Effects of sequential ligation of molybdenum cation by chalcogenides on electronic structure and gas-phase reactivity

The molybdenum chalcogenide cations MoXn+ (X = O, S; n = 1-3) are studied by a combined experimental and theoretical approach. The monoligated species MoO+ and MoS+ both have ((4) Sigma(-)) ground states that formally arise from spin-pairing of Mo+ (S-6) With O (P-3) and S (P-3), respectively. Similarly, the bent triatomic MoX2+ cations exhibit doublet ground states ((2)A(1)). The trichalcogenides MoO3+ and MoS3+ also have doublet ground states and exhibit similar C-3v-symmetrical structures; however, distinct energetic differences are found in that MoO3+ is much less stable than MoS3+, due to the necessity to ionize a strong Mo-O double bond in neutral MoO3. 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 (Ifs) increase with the electronegativity and the number of the chalcogenide atoms added: IE(MoO) = 7.9 +/- 0.3 eV, IE(MoO2) = 8.7 +/- 0.3 eV, IE(MoO3) = 11.7 +/- 0.3 eV, IE(MoS) = 7.7 +/- 0.3 eV, IE(MoS2) = 8.6 +/- 0.3 eV, and IE(MoO2) = 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, O2Mo+-O 62 +/- 17, Mo+-S 88 +/- 14, SMo+-S 100 +/- 14, and S2Mo+-S 88 +/- 14. Notable differences are observed in the gas-phase reactivity of the MoXn+ 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 MoX2+ cations and also MoS3+ are somewhat less reactive. The high-valent transition-metal oxide MoO3+ is the most reactive species and is even capable of activating methane.