[Os(η⁵-B₅H₁₀)₂]: A carbon-free analog of ferrocene.

The synthesis of a carbon-free analog of ferrocene has been a long-standing goal in inorganic chemistry. We report here the preparation and crystallographic characterization of the carbon-free sandwich complex [Os(η5-B5H10)2], which is qualitatively similar to ferrocene in structure and bonding. The metal-ring bonding is much stronger in [Os(η5-B5H10)2], owing to rehybridization of the π molecular orbitals brought in by the bridging B-H-B hydrogens and the bent B-Ht bonds of the larger B5H10 ring (where B-Ht is the terminal B-H bond). This also leads to an unusually short ring-ring separation of 3.071 angstroms and a staggered conformation. The isomeric [Os(η5-B5H10)(η3-B5H10)], which features a coordination mode unknown for ferrocene, has also been isolated. A similar attempt for [Ru(η5-B5H10)2] resulted only in [Ru(η5-B5H10)(η3-B5H10)]. Editor's summary: Elucidation of the structure of ferrocene was a foundational event in the development of organometallic chemistry. Once it was clear that iron could be sandwiched between two flat rings of carbon, all manner of other sandwich compounds with numerous catalytic and electrochemical applications were produced. Inorganic chemists have long sought to replicate the bonding motif using elements other than carbon. Mohapatra et al. now report a ferrocene analog in which two boron rings sandwich an osmium ion. Each boron center in the pentagonal rings is bonded to its neighboring borons, as well as to one terminal and two bridging hydrides. —Jake S. Yeston [ABSTRACT FROM AUTHOR]