Replacement of the naphthalene ligand in ruthenium complex [CpRu(C10H8)]+ (1) by halide anions readily proceeds at room temperature to give insoluble oligomeric species [CpRuX]n (X = Cl, Br, I). Similar reactions in the presence of mono- or bidentate ligands afford complexes [CpRuL2X] where L = CO, P(OMe)3, tBuNC; L2 = dppm, dppe, dppp, bipy, phen, cod, nbd, or 1,4-diphenylbutadiene. Useful catalysts [CpRu(cod)X] were obtained by this method in 70–90 % yields. The structure of [CpRu(cod)I] was determined by X-ray diffraction. Reaction of 1 with Br– and allyl bromide afforded RuIV complex [CpRu(η3-C3H5)]Br2. Cation 1 also was found to react with azide anion in the presence of bidentate phosphanes to afford [CpRuL2N3] (L2 = dppm, dppe). Reaction of 1 with neutral ligands in the absence of nucleophilic anions proceeded under visible-light irradiation to give cationic complexes [CpRuL3]+ [L = CO, P(OMe)3, P(OEt)3, tBuNC] in 80–90 % yields. Complex 1 (2 mol-%) catalyzed cyclotrimerization of dipropargyl Meldrum's acid with various alkynes RC≡CH [R = H, Bu, Hex, Ph, SiMe3, (CH2)4C≡CH, (CH2)3OH, (CH2)2Br, CH2OMe, CH2OAc, CH2NMeBoc] producing benzene derivatives in 50–85 % yields. According to DFT calculations, the attack of the first ligand (Cl– or L) is a rate-determining step in the naphthalene replacement in 1. The activation barrier for attack of the Cl– anion is ca. 10 kcal mol–1 lower in energy than that of the neutral ligands L = CO, MeNC, MeCN, thus providing a rationale for the faster reaction in the presence of halide anions. The barriers for naphthalene replacement in 1 were also found to be ca. 10–15 kcal mol–1 lower in energy than those for the benzene replacement in [CpRu(C6H6)]+.