η5‐P‐ or η4‐P‐Coordination in apically oxygenated phosphoranes? An ab initio study of PH4O−, PH4O− · E (E = Li+, NH4+, and HF) and related fluorinated oxyphosphoranes

Abstract

AbstractStructural optimizations of the apically substituted isomer of PH4O− and the diapically substituted isomer of PH3FO− with diffuse‐function augmented 3–21G* basis sets and with the 6–31 + G* and 6–31 + + G* basis sets result in P‐η4‐coordination in these anions. The structures obtained are those of a hydride or fluoride ion “solvated” by or complexed with phosphine oxide, rather than phosphoranes. In contrast, 3–21G* basis sets without diffuse functions on the atom in the trans‐apical position with regard to the oxy‐substituent yield P‐η5‐phosphorane structures that appear to be computational artifacts of the small basis set; the formation of the P‐η4‐geometries is curtailed by the insufficient functional description of the potential trans‐apical nucleophilic leaving group. The overall neutral apical isomers of PH4O− · E(E = Li+, NH4+), the diapical isomer of PH3FO−Li+, as well as the model‐solvated apical isomer of PH4O− · HF favor P‐η5‐phosphorane geometries at all of these computational levels. The mechanism by which the E‐group alters the electronic structures within PH4Oa− is discussed based on the geometries, the molecular orbitals, and electron density analysis techniques.