Abstract Addition of mono-dentate phosphines PMe 3 and PMe 2 Ph to the W(VI) alkyl alkylidyne complex W(CH 2 SiMe 3 ) 3 ( CSiMe 3 ) ( 1 ) is reversible, each reaching equilibrium. Thermodynamic studies of the equilibria have been conducted, giving Δ H ° = −10.0(1.1) kcal/mol and Δ S ° = −23(4) eu for the addition of PMe 3 and Δ H °′ = −3.0 (0.7) kcal mol −1 and Δ S °′ = −6(3) eu for the addition of PMe 2 Ph, indicating that the addition is exothermic. The experimental measurement allows a benchmarking study to select a proper DFT method to describe the current system. Of the DFT methods tested, M06 has demonstrated superior performance in calculating binding energy of a bimolecular reaction. The calculated reaction pathways show that W(CH 2 SiMe 3 ) 3 ( CSiMe 3 ) ( 1 ) reacts with PR 3 to form W(CH 2 SiMe 3 ) 3 ( CSiMe 3 )(PR 3 ) (PR 3 = PMe 3 , 3a ; PMe 2 Ph, 3b ), and the adduct then undergoes α-H migration to form W(CH 2 SiMe 3 ) 2 ( CHSiMe 3 ) 2 (PR 3 ) ( 4a , 4b ). 4a and 4b are found to be thermodynamically and kinetically stable intermediates. The calculations also suggest a pathway in the formation of the alkyl alkylidene alkylidyne complex W(CH 2 SiMe 3 )-( CHSiMe 3 )( CSiMe 3 )(PR 3 ) 2 ( 5a ).