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Reactions between tungsten alkylidyne [tBuOCO]W≡CtBu(THF)2 1 and sulfur containing small molecules are reported. Complex 1 reacts with CS2 to produce intermediate η2 bound CS2 complex [O2C(tBuC═)W(η2-(S,C)-CS2)(THF)] 8. Heating complex 8 provides a mixture of a monomeric tungsten sulfido complex 9 and a dimeric complex 10 in a 4:1 ratio, respectively. Heating the mixture does not perturb the ratio. Addition of excess THF in a solution of 9 and 10 (4:1) converts 10 to 9 (>96%) with concomitant loss of (CS)x. Both 9 and 10 can be selectively crystallized from the mixture. An alternative synthesis of exclusively monomeric 9 involves the reaction between 1 and PhNCS. Demonstrating ring expansion metathesis polymerization (REMP), tethered tungsten alkylidene 8 polymerizes norbornene to produce cis-selective syndiotactic cyclic polynorbornene (c-poly(NBE)). 

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.