We have used an electromigration technique to prepare single-molecule transistors incorporating a family of transition metal coordination complexes. In a number of these devices, we observe the Kondo effect, the formation of a correlated many-body state comprising a spin localized to the molecule coupled to the conduction electrons in the Au leads. We will discuss the temperature dependence of the width and height of the Kondo resonance, and its variation as a function of transition metal. In devices based on Co-containing molecules, Kondo temperatures (approx 70 K) comparable to those seen in scanning tunneling microscopy of Co atoms on bare Au are observed. The implications of these results, and the prospects of using single-molecule transistors as model quantum impurity systems will be discussed.
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