The structure of the quadruply bonded beta-Mo2Cl4(H2PCH2CH2PH2)(2) complex, with two bridging diphosphine ligands, is studied by means of DFT calculations with the B3LYP functional. Fun geometry optimization is first performed for both the lowest singlet (delta(2)) and triplet ((3)delta delta*) states. In agreement with the data on related experimental complexes, an essentially staggered conformation is found, the singlet state being the electronic ground slate. The potential energy curves associated with the internal rotation around the Mo-Mo bond are then computed for both the singlet and the triplet states, the geometry being fully optimized for each value of the rotational angle. The singlet state is always found to be the electronic ground state, but the singlet-triplet energy separation (Delta E(S-T) depends on the rotational angle. Comparison with the available experimental data on beta-Mo2Cl4(P-P)(2) complexes for seven complexes with an internal rotational angle ranging from 24.7 degrees to 69.4 degrees reveals a satisfactory agreement between theoretical and experimental values provided approximate spin-projected broken-symmetry calculations an used for the singlet state.