The coordination chemistry of the previously neglected 2,4-di(tert-butyl)pentadienyl (Pdl′) ligand with transition metals is explored. Several open metallocenes [(η5-Pdl′)2M] (M=Ti (3), V (4) and Cr (5)) were prepared and characterized by various spectroscopic techniques and X-ray diffraction. Attempts to prepare the open ferrocene [(η5-Pdl′)2Fe] were hampered by its high thermal instability, which leads to the formation of iron metal and 2,4,7,9-tetra-tert-butyl-1,3,7,9-decatetraene (2). Nevertheless, the half-open ferrocene [(η5-Pdl′)Fe(η5-Cp″)] (Cp″=1,2,4-(Me3C)3C5H2, 6) and mono(pentadienyl) iridium complex [(η5-Pdl′)Ir(η4-cod)] (9) were successfully synthesized. The electronic and molecular structures of the half-open ferrocene [(η5-Pdl′)Fe(η5-Cp″)] (6) and its closed analogue [(η5-Cp′)Fe(η5-Cp″)] (7, Cp′=1,3-(Me3C)2C5H3) were compared. Complex 6 is significantly more encumbered than 7, as shown by the short Pdl′cent–Fe distance. Clean oxidation of 6 and 7 can be accomplished electrochemically and chemically to give the half-open ferrocenium 6+ and closed ferrocenium 7+, respectively, and in both cases steric strain is released upon oxidation. On heating of 6+ to 152°C in the solid state H2 and isobutene is eliminated to give [(η5-Cp′)2Fe]+ (8).