This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Transition metal dioxygen complexes are important in several fields of biology and chemistry, which include aerobic oxidation, oxidative catalysis, and biological O2 activation. However, very few reports of Ni-O2 complexes are available, partly due to the difficulty in the isolation of NiI complexes, which have the target Ni oxidation state for O2 reactivity. Recently, we synthesized and spectroscopically characterized a Ni-O2 complex (1) ([Ni(14-tmc)O2](OTf);(14-tmc)=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane;(OTf)=CF3SO3), with an O-O stretching frequency of 1131 cm-1. Ni K-edge XAS and EXAFS data were combined with optical spectroscopies to arrive at a NiII-superoxide description of 1. In the lack of a crystal structure, EXAFS and DFT calculations were used to predict that O2 binds in an end-on fashion. Experiments with modified axial ligand systems have led to surprising results. For example, the choice of 13-tmc (1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclotridecane) as a ligand results in a species (2) with similar spectroscopic features as 1, while 12-tmc (1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) results in a species (3). 3 has been crystallographically characterized and has a unique side-on bound Ni-O2 conformation. 3 also demonstrates starkly different spectroscopic properties with a decreased O-O stretching frequency (1002 cm-1). The decrease in O-O frequency indicates a more peroxide like character in 3, which would suggest a NiIII-like character in 3 compared to a NiII-like character in 1. It is likely that the change in the axial ligand in 1 (14-tmc) to 12-tmc (3) tunes the bonding between the central Ni atom and dioxygen. To understand the role of the different tmc ligands in Ni-O2 bonding, we propose to undertake Ni K-edge XAS experiments on 1(for comparison purposes), 2 and 3.