A C≡C Triple Bond as a Structural Anchor of Planar Pentacoordinate Carbon

The potential energy surface of C2Li4H3- was examined to identify stable structures containing planar hypercoordinate carbon centers. The lowest-energy form corresponds to a C2v-symmetric arrangement in which a Li4H3 framework encloses a C≡C triple bond. Bonding analyses (NBO, AdNDP, EDA, and IQA) show that the system consists of a C22- fragment interacting mainly through electrostatic attraction and weak covalent contributions with the Li4H3+ unit. Magnetic response calculations indicate localized electron density supported by the C≡C bond, contrasting with the delocalization mechanisms typically invoked for planar hypercoordinate carbons. Born-Oppenheimer molecular dynamics simulations confirm structural stability under thermal conditions. These results outline an alternative electronic route to achieve planar pentacoordination in carbon systems.