Comparative analysis of the multicenter, long bond in [TCNE]*- and phenalenyl radical dimers: a unified description of multicenter, long bonds.

The nature of the multicenter, long bond in neutral phenalenyl dimers is analyzed in detail and compared to the multicenter, long bond in [TCNE](2)(2-). These dimers are prototypes of multicenter, long bond in dimers of neutral and anion radicals. This was done by examining the number of electrons (m) and atomic centers (c) involved in the long bond for these dimers, as well as identifying the dominant attractive components of their interaction energy (SOMO-SOMO bonding, dispersion, and the sum of the exchange-repulsion and electrostatic components) in accord with Pauling's focus on total bond energies. The long bond in [TCNE](2)(2-) is a 2e(-)/4c bond, the electrostatic component is repulsive, and the dominant attractive component is the dispersion component (-27.7 kcal/mol), about two times larger than the bonding component. In phenalenyl dimers the dispersion component (-31.7 kcal/mol) is about 2.5 times stronger (than the SOMO-SOMO bonding component; hence, the multicenter, long bond in these dimers is closer to a van der Waals bond than to a covalent bond. Consequently, it possesses a two-electrons/fourteen center 2e(-)/14c bond, rather than the 2e(-)/12c bond suggested by the SOMO-SOMO bonding component. The covalent-like properties in phenalenyl dimers result from the dominant dispersion component that enable the fragments to approach each other so that their SOMOs overlap and produce a qualitative MO diagram identical to that found in conventional covalent bonds.