Competition between π Donation and α-C−H Agostic Interactions in Complexes of the Type Tp‘Ta(CH-t-Bu)(X)(Y) (X = Halide; Y = Halide, NR₂, OR; Tp‘ = Hydrotris(3,5-dimethylpyrazolyl)borate)

The compounds Tp‘Ta(CHC(CH₃)₃)X₂ (2 (X = Cl), 3 (X = Br)) (Tp‘ = hydrotris(3,5-dimethylpyrazolyl)borate), have been synthesized by the reaction of KTp‘ with Ta(CHC(CH₃)₃)X₃(thf)₂ in THF. One Cl ligand in 2 can be substituted with OR^- (R = i-Pr, Me) or NMe₂^-. The values of ¹J_C-H for the alkylidene carbon are uniformly low due to the α-H agostic interaction between the metal center and the alkylidene proton. The magnitude of ¹J_C-H in these compounds depends upon the π donor properties of the remaining ligands. Attempted synthesis of monoarylamide complexes by displacement of Cl^- from 2 with N(H)Ar^- results in proton transfer from N to C and formation of the imido complexes Tp‘Ta(CH₂C(CH₃)₃)(NAr)Cl (Ar = 2,6-i-PrC₆H₃ (7), Ph (8)). When 2 reacts with KN(Me)Ph, proton transfer occurs from the Me group to the alkylidene carbon giving the imine complex Tp‘Ta(η²-CH₂NPh)(CH₂CMe₃)Cl (9). A single-crystal X-ray diffraction study of 9 confirmed the structure and suggested that 9 is best described as an azametallacyclopropane complex.