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Abstract The reactivity of phosphaalkynes, the isolobal and isoelectronic congeners to alkynes, with metal alkylidyne complexes is explored in this work. Treating the tungsten alkylidyne [^tBuOCO]W≡C^tBu(THF)₂(1) with phosphaalkyne (10) results in the formation of [O₂C(^tBuC=)W{η²‐(P,C)−P≡C−Ad}(THF)] (13‐^tBu_THF) and [O₂C(AdC=)W{η²‐(P,C)−P≡C−^tBu}(THF)] (13‐Ad_THF); derived from the formal reductive migratory insertion of the alkylidyne moiety into a W−C_arenebond. Analogous to alkyne metathesis, a stable phosphametallacyclobutadiene complex [^tBuOCO]W[κ²‐C(^tBu)PC(Ad)] (14) forms upon loss of THF from the coordination sphere of either13‐^tBu_THFor13‐Ad_THF. Remarkably, the C−C bonds reversibly form/cleave with the addition or removal of THF from the coordination sphere of the formal tungsten(VI) metal center, permitting unprecedented control over the transformation of a tetraanionic pincer to a trianionic pincer and back. Computational analysis offers thermodynamic and electronic reasoning for the reversible equilibrium between13‐^tBu/Ad_THFand14.