Avoided spin coupling: An unexpected σ-σ diradical in global planar pentacoordinate carbon

We present a global planar pentacoordinate carbon (ppC) featuring a hitherto unreported σ-σ diradical characteristic. Using the multi-reference approach combined with CCSD(T)/aug-cc-pVTZ level, 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 2pz π electrons of C3Li3- are fully located at C3 ring formed by C-C multiple bonds, in contrast to the perfect 2pz π-delocalization found in the well-known ppCs.