Aspects of the chemistry of zero-valent ruthenium complexes having a cyclic diene ligand, and their use for homo- and cross-dimerizations are reviewed. The naphthalene ligand in [Ru(η⁶-naphthalene)(η⁴-1,5-COD)] is readily removed to generate a formal 12e species, “Ru(η⁴-1,5-COD)”, having 6e coordination sites. These 6e coordination sites can be divided into 4e and 2e sites, and a conjugated compound and substituted alkene therefore selectively coordinate to them to satisfy the 18e rule. The zero-valent ruthenium center is highly Lewis basic and the oxidative coupling reaction readily occurs to form a ruthenacycle. The ruthenium(0) complexes catalyze homo- and cross-dimerizations by the oxidative coupling mechanism and some of the solid evidence in support of the mechanism is summarized. The coordination of substrates is prochiral-selective and when a chiral cyclic diene is incorporated in the catalyst, the first enantioselective cross-dimerizations are achieved.