Dative versus electron-sharing bonding in N-oxides and phosphane oxides R3EO and relative energies of the R2EOR isomers (E = N, P; R = H, F, Cl, Me, Ph). A theoretical study

Quantum chemical calculations using ab initio methods at the CCSD(T)/def2-TZVPP level and density functional theory using BP86 and M06-2X functionals in conjunction with def2-TZVPP basis sets have been carried out on the title molecules. The calculated energies suggest that the N-oxides R3NO with R = F, Cl are lower in energy than the amine isomers R2NOR, but the latter form is more stable than the N-oxides when R = H, Me, Ph. In contrast, the phosphane oxides R3PO are always more stable than the phosphanyl isomers R2NOR except for the parent system with R = H, where the two isomers are close in energy. The energy decomposition analysis suggests that the best description of the N-O bond in N-oxides R3NO depends on the nature of the substituent R. The halogen systems F3NO and CI3NO and the triphenyl species Ph3NO possess dative bonds R3N -> O, which are enhanced by R3N <- O pi backdonation. The contribution of the pi backdonation is only 10% of the total orbital interactions Delta E-orb in Ph3NO, but it amounts to similar to 22% of Delta E-orb in F3NO and CI3NO. The N-O bonds H3NO and Me3NO are better described in terms of electron-sharing single bonds between charged fragments R3N+-O, which are supported by modest R3N+<- O- pi backdonation that comprise 13-16% of Delta E-orb. In contrast, all phosphane oxides R3PO are best depicted with electron-sharing single bonds between charged fragments R3P+-O-, which are significantly supported by R3P+<- O- pi backdonation contributing 22-32% of Delta E-orb.