COMMUNICATION Dehydrogenation, Methyl Elimination and Insertion Reactions of the Agostic Methyl-Bridged Complex [Mo2Cp2(- 1 : 2 -CH3)(- PtBu2)(-CO)] M. Angeles Alvarez, Melodie Casado-Ruano, M. Esther García, Daniel García-Vivó* and Miguel A. Ruiz* [a] Abstract: The high unsaturation of the title complex enabled it to react with a wide variety of molecules under mild conditions, whereby the agostic methyl ligand underwent unusual or unprecedented processes. Methane elimination occurred in the reactions with PPh2H and SiPh2H2, this being followed in the latter case by SiH bond oxidative addition to give the hydride silylene derivative [Mo2Cp2H(-PtBu2)(-SiPh2)(CO)]. Dehydrogenation, however, was the dominant process in the room temperature reaction with [Fe2(CO)9], to give the unsaturated methylidyne cluster [Mo2FeCp2(3-CH)(-PtBu2)(CO)5] (MoMo = 2.6770(8) Å). In contrast, PMe elimination took place in the reaction with P4, to give the unsaturated triphosphorus complex [Mo2Cp2(- 3 : 3 -P3)(- PtBu2)] (MoMo = 2.6221(3) Å). Yet a most remarkable reaction occurred with BH3·THF, this involving insertion of two BH3 units and dehydrogenation to yield [Mo2Cp2(-B2H4Me)(-PtBu2)(CO)], with the novel methyldiboranyl ligand acting as a 5-electron donor due to the presence of two 3-centre, 2-electron BHMo interactions, according to spectroscopic data and DFT calculations (MoMo ca. 2.65 Å). Alkyl complexes are a most important class of compounds within organometallic chemistry, and the reactivity of the corresponding  MC bonds is a central matter of the discipline, extensively studied in the case of mononuclear complexes. [1] The chemistry of binuclear complexes featuring bridging alkyl ligands, however, is comparatively less developed, even if such complexes might serve as models for the intermediate species involved in several processes of interest, these including olefin oligomerization and polymerization, alkyl transfer reactions, and heterogeneously catalyzed CO hydrogenation. [2,3] When compared to electron- precise analogues, it is expected that the reactivity of alkyl- bridged complexes might be further increased in molecules bearing multiple metal-metal bonds, because of the coordinative and electronic unsaturation of the dimetal centre in such species. However, complexes displaying alkyl ligands bridging over metal-metal multiple bonds are scarce, and their reactivity has been little explored. [3,4,5] This situation changed upon our synthesis of the dimolybdenum complexes [Mo2Cp2(- 1 : 2 - CH2R)(-PCy2)(CO)2] (R = H, Ph), which feature agostic alkyl ligands over shortened MoMo double bonds, [6] and turned to be highly reactive, they being able to undergo coupling reactions to CO, CNR and Cp ligands, as well as photochemical dehydrogenation and more complex processes. [7,8] It was then of interest to examine the reactivity of related agostic complexes at even more unsaturated centres, as it is the case of the monocarbonyl derivative [Mo2Cp2(- 1 : 2 -CH3)(-PCy2)(-CO)], which features an agostic methyl ligand bridging over an intermetallic triple bond, [8d] but these studies were hampered by the high air-sensitivity of this molecule. Recently, however, we found that the stability of this type of complexes could be significantly improved by using the bulkier PtBu2 ligand as support of the dimetal centre. [9] Indeed we found that the targeted methyl complex [Mo2Cp2(- 1 : 2 -CH3)(-PtBu2)(-CO)] (1) could be conveniently prepared in situ upon photolysis of the corresponding dicarbonyl precursor [Mo2Cp2(- 1 : 2 -CH3)(- PtBu2)(CO)2] (Scheme 1), and this gave us the opportunity to explore in detail its chemical behaviour. At the time, it was clear that compound 1 was significantly activated with respect to CH bond cleavage, since it undergoes clean intramolecular dehydrogenation upon gentle heating at 353 K to yield the corresponding methylidyne derivative, in contrast to the behaviour of dicarbonyl complex [Mo2Cp2(- 1 : 2 -CH3)(- PCy2)(CO)2], which only yielded methylidyne derivatives under photochemical activation. [8b] In this paper we report our preliminary results on the general reactivity of 1 which involves, inter alia, several unusual or unprecedented transformations of the methyl ligand, such as room temperature dehydrogenation, reductive elimination with phosphorus, and migratory insertion of borane. Scheme 1. Synthesis and thermal dehydrogenation of compound 1. The unsaturated nature of compound 1 enables the addition of a variety of donor molecules under mild conditions, with the methyl ligand being actively involved in all cases (Scheme 2). For instance, 1 reacts rapidly with PPh2H at room temperature, with concomitant methane elimination, to give the known phosphanide complex [Mo2Cp2(-PPh2)(-PtBu2)(-CO)] in high yield. [10,11] Even a weak donor molecule as diphenylsilane adds to compound 1 upon moderate heating (343 K). Methane elimination again takes place in this case, but also the oxidative addition of the second SiH bond, to give the hydride silylene [a] Dr. M. A. Alvarez, Ld. M. Casado-Ruano, Prof. Dr. M. E. García, Dr. D. García-Vivó, Prof. Dr. M. A. Ruiz Departamento de Química Orgánica e Inorgánica/IUQOEM Universidad de Oviedo E-33071 Oviedo, Spain E-mail: garciavdaniel@uniovi.es (D.G.V.), mara@uniovi.es (M.A.R.) Supporting information for this article is given via a link at the end of the document.