On the basis of DFT-BS calculations an additive contribution of the mixed bridges to magnetic super-exchange coupling in the Cr(III) dimer, [(tren)Cr(μ-CO3)(μ-OH)Cr(tren)](ClO4)3·2H2O (1) was investigated through variation of the exchange coupling constant J with stretch and compress of the metal-bridge bond near its balance position. The qualitative orbital model indicated the Cr(III) dimers (1) and (4) behave a complicated magnetic exchange nature. Further the dependance of magnetic exchange behaviours on the electronic structure was established by using the overlap spin population analysis. On the other hand, the atomic net spin distribution shows there is a spin polarization mechanism in both hydroxo and carbonato bridge pathways, thus only the weak exchange coupling interaction occurs in the Cr(III) dimer.