The reactions of [Ru₃(CO)₁₂] with four aromatic diazines have been studied in THF at reflux temperature. With phthalazine (L¹), the compound [Ru₃(μ-κ²N²N³-L¹)(μ-CO)₃(CO)₇] (1), which contains an intact phthalazine ligand in an axial position bridging an Ru–Ru edge through both N atoms, is initially formed but it reacts with more phthalazine to give [Ru₃(κN²-L¹)(μ-κ²N²N³-L¹)(μ-CO)₃(CO)₆] (2), in which a π–π stacking interaction between the aromatic rings of both ligands determines their position in cluster axial sites on the same face of the Ru₃ triangle. With quinazoline (HL²), the cyclometalated hydrido decacarbonyl derivative [Ru₃(μ-H)(μ-κ²N³C⁴-L²)(CO)₁₀] (3) is initially produced but it partially decarbonylates under the reaction conditions to give [Ru₆(μ-H)₂(μ-κ²N³C⁴-L²)(μ₃-κ³-N¹N³C⁴-L²)(CO)₁₉] (4), which results from the displacement of a CO ligand of 3 by the uncoordinated N¹ atom of another molecule of 3. With 4,7-phenanthroline (H₂L³), the stepwise formation of the cyclometalated derivatives [Ru₃(μ-H)(μ-κ²N⁴C³-HL³)(CO)₁₀] (5) and two isomers of [Ru₆(μ-H)₂(μ₄-κ⁴N⁴C³N⁷C⁸-L³)(CO)₂₀] (6a, 6b) takes place. In compounds 6a and 6b, two Ru₃(μ-H)(CO)₁₀ trinuclear units are symmetrically (C₂ in 6a or C_S in 6b) bridged by a doubly-cyclometalated 4,7-phenanthroline ligand. With 2,3′-bipyridine (HL⁴), two products have been isolated, [Ru₃(μ-H)(μ-κ²N^3′C^4′-L⁴)(CO)₁₀] (7) and [Ru₃(μ-H)(μ-κ³N²N^3′C^2′-L⁴)(CO)₉] (8). While compound 7 contains an N^3′C^4′-cyclometalated 2,3′-bipyridine, in compound 8 an N^3′C^2′-cyclometalation is accompanied by the coordination of the N² atom of the remaining pyridine fragment. The structures of compounds 2, 3, 4, 6a and 8 have been determined by X-ray diffraction crystallography.