To check the unusual oxidation state II of the gold ion in the studied complexes, QTAIM (Quantum Theory of Atoms in Molecules) analysis of the electronic structure has been carried out employing the localization index, λ(A) as the central property. The oxidation state of an electropositive element in contact with elements that are more electronegative can be defined as the difference between its atomic number and its λ(A). This quantity is consistent with IUPAC definition of oxidation state that is atomic charge after "ionic approximation" of heteronuclear bonds. The QTAIM analysis of [(bipy)MX] + (M = Cu, Au; X = Cl, Br, I) complexes in Table shows that while the atomic charges of M vary from 0.4 for [(bipy)AuI] + to 1.0 for [(bipy)CuCl] + , the λ(A) index remains rather constant among all species, confirming the +2 oxidation state of the metal. The differences between the λ(A) and atomic numbers for copper and gold complexes are similar to each other, which is consistent with their similar oxidation numbers. The bonding between Au(II) and the other atoms in the molecule can be described by the delocalization index, DI, that expresses the level of electron sharing between two atoms. The results in Table point to the covalent M-X bonds with DIM-X values varying between 1.1 ([(bipy)CuBr] + ) and 1.5 ([(bipy)AuCl] + ). The DIM-X higher than 1 denotes multiple bonding, see NBO analysis below. Dative bonds are predicted between M and nitrogen atoms, with DIM-N about 0.5-0.7 for each nitrogen in Table . The multiple bonding character of M-X bonds (DIM-X >1 in Table ) is reflected in the NBO analysis of ([(bipy)AuCl] + ) that finds three one-electron bonding NBOs between Au and X. Two of these one-electron bonds share similar spatial shape but one host an alpha-the other host a beta electron (Figure ) which basically corresponds to a two-electron (M-X) bond. The third M-X bonding NBO corresponds to one-electron LP𝞹 (X) donation to d𝞹 * (M), Figure . The Au-N interactions are also observed in all systems. The NBO procedure finds LP(N) NBOs that strongly interact with the M-X antibonding NBOs. Table . The atomic charges, Q, on metal, halogen, and nitrogen atoms, localization index of the metals (LI) and delocalization index (DI) between metal and halogen, and metal and nitrogen atoms. Spin densities at metal, halogen and nitrogen atoms; atomic properties of the nitrogen atoms in the asymmetric structures are reported for N1 and N2 as marked in Figure , respectively.