Effects of Sequential Ligation of Molybdenum Cation by Chalcogenides on Electronic Structure and Gas-Phase Reactivity

The molybdenum chalcogenide cations MoX n + (X = O, S; n = 1−3) are studied by a combined experimental and theoretical approach. The monoligated species MoO + and MoS + both have ( 4 Σ - ) ground states that formally arise from spin-pairing of Mo + ( 6 S) with O ( 3 P) and S ( 3 P), respectively. Similarly, the bent triatomic MoX 2 + cations exhibit doublet ground states ( 2 A 1 ). The trichalcogenides MoO 3 + and MoS 3 + also have doublet ground states and exhibit similar C 3 v -symmetrical structures; however, distinct energetic differences are found in that MoO 3 + is much less stable than MoS 3 +, due to the necessity to ionize a strong Mo−O double bond in neutral MoO 3 . Sequential addition of chalcogenides to molybdenum goes hand in hand with an increase of the formal oxidation state of the metal. As a result, the ionization energies (IEs) increase with the electronegativity and the number of the chalcogenide atoms added: IE(MoO) = 7.9 ± 0.3 eV, IE(MoO 2 ) = 8.7 ± 0.3 eV, IE(MoO 3 ) = 11.7 ± 0.3 eV, IE(MoS) = 7.7 ± 0.3 eV, IE(MoS 2 ) = 8.6 ± 0.3 eV, and IE(MoS 3 ) = 8.9 ± 0.3 eV. Thermochemical considerations in conjunction with the ion/molecule reaction bracketing technique and theoretical results provide reevaluated values for the bond dissociation energies (in kcal/mol): Mo + −O 118 ± 2, OMo + −O 131 ± 5, O 2 Mo + −O 62 ± 17, Mo + −S 88 ± 14, SMo + −S 100 ± 14, and S 2 Mo + −S 88 ± 14. Notable differences are observed in the gas-phase reactivity of the MoX n + cations. In general, the molybdenum sulfides are less reactive than the corresponding oxides. The monoligated MoX + cations promote C−H bond activation of hydrocarbons, while the MoX 2 + cations and also MoS 3 + are somewhat less reactive. The high-valent transition-metal oxide MoO 3 + is the most reactive species and is even capable of activating methane.