Dative versus electron-sharing bonding in N-imides and phosphane imides R ₃ ENX and relative energies of the R ₂ EN(X)R isomers (E = N, P; R = H, Cl, Me, Ph; X = H, F, Cl) ^*

Quantum chemical calculations using density functional theory BP86 and M06-2X functionals in conjunction with def2-TZVPP basis sets have been carried out on the title molecules. The calculation results reveal that the N-imides R ₃ NNX are always clearly higher in energy than the imine isomers R ₂ NN(X)R. In the case of phosphane imides R ₃ PNX and the isomers R ₂ PN(X)R, the substituent R plays a critical role in determining their relative stabilities. When R is hydrogen or phenyl group, R ₃ PNX are always higher in energy than R ₂ PN(X)R but the former are more stable than the latter when R = Cl. Interestingly, the Me ₃ PNX and Me ₂ PN(X)Me are quite close in energy. The energy decomposition analysis suggests that the P–N bond in the phosphane imides R ₃ PNX (R = H, Cl, Me, Ph; X = H, F, Cl) should be described in terms of an electron-sharing single bond between two charged fragments R ₃ P ⁺ -NX ⁻ that is supported by (R ₃ P) ⁺ ←(NX) ⁻ π-backdonation. The π-bond contributes 14–21% of the total orbital interactions while the σ-bond provides 60–68% of ΔE _orb .