Catalytic oxygen atom transfer (OAT), which frequently employs molybdenum oxo species, is an important reaction for both nature and industry. The mechanistic details of oxygen atom transfer from Tp(R)MoO(2)(XPh) to PMe3 were investigated for R = 3-iPr and 3-Me and X=O and S by density functional theory (DFT) calculations of the enthalpies, free energy with solvent corrections, and natural bond orbital (NBO) analysis. The mechanism for both systems proceeds via rate-determining attack of PMe3 to form a stable intermediate with a bound OPMe3 ligand. From this intermediate the reaction proceeds through a substitution involving loss of OPMe3 and coordination of a single CH3CN solvent molecule. The solvent corrected free energy barriers of the rate-determining OAT step for the O and S systems were found to be energetically more favorable for the S systems by 6.2 and 2.2 kcal/mol (for the R=3-iPr and 3-Me, respectively). This lower energy barrier is the result of better stabilization by the SPh ligand of the Mo-IV products and the transition states, which are the unexpectedly later and more product-like. Additional examination of the NBO analysis emphasizes the role of the local acidity of the Mo and by extension the character of the ligands. The decreased electronegativity and softer character of the S atom result in an increased covalent character in the Mo-X bond which leads to the stabilization of a later (and lower energy) transition state and the corresponding product of the S system relative to O system. (C) 2010 Elsevier B.V. All rights reserved.