The mechanism of tropo-inversion of the Ru(II) complex bearing a chirally flexible biphenylphosphine (BIPHEP) ligand has been studied by the B3LYP and the ONIOM(B3LYP: HF) methods. The relative energy difference between the favorable (S)I(SS) and the less favorable (R)I(SS) pairs of BIPHEP-RuCl2/DPEN complexes is well comparable to the experimental results. The tropo-inversion of the (S)I(SS) pair to the (R)I(SS) pair is found to be achieved by solvent-assisted rotation around the biphenyl single bond and subsequent recoordination of phosphine to the Ru center. The sterically demanding phenyl groups attached to phosphorus atoms in the BIPHEP-RuCl2 moiety prevented the internal rotation without dissociation of P-Ru coordination.