Synthesis and Structure of {η5-C5H5Cr}2{μ-η6:η6-(μ-1,2-C3H6-1,2-C2B4H4)}. An Unusual 24-Electron Triple-Decker Sandwich Complex Containing a Metal-Stabilized, Planar Tetraborabenzene

The 24-valence-electron triple-decker sandwich complex {η5-C5H5Cr}2{μ-η6:η6-(μ-1,2-C3H6-1,2-C2B4H4)}, 2, with an unprecedented C2B4 planar central ring, has been characterized. Isolated from a reaction analogous to that which produces the cluster {η5-C5Me5Cr}2B4H8, 2 provides an example of the stabilization of a planar C2B4H4R2 acceptor fragment by coordination to two donor fragments.