The reactivity of a number of two-coordinate [Pd(L)(L')] (L = N-heterocyclic carbene (NHC) and L' = NHC or PR3) complexes with O-2 has been examined. Stopped-flow kinetic studies show that O-2 binding to [Pd(IPr)(P(p-tolyl)(3))] to form cis-[Pd(IPr)(P(p-tolyl)(3))-(eta(2)-O-2)] occurs in a rapid, second-order process. The enthalpy of O-2 binding to the Pd(0) center has been determined by solution calorimetry to be -26.2(1.9) kcal/mol. Extension of this work to the bis-NHC complex [Pd(IPr)(2)], however, did not lead to the formation of the expected diamagnetic complex cis-[Pd(IPr)(2)(eta(2)-O-2)] but to paramagnetic trans-[(Pd(IPr)(2)(eta(1)-O-2)(2)], which has been fully characterized. Computational studies addressing the energetics of O-2 binding have been performed and provide insight into reactivity changes as steric pressure is increased.