text: Starting from the late 1980s, more modern calculations and the lack of spectroscopic evidence reveal that the P 3d contribution is negligible, invalidating the earlier hypothesis. Instead, a charge separated model is generally accepted.
  According to this description, the P–N bond is viewed as a very polarised one (between notional  and ), with sufficient ionic character to account for most of the bond strength.
  The rest (~15%) of the bond strength may be attributed to a negative hyperconjugation interaction: the N lone pairs can donate some electron density into π-accepting σ* molecular orbitals on the P.