Bis(diphenylphosphino)methane (1 equiv.) and dimethyl acetylenedicarboxylate (1 equiv.) react to give the 5H-diphosph(V)ole (6). In contrast to the behaviour of the acyclic analogues (1,2-alkylidenediphosphoranes of the type PC–CP), prototropy involving the methylene group allows the phosphoranyl groupings to attain PC–PC conjugation. Solvent and variable temperature effects on the ¹H n.m.r. spectra indicate the presence of two conformers resulting from restricted rotation of one ester group. The ratio of the conformers is solvent dependent. Evidence is also presented for the formation of an unstable derivative of the 1,4-diphosph(V)orin (12a) from cis-1,2-bis(diphenylphosphino)ethylene and dimethyl acetylenedicarboxylate.