Synthesis and Crystal Structure of the “Open” Calcocene [( t Bu 2 C 5 H 5 ) 2 Ca · thf]

Calcium behaves more like a transition metal than a main‐group element when it forms an “open” metallocene with the 2,4‐di‐ tert ‐butylpentadienyl ligand. Like most transition metal pentadienyl complexes, the title compound 1 (shown C on the right) contains pentahapto‐ rather than monohapto‐bound ligands. Unlike its transition metal counterparts, however, the metal–carbon bonds in 1 are noticeably longer than those in the analogous cyclopentadienyl complexes. Furthermore, the pentadienyl ligands in 1 have an anti ‐eclipsed arrangement, which is unprecedented for open metallocenes.