Computational and experimental approaches to the molecular structure of the HCl adduct of Me3PO by Andreas Orthaber and others (2010)

Andreas Orthaber, Ferdinand Belaj and Rudolf Pietschnig, Comptes Rendus Chimie 13 (2010), 923–928. Section 3.2 analyses the strongly polar P–O sigma bond and oxygen lone-pair negative hyperconjugation; this is not a claim that every orbital-localization scheme assigns the same bond order.