The 1,2-addition reactions involving CS2 and oxygen-bridged intramolecular G13/P-based (G13 = group 13 element) and B/G15-based (G15 = group 15 element) frustrated Lewis pairs (FLPs) have been theoretically analyzed through density functional theory (DFT). Our DFT calculations suggest that of the nine FLP-assisted compounds, only B/P-Rea, Al/P-Rea, Ga/P-Rea, and In/P-Rea can kinetically and thermodynamically initiate energetically favorable 1,2-addition reactions with CS2, forming five-membered heterocyclic adducts. Our findings from the activation strain model suggest that the atomic radius of the Lewis acceptor (G13) and donor (G15) is critical in setting the barrier heights needed for optimal orbital interactions between G13/P-Rea, B/G15-Rea, and CS2. The EDA-NOCV (energy decomposition analysis-natural orbitals for chemical valence) analysis we conducted indicates that donor-acceptor bonding (singlet-singlet) plays a more dominant role than electron-sharing bonding (triplet-triplet) in the transition states G13/P-TS and B/G15-TS. Based on frontier molecular orbital theory and EDA-NOCV analyses, the bonding in the 1,2-addition reactions of oxygen-bridged intramolecular G13/P-Rea and B/G15-Rea with CS2 is primarily influenced by the forward interaction (lone pair (G15) → p-π*(CS2)), while the backward interaction (p-π*(G13) ← p-π(CS2)) has a relatively minor influence. Several significant findings derived from the current theoretical analyses are discussed in this work.