This chapter summarizes developments in the study of (transition) metal clusters held by metal-metal bonds. Similar to the case of boranes and carboranes, electron-counting rules have been devised to correlate the bonding and structure of metal clusters. Three sections are devoted to clusters with two to seven metal atoms, followed by a section on iso-bond valence and iso-structural series. The final section covers selected areas of metal-metal interaction, including aurophilicity, argentophilicity, metal string molecules, and metal-based infinite chains and networks.
Keywords: argentophilicity, aurophilicity, bond valence, metal-metal bond, high-nuclearity clusters, interstitial heteroatoms, iso-bond valence, iso-structural clusters, quadruple bond, quintuple bond