Thermolysis of Ru 3 (CO) 12 with carboxylic acids directly accessed a large variety of low valence ruthenium carbonyl complexs. Under the conditions of the reaction firstly reported by J. Lewis, 3 equivalents of acids were employed to assembly sawhorsetype dirutheniumtetra carbonyl complex. Six Ru 3 -Ru 6 ruthenium clusters were unexpectedly isolated in the reaction of Ru 3 (CO) 12 with 1 equivalent of carboxylic acids under thermal condition. The reaction of Ru 3 (CO) 12 with ten kind of carboxylic acids were systematically investigated. The reactions of Ru 3 (CO) 12 with non-coordinative carboxylic acids afforded b-H 2 Ru 4 (CO) 13 (2), Ru 6 (m 6 -C)(m-CO)(CO) 16 (3) and then to Ru 6 (m 6 -C)(m-CO)(CO) 13 (h 6 -C 6 H 5 CH 3 ) (4), respectively. The further experiments indicated that the formation of 3 was attributed to the assembly of tetraruthenium carbonyl dihydrid 2 with polymeric sawhorse diruthenium intermediate [Ru 2 (CO) 4 (OOR) 2 ] n (R = alkyl, aryl). The thermolysis of phenylpropiolic acid with Ru 3 (CO) 12 unveiled an alternative pathway to access Ru 6 skeleton. The triruthenium cluster [(m-H)Ru 3 (CO) 9 (m 3 -h 2 -(?)-C CC 6 H 5 )] (5) and butterfly tetraruthenium compound Ru 4 (CO) 12 (HC CC 6 H 5 ) (6) were identified as the key intermediates in the construction of compound 3.[a] J.