Transition-metal cyclopentadienyl sandwich complexes, better known as metallocenes, have given organometallic chemists a rich assortment of electronic and magnetic properties to explore ever since ferrocene was discovered a half-century ago. Jennifer C. Green, Dermot O'Hare, and colleagues at the University of Oxford have now doubled chemists' pleasure by conducting the first comprehensive study of double metallocenes across a periodic row (J. Am. Chem. Soc., DOI: 10.1021/ja8057138). Double metallocenes, M2(C8H6)2, which were first reported in the 1970s, consist of two metal atoms flanked by pentalene ligands (fused cyclopentadienyls). The Oxford team worked through what proved to be difficult syntheses to make V, Cr, Mn, Co, and Ni permethylpentalene complexes and extensively studied their electronic, magnetic, and structural trends. Structurally, the vanadium complex (shown) displays the classic metallocene fivefold bonding interaction between the metal and ligand. It also forges an unusual V≡V bond. At the other extreme, nickel (shown) exhibits only a threefold bonding interaction with the pentalenes and does not appear to form a metal-metal bond. Iron surprisingly doesn't form a double ferrocene, the researchers found, which is ironic because ferrocene is the prototypical metallocene and its study gave birth to modern organometallic chemistry.