Kondo coupling of $f$ and conduction electrons is a common feature of $f$-electron intermetallics. Similar effects should occur in carbon ring systems (metallocenes). Evidence for Kondo coupling in $\mathrm{Ce}({\mathrm{C}}_8{\mathrm{H}}_8{)}_2$ (cerocene) and the ytterbocene $\mathrm{Cp}^{*}{}_2\mathrm{Yb}(\mathrm{bipy})$ is reported from magnetic susceptibility and $L_{\mathrm{III}}$-edge x-ray absorption spectroscopy. These well-defined systems provide a new way to study the Kondo effect on the nanoscale, should generate insight into the Anderson Lattice problem, and indicate the importance of this often-ignored contribution to bonding in organometallics.

• Received 12 May 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.267202