The experimental charge density determination (T = 120 K) of a second-row transition metal cluster and subsequent topological analysis elucidate the bonding of the Ru-3(CO)(12) molecule. The Ru-Ru bond critical point features were found to be close to those of the first-row transition metal-metal bonds. Furthermore, the experiment reveals the presence of C. . .C interactions among the axial carbonyl ligands. They may justify the sterically hindered D-3h configuration of the molecule and the distortion of axial COs from linearity. © 2005 Elsevier B.V. All rights reserved.