In this paper, we investigate the second inverse sum indeg index ISI2 of graphs, a topological index that has signi cant applications in chemical graph theory. Upper and lower bounds for ISI2 of graphs and trees with a speci ed number of pendent edges are established. Furthermore, ISI2 of various bridge graphs are computed. The main contribution of this work lies in presenting precise bounds and exact expressions for particular families of graphs, o ering resources for researchers and engineers in mathematical chemistry and applied graph theory.
