Reaction of [Pd(IPr)(2)] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and O-2 leads to the surprising discovery that at low temperature the initial reaction product is a highly labile peroxide complex cis-[Pd(IPr)(2)(eta(2)-O-2)]. At temperatures greater than or similar to -40 degrees C, cis-[Pd(IPr)(2)(eta(2)-O-2)] adds a second O-2 to form trans-[Pd(IPr)(2)(eta(1)-O-2)(2)]. Squid magnetometry and EPR studies yield data that are consistent with a singlet diradical ground state with a thermally accessible triplet state for this unique bis-superoxide complex. In addition to reaction with O-2 , cis-[Pd(IPr)(2)(eta(2)-O-2)] reacts at low temperature with H2O in methanol/ether solution to form trans-[Pd(IPr)(2)(OH)(OOH)]. The crystal structure of trans-[Pd(IPr)(2)(OOH) (OH)] is reported. Neither reaction with O(2 )nor reaction with H2O occurs under comparable conditions for cis-[Pd(IMes)(2)(eta(2)-O-2)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene). The increased reactivity of cis-[Pd(IPr)(2)(eta(2)-O-2)] is attributed to the enthalpy of binding of O-2 to [Pd(IPr)(2)] (-14.5 +/- 1.0 kcal/mol) that is approximately one-half that of [Pd(IMes)(2)] (-27.9 +/- 1.5 kcal/mol). Computational studies identify the cause as interligand repulsion forcing a wider C-Pd-C angle and tilting of the NHC plane in cis-[Pd(IPr)(2)(eta(2)-O-2)]. Arene-arene interactions are more favorable and serve to further stabilize. cis-[Pd(IMes)(2)(eta(2)-O-2)]. Inclusion of dispersion effects in DFT calculations leads to improved agreement between experimental and computational enthalpies of O-2 binding. A complete reaction diagram is constructed for formation of trans-[Pd(IPr)(2)(eta(1)-O-2)(2)] and leads to the conclusion that kinetic factors inhibit formation of trans-[Pd(IMes)(2)(eta(1)-O-2)(2)] at the low temperatures at which it is thermodynamically favored. Failure to detect the predicted T-shaped intermediate trans-[Pd(NHC)(2)(eta(1)-O-2)] for either NHC = IMes or IPr is attributed to dynamic effects. A partial potential energy diagram for initial binding of O-2 is constructed. A range of low-energy pathways at different angles of approach are present and blur the distinction between pure "side-on" or "end-on" trajectories for oxygen binding.