A synthetic heavier p-block bismuth (Bi) complex with a rigid pincer ligand, is shown to be a Bi-based molecular electrocatalyst for proton reduction under weak acid conditions. High activity of the complex is observed in catalysis, with a i(cat)/i(p) value of 95 with CH3CO2H. Experimental and theoretical studies indicate that the hydrogen evolution reaction (HER) mechanism involves generating a low-valent Bi I complex and its reactivity toward acid to form a Bi-III-hydride species. Subsequent protonolysis of the Bi-III-hydride eliminates H-2 through a heterolytic route. The HER cycling through the low-valent metal species and its hydride has been proposed previously in transition-metal (TM) centered catalysts but rarely validated in main-group catalytic systems. The finding brings insight into heavier main-group Bi-catalyzed H-2 evolution and the role of low-valent main-group metal centre during catalysis. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.