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Keywords:

  • bonding analysis;
  • carbon;
  • density functional calculations;
  • donor–acceptor systems;
  • protonation

Abstract

Quantum-chemical calculations with DFT (BP86) and ab initio methods (MP2, SCS-MP2) were carried out for protonated and diprotonated compounds N-H+ and N-(H+)2 and for the complexes N-BH3, N-(BH3)2, N-CO2, N-(CO2)2, N-W(CO)5, N-Ni(CO)3 and N-Ni(CO)2 where N=C(PH3)2 (1), C(PMe3)2 (2), C(PPh3)2 (3), C(PPh3)(CO) (4), C(CO)2 (5), C(NHCH)2 (6), C(NHCMe)2 (7) (Me2N)2C[DOUBLE BOND]C[DOUBLE BOND]C(NMe2)2 (8) and NHC (9) (NHCH=N-heterocyclic carbene, NHCMe=N-substituted N-heterocyclic carbene). Compounds 14 and 69 are very strong electron donors, and this is manifested in calculated protonation energies that reach values of up to 300 kcal mol−1 for 7 and in very high bond strengths of the donor–acceptor complexes. The electronic structure of the compounds was analyzed with charge- and energy-partitioning methods. The calculations show that the experimentally known compounds 25 and 8 chemically behave like molecules L2C which have two L→C donor–acceptor bonds and a carbon atom with two electron lone pairs. The behavior is not directly obvious when the linear structures of carbon suboxide and tetraaminoallenes are considered. They only come to the fore on reaction with strong electron-pair acceptors. The calculations predict that single and double protonation of 5 and 8 take place at the central carbon atom, where the negative charge increases upon subsequent protonation. The hitherto experimentally unknown carbodicarbenes 6 and 7 are predicted to be even stronger Lewis bases than the carbodiphosphoranes 13.