Stable ruthenium nanoparticles were prepared by passivation of the metal cores (diameter 2.7-3.2 nm by transmission electron microscopy) with ruthenium-carbon covalent bonds. Electrochemical study showed that the electronic conductivity of the particle films exhibited metal-like temperature dependence, and it decayed exponentially with the length of the alkyl spacer of the aliphatic protecting ligands, with an electronic coupling coefficient (beta) of 0.48 angstrom (1). This was ascribed to the strong Ru-C bonding interaction and low interfacial contact resistance where the spilling of core electrons into the organic protecting shell led to enhanced interparticle charge transfer. (c) 2008 Elsevier B.V. All rights reserved.