Phosphorus(III) double bonded systems possess two energetically closely spaced HOMOs, π and σ, and if the orbital sequence HOMO/LUMO is π/π*, a [2 + 2]cycloaddition route is followed, while with a σ/π* sequence [2 + 2] or [4 + 1] cycloadducts are formed; owing to σ and π orbital crossing, increasing the electronegativity of a π-donating ligand X favours the latter paths.
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Schoeller et al. (1982) studied this question.