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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 101; no. 35; pp. 6252 - 6264
• Main Authors: Kretzschmar, Ilona, Fiedler, Andreas, Harvey, Jeremy N, Schröder, Detlef, Schwarz, Helmut
• Format: Journal Article
• Language: English
• Published: American Chemical Society 28.08.1997
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp971941+

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 (4Σ-) ground states that formally arise from spin-pairing of Mo+ (6S) with O (3P) and S (3P), respectively. Similarly, the bent triatomic MoX2 + cations exhibit doublet ground states (2A1). The trichalcogenides MoO3 + and MoS3 + also have doublet ground states and exhibit similar C 3 v -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 (IEs) 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(MoS3) = 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 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 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.