• Ab initio calculations;
  • Density functional calculations;
  • Anions;
  • Homoaromaticity;
  • Molecular orbitals


A series of anions with general formula (R2C=P)2Pn (Pn = N, P, As, Sb; R = H, Me, CF3, SiH3, SiMe3, SiF3) was investigated theoretically at the DFT (RI-BP86/TZVP) and RI-MP2/TZVP levels of approximation. Significantly sharpened central PPnP bond angles were predicted for the species with π-acceptor R substituents. The corresponding bond angle values decrease in the order Pn = N >> P > As > Sb. This finding is in agreement with the previously published structural data for [(Me3Si)2C=P]2P. An island homoaromaticity was proposed as a suitable explanation for the observed effect. In more detail, it consists of shifting negative charge to the electron-withdrawing terminal groups with formation of a homologue of an aromatic heterocyclopropylium cation as the central moiety. Calculated structures and charge distributions are in agreement with the proposed model as well as with the molecular structure of the newly synthesized 2,4-diphospha-3-arsapentadienyl anion.