those of ((BuCH2)-C-t)(3)Ta=CHBut (2), (BuCH2Li)-C-t, ((BuCH2)-C-t)(3)TaCl2 (3), ((BuCH2)-C-t)(4)TaCl (4), and CMe4 in a fairly narrow region. In the current work, deuterium-labeled Ta(CD2But)(5) (1-d(10)) has been prepared from the reactions of ((BuCD2)-C-t)(3)TaCl2 (3-d(6)) with 2 equiv of (BuCD2Li)-C-t as well as ((BuCD2)-C-t)(4)TaCl (4-d(8)) with 1 equiv of (BuCD2Li)-C-t. Due to a kinetic isotope effect, Ta(CD2But)(5)(1-d(10)) has a much longer life than 1. In addition, there are fewer peaks in the H-1 NMR spectra of Ta(CD2But)(5) (1-d(10)). H-2 NMR spectroscopy can also be used to characterize 1-d(10). These properties provide an opportunity to identify and study 1-d(10) in detail. Kinetic studies of the Ta(CD2But)(5) (1-d(10)) --> ((BuCD2)-C-t)(3)Ta=CDBut (2-d(7)) and Ta(CH2But)(5) (1) --> ((BuCH2)-C-t)(3)Ta=CHBut (2) conversions yield a kinetic isotope effect (KIE) = 14.1(0.8) at 273 K. In addition, kinetic studies of the 1-d(10) --> 2-d(7) conversion at 273-298 K give Delta H-D double dagger = 21.1(1.5) kcal/mol and Delta S-D double dagger = -4(6) eu for the alpha-deuterium abstraction reaction.