Understanding electron flow during chemical reactions is fundamental to ultrafast chemistry, particularly in transition-metal complexes where redox processes involve intricate coupling between electronic and nuclear dynamics. While time-resolved X-ray spectroscopy offers insight into these dynamics, interpreting spectral data to identify transient intermediates and electron transfer mechanisms remains challenging. We employ a dual-edge strategy that simultaneously simulates O K-edge and Cu L-edge X-ray absorption spectra for the paradigmatic \({{{{\rm{CuO}}}}}_{2}^{+}\) system. We show that symmetric Cu–O bond shortening drives metal-to-ligand electron transfer, converting Cu(I):O2 to Cu(II): \({{{{\rm{O}}}}}_{2}^{\bullet -}\) . Peak-by-peak analysis along the binding coordinate resolves concurrent dioxygen reduction and copper oxidation, leveraging the interpretable ligand K-edge to decode the complex metal L-edge spectrum. A Born-Oppenheimer molecular dynamics simulation further captures thermally-driven transitions between side-on and end-on configurations, showing distinct spectral signatures, and identifies the O K-edge as a sensitive probe for Cu-O bond fluctuations. We establish a dual-edge protocol for decoding metal L-edge spectra and demonstrate the complementary power of static and dynamical simulation: the former offers a practical route to statistically averaged spectral trends, while the latter delivers explicit time-resolved insight into stochastic events. They provide a robust framework for mapping atomic-level electron flow in ultrafast X-ray studies of catalysis and energy science.
