An exotic 3-center/4-electron carbon–carbon pi long-bond: Is it tangible?

Abstract

Centrosymmetric species with AB2 boding motif where central atom (A) is relatively more electronegative than the end ligands (B atoms) have been predicted to exhibit an unusual form of three-center/four-electron (3c/4e) long-bonding where bonding/antibonding interactions occur between the two end ligands (B–B) bypassing the central atom albeit via sharing the latter’s lone pair. Such a long-bond of sigma- and pi-type has so far been predicted as single bonds and mainly as linear (B–A–B) bonding motifs. This work, through computations based on density-functional theory (DFT) and natural bond orbital (NBO) analysis, reveals pi-type long-bonding between carbon atoms separated by more than 2 angstroms in the gas-phase chain species, namely CnX (X = O,N; n = 2,4,6,8). Notably, this work, through DFT and NBO analysis, not only successfully predicts pi-type long-bonding in linear and nonlinear CnX species (mainly those where X is the central atom) but is also able to reveal multiple pi long-bonds particularly in the triplet-C2O as well as a pi long-bond in an isomer of C8N where N is not the central atom. However, preliminary computations using complete active space self-consistent field (CASSCF) multi-reference method performed in this work could only confirm antibonding pi long-bond orbital, whereas the validity of bonding pi long-bond orbital of the proposed long-bond could not be ascertained through the CASSCF.