Quantum chemical calculations at the BP86/TZVPP//BP86/SVP level of theory have been performed for the isoelectronic series of compounds [(PPh3 )2 C→EH2 ]q (Eq =Be, B+ , C2+ , N3+ , O4+ ). The equilibrium geometries and bond dissociation energies were calculated and the nature of the C→E bond was investigated with charge and energy decomposition methods. The dication [(PPh3 )2 C→CH2 ]2+ could become isolated as a salt compound with two counter ions [AlBr4 ]- . The X-ray structure analysis of [(PPh3 )2 C→CH2 ]2+ gave bond lengths and angles that are in good agreement with the calculated data. The geometry optimization of [(PPh3 )2 C→OH2 ]4+ gave [(PPh3 )2 C→OH]3+ as the equilibrium structure. Bonding analysis of [(PPh3 )2 C→EH2 ]q shows that [(PPh3 )2 C→BeH2 ] and [(PPh3 )2 C→BH2 ]+ possess donor-acceptor bonds in which the σ and π lone-pair electrons of (PPh3 )2 C donate into the vacant orbitals of the acceptor fragment. The multiply charged compounds are better described as substituted olefins [(PPh3 )2 CCH2 ]2+ , [(PPh3 )2 CNH2 ]3+ , and [(PPh3 )2 COH]3+ , which possess electron-sharing σ and π bonds that arise from the interaction between the triplet states of [(PPh3 )2 C]2+ and the respective fragment CH2 , (NH2 )+ , and (OH)+ . The multiply charged cations [(PPh3 )2 CCH2 ]2+ , [(PPh3 )2 CNH2 ]3+ , and [(PPh3 )2 COH]3+ are calculated to be stable toward dissociation.
Keywords: bonding analysis; carbodiphosphoranes; cations; density functional calculations; donor-acceptor systems.
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