The first examples of half-sandwich boratabenzene ruthenium complexes have been prepared and well-characterized. Treatment of RuCl2(PPh3)3 with the anionic boratabenzene ligands C5H5BORNa (R = Et, Me) produced the B-alkoxy complexes (η6-C5H5BOR)RuCl((PPh3)2 (R = Et, 1a; Me, 1b), while the reaction of RuCl2(PPh3)3 with the neutral borabenzene–PPh3 adduct C5H5BPPh3 led to the formation of the dinuclear complex (μ2-η6,η6-C5H5BOBC5H5)[RuCl(PPh3)2]2 (2), with an oxo-bis(boratabenzene) bridging ligand connected by the B–O–B linkage. The B-hydroxy complex (η6-C5H5BOH)RuCl(PPh3)2 (3) was obtained from the nucleophilic substitution of complexes 1a, 1b, or 2 with water at the boron atoms, and the complexes 1a, 1b, 2, and 3 could interconvert through the nucleophilic substitution reactions. While the reaction of 1a with phenylacetylene did not successfully afford the vinylidene complex as expected, reactions of 1a with terminal ω-alkynols HC≡C(CH2)nCH(R)OH (R = H, n = 1, 2; R = Me, n = 1) proceeded smoothly to provide the neutral oxacyclocarbene complexes (η6-C5H5BOEt)RuCl(═CCH2CH2CH2O)(PPh3) (4), (η6-C5H5BOEt)RuCl(═CCH2CH2CH2CH2O)(PPh3) (5), and (η6-C5H5BOEt)RuCl(═CCH2CH2CH(Me)O)(PPh3) (6) via intramolecular nucleophilic attack of the hydroxyl group to the corresponding vinylidene intermediates.