Reaction of d(0) Ta(NMe2)(5) (1) with O-2 yields two aminoxy complexes (Me2N)(n)Ta(eta(2)-ONMe2)(5-n) (n = 4, 2; 3, 3) as well as (Me2N)(4)Ta-2[eta(2)-N(Me)CH2NMe2](2)(mu-O)(2) (4) and (Me2N)(6)Ta-3[eta(2)-N(Me)CH2NMe2](2)-(eta(2)-ONMe2)(mu-O)(3) (5) containing novel chelating (aminomethyl)amide-N(Me)CH2NMe2 ligands. The crystal structures of 2-5 have been determined by X-ray crystallography. (Me2N)(4)Ta(eta(2)-ONMe2) (2) converts to (Me2N)(3)Ta(eta(2)-ONMe2)(2) (3) in its reaction with O-2. In addition, the reaction of Ta(NMe2)(5) with 3 gives 2 only at elevated temperatures. Density functional theory (DIFT) calculations have been used to investigate the mechanistic pathways in the reactions of Ta(NMe2)(5) (1) with triplet O-2-Monomeric reaction pathways in the formation of 2-5 are proposed. A key step is the oxygen insertion into a Ta-N bond in 1 through an intersystem conversion from triplet to singlet energy surface to give an active peroxide complex (Me2N)(4)-Ta(eta(2)-O-O-NMe2) (A4). The DFT studies indicate that the peroxide ligand plays an important role, including oxidizing an amide to an imine ligand through the abstraction of a hydride. Insertion of Me-N=CH2 into a Ta-amide bond yields the unusual -N(Me)CH2NMe2 ligands.