Synthesis and structural characterization of PNNP ligand and dicobalt complexes by single-crystal XRD and EXAFS are described in reference . Co K-edge X-ray absorption spectra were measured at the Stanford Synchrotron Radiation Lightsource (SSRL) on the unfocused 20-pole 2T wiggler beamline 7-3 under standard ring conditions (3 GeV, ~500 mA). A liquid N2-cooled Si(220) double crystal monochromator was used for energy selection with crystal orientation of φ = 90°. Samples were handled in a dry N2-filled glovebox. Bulk solid samples were prepared by grinding ~5 mg sample (mass calculated to yield ~1 absorption length above the Co K-edge) with ~25 mg dry boron nitride in an agate mortar and pestle to form a uniformly colored, fine powder. The sample was pressed into a 7 mm diameter cylindrical pellet and held between 64 μm pieces of Kapton tape. During measurement, samples were loaded into a Cryo Industries closed-cycle liquid He cryostat and maintained at ~10 K throughout the measurement. Data was collected in transmission mode using N2-filled ionization chambers. The X-ray beam size was 1 mm (height) × 5 mm (width) for all measurements. Spectra were measured to k = 14.1 Å -1 and a Co foil was measured simultaneously for energy calibration, with the first inflection point in the Co foil spectrum fixed at 7709 eV. Spectra were collected over three regions: (1) 7390-7690 eV, 5 eV/step; (2) 7690-7750 eV, 0.15 eV/step; (3) 7750-8497 eV (k = 1.6-14.1 Å -1 ), 0.05 Å -1 step in k.