Structural Variability of R2C Adducts of 3a,6a-Diaza-1,4-diphosphapentalene: Tuning the NP Bonding

3a,6a-Diaza-1,4-diphosphapentalene (DDP) reacts with hexachlorocyclopentadiene to form a stable adduct (ClC)(4)C=DDP (4). The phosphorus atom involved into coordination has a pyramidal arrangement but retains partial double bonding with carbon [1.752(3) angstrom]. At the same time, the P-N bond remains covalent [1.824(3) angstrom]. The adduct 4 is better described as a zwitterionic compound with strongly delocalized positive and negative charges. A similar zwitterionic adduct DDP=C(CN)(2) was prepared by the reactions of dichloro-DDP (7) with malononitrile in the presence of Et3N. DFT calculations showed that related structures are formed in the case of the substituents (ClC)(4)C=, (HC)(4)C=, (NC)(2)C=, and (MeCO)(2)C=, possessing electron-delocalizing properties. Compounds with other R2C groups (R = Ph, Me, C6F5, Cl), possessing electronegative properties as well, but insufficient e-delocalization, demonstrate the noncovalent P-N bonding and a little shorter R2C-P bond lengths (ca. 1.70 angstrom).