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Vanadium‐based catalysts have shown activity and selectivity in ring‐opening metathesis polymerization of strained cyclic olefins comparable to those of Ru, Mo, and W catalysts. However, the application of V alkylidenes in routine organic synthesis is limited. Here, we present the first example of ring‐closing olefin metathesis catalyzed by well‐defined V chloride alkylidene phosphine complexes. The developed catalysts exhibit tolerance to various functional groups, such as an ether, an ester, a tertiary amide, a tertiary amine, and a sulfonamide. The size and electron‐donating properties of the imido group and the phosphine play a crucial role in the stability of active intermediates. Reactions with ethylene and olefins suggest that both β‐hydride elimination of the metallacyclobutene and bimolecular decomposition are responsible for catalyst degradation.

3,3′,5,5′‐Tetra‐tert‐butyl‐2′‐sulfanyl[1,1′‐biphenyl]‐2‐ol (H₂[^tBu₄OS]) was prepared in 24 % yield overall from the analogous biphenol using standard techniques. Addition of H₂[^tBu₄OS] to Mo(NAr)(CHCMe₂Ph)(2,5‐dimethylpyrrolide)₂led to formation of Mo(NAr)(CHCMe₂Ph)[^tBu₄OS], which was trapped with PMe₃to give Mo(NAr)(CHCMe₂Ph)[^tBu₄OS](PMe₃) (1(PMe₃)). An X‐ray crystallographic study of1(PMe₃) revealed that two structurally distinct square pyramidal molecules are present in which the alkylidene ligand occupies the apical position in each. Both1(PMe₃)_Aand1(PMe₃)_Bare disordered. Mo(NAd)(CHCMe₂Ph)(^tBu₄OS)(PMe₃) (2(PMe₃); Ad=1‐adamantyl) and W(NAr)(CHCMe₂Ph)(^tBu₄OS)(PMe₃) (3(PMe₃)) were prepared using analogous approaches.1(PMe₃) reacts with ethylene (1 atm) in benzene within 45 minutes to give an ethylene complex Mo(NAr)(^tBu₄OS)(C₂H₄) (4) that is isolable and relatively stable toward loss of ethylene below 60 °C. An X‐ray study shows that the bond distances and angles for the ethylene ligand in4are like those found for bisalkoxide ethylene complexes of the same general type. Complex1(PMe₃) in the presence of one equivalent of B(C₆F₅)₃catalyzes the homocoupling of 1‐decene, allyltrimethylsilane, and allylboronic acid pinacol ester at ambient temperature.1(PMe₃),2(PMe₃), and3(PMe₃) all catalyze the ROMP ofrac‐endo,exo‐5,6‐dicarbomethoxynorbornene (rac‐DCMNBE) in the presence of B(C₆F₅)₃, but the polyDCMNBE that is formed has a random structure.