The mechanisms of the reactions of W and W ⁺ with CO _x (x = 1, 2): A computational study

The mechanisms of the reactions of W and W ⁺ with CO _x (x = 1, 2) were studied at the CCSD(T)/[SDD + 6-311G(d)]//B3LYP/[SDD + 6-31G(d)] level of theory. It was shown that the gas-phase reaction of W with CO ₂ proceeds with a negligible barrier via an insertion pathway, W( ⁷S) + CO ₂( ¹A ₁) -W(η ²-OCO)( ⁶A′) ′ OW(η ¹-CO) ( ¹A) → WO ( ³Σ ⁺) + CO( ¹Σ). This oxidation process is calculated to be exothermic by 32.4 kcal/mol. Possible intermediates of this reaction are the W(η ²-OCO) and OWCO complexes, among which the latter is 37.4 kcal/mol more stable and lies 39.7 and 7.3 kcal/mol lower than the reactants, W( ⁷S) + CO ₂( ¹A ₁), and the products, WO ( ³Σ ⁺) + CO( ¹Σ), respectively. The barrier separating W(η ²-OCO) from OWCO is 8.0 kcal/mol (relative to the W(η ²-OCO) complex), which may be characterized as a W ^+δ-(CO ₂) ^-δ charge-transfer complex. Ionization of W does not change the character of the reaction of W with CO ₂: the reaction of W ⁺ with CO ₂, like its neutral analog, proceeds via an insertion pathway and leads to oxidation of the W-center. The overall reaction W ⁺( ⁶D) + CO ₂( ¹A ₁) → W(η ¹-OCO) ⁺( ⁶A) → OW(η ¹-CO) ⁺( ⁴A) → WO ⁺-( ⁴S ⁺) + CO( ¹Σ) is calculated to be exothermic by 25.4 kcal/mol. The cationic reaction proceeds with a somewhat large (9.9 kcal/mol) barrier and produces two intermediates, W(η ¹-OCO) ⁺( ⁶A) and OW(η ¹-CO) ⁺-( ⁴A). Intermediate W(η ¹-OCO) ⁺( ⁶A) is 20.0 kcal/mol less stable than OW(η-CO) ⁺( ⁴A), and separated from the latter by a 35.2 kcal/mol barrier. Complex W(η ¹-OCO) ⁺( ⁶A) is characterized as an ion-molecular complex type of W ⁺-(CO ₂). Gas-phase reactions of M=W/W ⁺ with CO lead to the formation of a W-carbonyl complex M(η ¹-CO) for both M=W and W ⁺. The C-O insertion product, OMC, lies by 5.2 and 69.3 kcal/mol higher than the corresponding M(η ¹-CO) isomer, for M=W and W ⁺, respectively, and is separated from the latter by a large energy barrier.