Synthesis, Solid‐State Structure, and Bonding Analysis of the Beryllocenes [Be(C5Me4H)2], [Be(C5Me5)2], and [Be(C5Me5)(C5Me4H)]

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Abstract
The beryllocenes [Be(C5Me4H)2] (1), [Be(C5Me5)2] (2), and [Be(C5Me5)(C5Me4H)] (3) have been prepared from BeCl2 and the appropriate KCp′ reagent in toluene/diethyl ether solvent mixtures. The synthesis of 1 is facile (20 °C, overnight), but generation of decamethylberyllocene 2 demands high temperatures (ca. 115 °C) and extended reaction times (3–4 days). The mixed-ring beryllocene 3 is obtained when the known [(η5-C5Me5)BeCl] is allowed to react with K[C5Me4H], once more under somewhat forcing conditions (115 °C, 36 h). The structures of the three metallocenes have been determined by low-temperature X-ray studies. Both 1 and 3 present η5/η1 geometries of the slip-sandwich type, whereas 2 exhibits an almost regular, ferrocene-like, sandwich structure. In the mixed-ring compound 3, C5Me5 is centrally bound to beryllium and the η1-C5Me4H ring bonds to the metal through the unique CH carbon atom. This is also the binding mode of the η1-ring of 1. To analyze the nature of the bonding in these molecules, theoretical calculations at different levels of theory have been performed on compounds 2 and 3, and a comparison with the bonding in [Be(C5H5)2] has been made. As for the latter molecule, energy differences between the η5/η5 and the η5/η1 structures of 2 are very small, being of the order of a few kcal mol−1. Constrained space orbital variations (CSOV) calculations show that the covalent character in the bonding is larger for [Be(C5Me5)2] than for [Be(C5H5)2] due to larger charge delocalization and to increased polarizability of the C5Me5 fragment.