Reactions of methylphosphaalkyne, P CMe, with a digermene, R '' Ge-2=GeR ''(2) (R '' = -CH(SiMe3)(2)), and two distannenes, R '' Sn-2 SnR ''(2) and Ar'Sn-2 SnAr'(2) (Ar' = C6H2Pr3i-2,4,6), have given moderate to high yields of the first bridged 2,3,5,6-tetraphospha-1,4-dimethylidenecyclohexanes, [R2E{C(Me)(H)PC( CH2)P}](2) (R = R '' or Ar', E = Sn or Ge), all of which have been structurally characterized. Their mechanisms of formation are thought to involve successive [2 + 1] and [2 + 2] phosphaalkyne cycloaddition, heterocycle rearrangement, phosphaalkene/vinylphosphine tautomerization, and intermolecular hydrophosphination reactions. In one reaction, two intermediates have been spectroscopically observed and one trapped by coordination to one or two W(CO)(5) fragments, yielding the first diphosphagermole complexes, {[W(CO)(5)}(1or2){R '' Ge-2[C(Me)PC(Me)P]}], which have been structurally characterized. Differences between the reactivities of P CMe and P CBut are highlighted.