Abstract Exploring the dynamic structural evolution of electrocatalysts during reactions represents a fundamental objective in the realm of electrocatalytic mechanism research. In pursuit of this objective, we synthesized PhenPtCl 2 nanosheets, revealing a N 2 -Pt-Cl 2 coordination structure through various characterization techniques. Remarkably, the electrocatalytic performance of these PhenPtCl 2 nanosheets for hydrogen evolution reaction (HER) surpasses that of the commercial Pt/C catalyst across the entire pH range. Furthermore, our discovery of the dynamic coordination changes occurring in the N 2 -Pt-Cl 2 active sites during the electrocatalytic process, as clarified through in situ Raman and X-ray photoelectron spectroscopy, is particularly noteworthy. These changes transition from Phen-Pt-Cl 2 to Phen-Pt-Cl and ultimately to Phen-Pt. The Phen-Pt intermediate plays a pivotal role in the electrocatalytic HER, dynamically coordinating with Cl - ions in the electrolyte. Additionally, the unsaturated, two-coordinated Pt within Phen-Pt provides additional space and electrons to enhance both H + adsorption and H 2 evolution. This research illuminates the intricate dynamic coordination evolution and structural adaptability of PhenPtCl 2 nanosheets, firmly establishing them as a promising candidate for efficient and tunable electrocatalysts.