Motivated by the exceptional interest of researchers in two-dimensional nanostructures, the current study deals with the structural, electronic, optical, and gas-sensing properties of recently synthesized monolayer phosphorene. Van der Waals induced first-principles calculations were performed to study the binding mechanism of pristine and defected phosphorene towards the toxic gases PH3 and AsH3. The preferential sites and orientations of these molecules on the phosphorene sheet were determined, and a detailed analysis of the adsorption energetics was performed. Both of the gas molecules interact weakly with the phosphorene sheet, with AsH3 the binding was slightly stronger than PH3. The creation of defects such as monovacancies and divacancies in the phosphorene sheet was found to significantly enhance the adsorption mechanism. The adsorption energies of both PH3 and AsH3 improved by factors of four and three, respectively, as compared to their values on pristine phosphorene. The adsorption mechanism was further investigated by plotting the band structure and density of states. We also studied the optical properties and the static dielectric matrices of these nanostructures using density functional perturbation theory. Our findings showed that defected phosphorene with vacancies can be considered as an efficient sensor for toxic gases.