Therefore, we have turned our attention to the multi-configurational method, namely CASSCF. For the [Re2-Model] 0 system, the active space consists of the metal-based orbitals that are 𝜎, 2𝜋, and 𝛿, and their antibonding counterparts (𝜎 * , 2𝜋 * , and 𝛿 * ). The Re2-core has four eight electrons which are distributed among the eight orbitals (Fig. bonding orbitals and the corresponding two antibonding molecular orbitals and them four electrons distributing among them. This yields CAS (12,12) and CAS(14,12) active space for [Re2-Model] 0 and [Os2-Model] 0 , respectively (see Fig. and Fig. ). For [Re2-Model] 0 , the Re-N 𝛿 orbitals are kept fully occupied with the occupation of 1.99 and 0.01 of bonding and antibonding orbitals, respectively. The occupations of the 𝜎 and 𝜋 manifolds are the same to that in the CAS(8,8) whereas the occupations of the 𝛿 manifold is slightly different from those of the CAS(8,8) with occupations of 1.58 and 0.42 of 𝛿 and 𝛿 * orbitals, respectively, compared to 1.56 and 0.44 in CAS(8,8). Resulting in slight increase in the strength of the 𝛿 bonding (𝛿 bond order of 0.56 with CAS(8,8) compare to 0.58 with CAS (12,12)) but this has almost no effect on the total bond order (3.30 vs 3.29). For [Os2-Model] 0 , the Os-N 𝛿 orbitals are preserved fully occupied with occupation number of 1.99 and 0.01 of bonding and antibonding orbitals, respectively, similar to those of Re-analogue. The active space orbitals of the [Os2-Model] 0 are shown in Fig. . These results show that including the M-N 𝛿 orbitals in the active space has no effect on the nature of the metalmetal bonding in these two complexes. orbital is fully occupied since if the electron is removed from this orbital, it would then be empty.