Differences in the crystal structures of two dialkyl diester triphenyl­phospho­nium ylids

Hydrogen bonding and crystal packing play major roles in determining the conformations of ethyl methyl 2‐(triphenyl­phospho­ranyl­idene)malonate, Ph₃P=C(CO₂CH₃)CO₂CH₂CH₃ or C₂₄H₂₃O₄P, (I), and dimethyl 2‐(triphenyl­phosphor­anyl­idene)malonate, Ph₃P=C(CO₂CH₃)₂ or C₂₃H₂₁O₄P, (II). In (I), the acyl O atom of the ethyl ester group is anti to the P atom, while the O atom of the methyl ester group is syn. In (II), the dimethyl diester is a 1:1 mixture of anti–anti and syn–anti conformers.