Molybdenum complexes Schrock cycle

Fig. 2. The Schrock cycle with its intermediates, based on the molybdenum(III) complex [Mo(HIPTN3N)] containing the triamidoa-mine ligand HIPTN3N (hexaisopropylterphenyl-triamidoamine) (2).

In mechanistic studies, the molybdacyclobutane of a MAP catalyst was found to break up to ethylene/methylidene intermediates 4500 times faster than the corresponding tungstacycle (at 40°C) [19]. Syn and anti proton exchange were also found to be significantly faster (up to lOOx s) for molybdacycles than for tungsta-cycles. Methylidene rotation about the M=C bond was determined to be comparatively slower for molybdenum complexes (<0.2 s ) than for tungsten complexes (3.6-230 s ). Schrock and coworkers proposed that many of these properties contribute to the superior efficiency of the tungsten MAP catalysts relative to their molybdenum counterparts. 

Guha AK, Phukan AK. Why vanadium complexes perform poorly in comparison to related molybdenum complexes in the catalytic reduction of dinitrogen to ammonia (Schrock cycle) a theoretical study. Inorg Chem. 2011 50 8826-8833. 

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