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The reaction proceeds via the unstable molybdenum(V) bromide, which releases bromine at room temperature.
Molybdenum(IV) bromide can also be prepared by oxidation of molybdenum(III) bromide with bromine.
References
Calderazzo, Fausto; Maichle-Mössmer, Cäcilie; Pampaloni, Guido; Strähle, Joachim (1993). "Low-Temperature Syntheses of Vanadium(III) and Molybdenum(IV) Bromides by Halide Exchange". J. Chem. Soc., Dalton Trans. (5): 655–658. doi:10.1039/DT9930000655.
Georg Brauer (Hrsg.) u. a.: Handbuch der Präparativen Anorganischen Chemie. 3., umgearbeitete Auflage. Band III, Ferdinand Enke, Stuttgart 1981, ISBN3-432-87823-0, S. 1537.