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W(NPh)[o-(Me3SiN)2C6H4](CH2CMe3)2, 2h, is quantitatively converted to a new metallacyclopentane complex, W(CH2CH2CH2CH2)(NPh)[o-(Me3SiN)2C6H4], 4, upon the addition of ethylene at T > 70 °C. The complex W(CH2CH2CH2CH2)(NPh)[o-(Me3SiN)2C6H4], 4, was also formed by the transmetalation reaction of W(NPh)[o-(Me3SiN)2C6H4](Cl)2 with the corresponding Grignard reagent, BrMg(CH2)4MgBr, and also when W(NPh)[o-(Me3SiN)2C6H4](PMe3)(C(H)t-Bu), 3, reacts with ethylene. Mechanistic studies suggest that 2h and 3 react with ethylene through a common base-free alkylidene intermediate which undergoes metathesis with ethylene and then generates a W(IV) intermediate which is converted to 4 by coupling two molecules of ethylene. This reaction is a deactivation pathway when 3 is used as an olefin metathesis catalyst for terminal olefins because ethylene is generated during the reaction. The tungsten(IV) complex W(NPh)[o-(Me3SiN)2C6H4](PMe3)2(η2-C2H4), 5, was observed during the reaction between 3 and ethylene. Complex 5 can also be synthesized from the diethyl complex W(NPh)[o-(Me3SiN)2C6H4](CH2CH3)2, 2b, in the presence of PMe3. Addition of excess ethylene to 5 gives 4. The X-ray structure of 4 was determined.

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