The reaction of excess P≡CMe with the triphosphabenzene, 1,3,5-P3C3But3, yields a phosphaalkyne pentamer, P5C5Me2But3, which displays a pentaphosphaisolumibullvalene core structure. Its treatment with [W(CO)5(THF)] gives a complex of this cage, [(W(CO)5)2(µ-η1:η1-P5C5Me2But3)], which has been structurally characterised. In contrast, the previously reported reaction of P≡CBut with 1,3,5-P3C3But3, affords, in addition to the known tetraphosphabarrelene, 1,3,5,7-P4C4But4, a new phosphaalkyne pentamer (P5C5But5), which has a partially unsaturated “open cage” core. Although P≡CBut does not react with 1,3,5,7-P4C4But4, the reaction of P≡CMe with the tetraphosphabarrelene is shown to give a mixture of products. Treatment of these with [W(CO)5(THF)] leads to the isolation of the tungsten carbonyl complex, [(W(CO)5)(W(CO)4)(µ-η1:η4-P5C5MeBut4)], which has been structurally characterised. This study suggests that P≡CMe has a significantly greater reactivity towards cycloadditions than its bulkier counterpart, P≡CBut.