The tri-nuclear heterometallic tetrahedral cluster [CoMoRu(µ3-S)(CO)8(Cp)COOCH3], (Cp = η5-C5H4) in question was studied using the atoms in molecules quantum theory (QTAIM) to examine bonding interactions such as M-M, M-S, M-CO, and M-Cp. The electron density of bonding interactions within the cluster has its topological properties calculated based on this theory. Interestingly, the local topological parameters calculated for the Mo-Ru bond differ considerably from those of Mo-Co, and Ru-Co interactions, for which bond critical points and bond paths were not observed. Bridging sul de ligands signi cantly impacted the electron density distribution of Mo…Co, and Ru…Co interactions more than Mo-Ru bond. The topological properties of the latter bond were characteristic of open-shell metal-metal interactions, with small positive values of ρb and ∇2ρb, and small negative values of Hb close to 0. The core part, [CoMoRu(µ3-S], was found to have a multicenter 4c-6e interaction. In this core, the three M-S bonds between the metal atoms and the sul de ligand showed similar topological parameters that were typical of open-shell (covalent) interactions. A signi cant CO to M π-back donation was found by performing the Source Function and δ(M…OCO) delocalization index calculations. 1.Introduction Transition metal cluster complexes are useful in synthetic reactions because they can convert inactive substrates into chemically active substances, which is why they are important in inorganic and organometallic chemistry[1]. Because they increase catalytic reactivity by means of the cooperative activation and synergistic interaction of metal centers with different electronic properties, heterometallic cluster complexes which comprise cluster cores with different kinds of metals, are especially intriguing[2, 3]. It has been demonstrated that trinuclear mixed heterometallic clusters signi cantly alter the reactivity of chemical bond cleavage, for instance, the C-H and C-C bond interactions of alkanes. This drastic change happens due to the electronic anisotropy circumstances that are induced as a result of the polarization of the bonds in the mixed metal atoms[4].Recently, the Bader group's quantum theory of atoms in molecules (QTAIM) [5] has been used extensively to study the metal-metal and metal-ligand bonds in organometallic complexes [6][7][8][9][10][11][12]. This method uses the appearance of a bond critical point and the bond path between two neighboring atoms to de ne the chemical bond that binds their nuclei together. The bond's properties can be inferred from the topological properties within this critical point, such as the electron density ρb and its Laplacian ∇ 2 ρb.Although the QTAIM approach has been used to study the characteristic of different kinds of M-M and M-L interaction, the topological nature of these bonds is still the subject of debate and requires further research. Previous QTAIM studies on clusters containing mixed heterometallic transition metals (Co, Mo, and Ru) bridged by sul de ligands are quite rare. In...