Molybdenum-Boron Bonds in the Crystal – Structural Characterization of K[(η-C5H5)Mo(CO)3] and [(η-C5H5) (OC)3Mo-{B(NMe2)-B(NMe2)Br}]

Alkali metal salts M′[(η5-C5H5)M(CO)n] (M′ = alkali metal, M(CO)n = e. g. Fe(CO)2, Ru(CO)2, Mo(CO)3, W(CO)3) [1 – 7] are important starting materials for a wide range of half-sandwich complexes with covalent metal-metal and metal-element bonds. However, since these highly reactive anionic species are commonly prepared and reacted in situ, little is known about their solid-state structures, and only a few of them have been crystallographically characterized so far, such as [Li(TMEDA)2][(η-C5H5)Mo(CO)3] [4] and K[(η-C5H5)Fe(CO)2] [1]. In the case of the former, the coordination of the alkali metal cation by suitable ligands (in this case: TMEDA) commonly imposes the formation of ion pairs in the solid state. In contrast, K[(η-C5H5)Fe(CO)2] [1] represents a very particular case as it forms a two-dimensional network in the crystal, due to the absence of coordinating ligands at the potassium cation. We and others have frequently employed the molybdenum species [Li(TMEDA)2][(η-C5H5)Mo(CO)3] for the preparation of corresponding half-sandwich complexes with Mo–B bonds [8 – 14]. While the bonding patterns such as boryl [4], diboran(4)yl [9, 15] as well as base-stabilized boryl complexes [16], have been unequivocally established in solution by IR and multinuclear NMR spectroscopy, very few crystallographic data have been published on molybdenum boryl complexes of the general formula