Avoided spin coupling: an unexpected σ–σ diradical in global planar pentacoordinate carbon

Abstract

We present a global planar pentacoordinate carbon (ppC) featuring a hitherto unreported σ–σ diradical characteristic. Using the multi-reference approach combined with the CCSD(T)/aug-cc-pVTZ method, the ppC C₃Li₃⁻ was found to be an intriguing triplet ground state, in which the unpaired density is mostly located at three Li ligands. Chemical bonding analysis reveals that the 2p_z π electrons of C₃Li₃⁻ are fully located at the C₃ ring formed by C–C multiple bonds, in contrast to the perfect 2p_z π-delocalization found in the well-known ppCs.