Credit: © 2008 ACS

Metallocenes — 'sandwich' compounds with metals facially bonded to two cyclopentadiene ligands — are very well-known. Their discovery prompted a renaissance in organometallic chemistry and they are now widely used in catalysis and materials chemistry. Examples include ferrocene in nonlinear optical materials, and cobaltocene in superconductors.

Now, Dermot O'Hare, Jennifer Green and colleagues at the University of Oxford have made1 a series of double metallocenes from first-row transition metals. The bread in the sandwich is permethylpentalene (analogous to two fused permethylated cyclopentadiene — Cp* — rings ) and the fillings range from vanadium to nickel. The interplay between atomic size and bonding requirements results in interesting structural differences. For example, in the vanadium compound the metal atoms bond to all five carbon atoms in each ring and triply bond to each other. At the other end of the scale, the nickel atoms bond to only three of the ring carbon atoms and not to each other. In between is a range of behaviour and metal–metal bond orders.

Surprisingly, given the stability of ferrocene compounds, the iron analogue could not be synthesized; instead a dimer of the ligands was formed. Density functional theory calculations revealed that this was because it would lead to an unstable monometallic radical and therefore loses the metal atom to form the dimer. In addition to the structural diversity discovered, a similarly wide range of magnetic, electronic and redox behaviour was observed.