[Os(η 5 -B 5 H 10 ) 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-H_t bonds of the larger B₅H₁₀ ring (where B-H_t 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)].