The nature of the multicenter bonding in π-[TCNE]22− dimer: 4c/2e, 12c/2e, or 20c/2e?

Abstract

The dianion dimer of tetracyanoethylene (TCNE), π-[TCNE]₂²⁻, represents an unusual class of organic compounds, that possess exceptionally long C–C bonding interactions (∼2.96 Å). It is twice that of conventional C–C bonds but shorter than the sum of the van der Waals radii. Experimental and computational studies best characterize the intradimer bonding as a multi-center C–C bond. A number of theoretical studies indicate that the π-[TCNE]₂²⁻ dimer exhibits long, four-centers/two-electron (4c/2e) C–C bonds. However, there is still some disputation about the number of centers involved in the multicenter bonding. This work focuses on quantitative understanding of the nature of the long, multicenter bonding in the π-[TCNE]₂²⁻ dimer. By the chemical bonding analysis, it is found that ∼68% of electrons in the long bond locate on the –CC– groups and the remaining locate on the –CN groups. The substituent effect is investigated by comparing the interaction energy curves of the [C₂X₄]₂²⁻ dimer (X = H, Cl and CN). The deep local well at 3.0 Å for X = CN indicates a strong bonding interaction. However, there is no obvious local well in the curves for X = H and Cl, which indicates that the [C₂Cl₄]₂²⁻ and [C₂H₄]₂²⁻ dimer are unstable. Herein, the –CN plays an important role in the bonding of the π-[TCNE]₂²⁻ dimer, and it is more reasonable to take the long bond as 20c/2e (12 carbon plus 8 nitrogen). Moreover, we found an abnormal charge redistribution from TCNE to TCNE⁻, and the charge distribution of [TCNE]₂²⁻ is very similar to that of TCNE⁻, which benefits the formation of the dianion dimer.