The syntheses and full characterizations of the peri-substituted naphthalenes (Nap) and acenaphthenes (Ace) 1-Br-8-(Ph2P)-Nap (1a) and 5-Br-6-(Ph2P)-Ace (1b), as well as their derivatives 1-Br-8-[Ph2P(E)]-Nap [E = CH3+ (counterion I–) (2a); E = O (3a); E = S (4a); E = Se (5a)] and 5-Br-6-[Ph2P(E)]-Ace [E = CH3+ (counterion I–) (2b); E = O (3b); E = S (4b); E = Se (5b)] are reported. In order to quantify the energetic and electronic effects of the peri-interactions, an additional set of molecules, 1c–5c, with the bromine atom and the Ph2P(E) fragment on opposite sides of the naphthalene group was generated, which serves as reference because 1c–5c exhibit negligible peri-interactions. The molecular arrangements of all 15 compounds were optimized at the B3PW91/6-311+G(2df, p) level of theory. The analysis of the peri-interactions was not only based on the inspection of the molecular arrangement and energies alone, but extended to a set of real-space bonding indicators (RSBI). These indicators were derived from theoretically calculated electron densities and pair densities, respectively. Particularly, the stockholder, Atoms-In-Molecules (AIM) and Electron-Localizability-Indicator (ELI-D) space partitioning schemes were used to produce Hirshfeld surfaces (HS), bond topological properties and basins of localized bonding and nonbonding electron pairs. Since 1c–5c are 35–58 kJ·mol–1 lower in energy than their counterparts 1a–5a, the hypothesis of a mainly repulsive peri-interaction in 1a/b–5a/b was confirmed. The shapes and contact patterns of the HSs of atoms and fragments involved in the peri-interactions (Br, P, E = CH3+, O, S, Se) reveal that only in 1a and 1b are peri-interactions exhibited between the bromine and the phosphorus atoms. In all other cases (2a/b–5a/b), the interaction mainly occurs between the bromine atom and the E atom/fragment. According to the bond topological properties and the electron populations within the (non)bonding ELI-D basins, which both are almost unaffected by the Br-P/E peri-interaction, sterical interactions are characterized essentially by geometrical and energetical changes.