A comparison between models for the substrate−catalyst adduct in hydrodenitrogenation (HDN) catalysis is made with respect to oxygen vs sulfur ancillary ligands. Reacting η2(N,C)-NC5 tBu3H2Ta(OAr)2Cl (1, Ar = 2,6-C6H3 iPr2) with KOtBu affords orange crystals of the alkoxide η2(N,C)-NC5 tBu3H2Ta(OAr)2(OtBu) (2), while 1 and LiStBu react to form the red thiolate analogue η2(N,C)-NC5 tBu3H2Ta(OAr)2(StBu) (3). Structural studies of both complexes 2 and 3 are reported and compared with other η2(N,C)-NC5 tBu3H2 derivatives. A trace of the bromide complex η2(N,C)-NC5 tBu3H2Ta(OAr)2Br (4) is isolated from reacting η2(N,C)-NC5 tBu3H2Ta(OAr)2Cl (1) with EtMgBr in THF/Et2O solution and is also structurally characterized for comparison. Complexes 2−4 reveal a severe interruption of aromaticity within the heterocycle, different rotational preferences of the pyridine NC5 plane with respect to the Ta(OAr)2X moiety, and various aryloxide ligand structural differences. From this comparison, arguments will be presented that support the ancillary ligand π-donor ability decreasing as OtBu > OAr > StBu > Cl ≈ Br > Et, although evidence suggests that the StBu ligand is a better σ + π donor overall than OAr or OtBu.