The complex Mo2Cl4(NHEt2)(4) (1) undergoes facile substitution reactions of the amine ligands by phosphines to give compounds with the same core structure, Mo2Cl4(phosphine)(4), where the phosphine is PMe3 (2), PMe2Ph (3), PHEt2 (4), dmpm (bis(dimethylphosphino)methane) (5), or dmpe (1,2-bis(dimethylphosphino)ethane) (6). Complexes of the type M2X4L4 are well-known with monodentate ligands, but there is no previous crystallographically verified example of the M2X4L4 type of compound in which L is a potentially bidentate ligand acting as a unidentate ligand. Mo2Cl4(eta(1)-dmpm)(4) (5) can be isolated in good yield as the kinetic product of the substitution reaction at room temperature. When a solution of 5 in THF is heated, transformation into the more stable compound Mo2Cl4(mu-dmpm)(2) (7) takes place. Mo2Cl4(eta(1)-dmpe)(4) (6) is stable in solution in the presence of free phosphine and can be detected by NMR. Over time it converts to a polymeric material which precipitates from the solution. For dppm (bis(diphenylphosphino)methane) (8) and dppa (bis(diphenylphosphino)amine) (9), only products of the stoichiometry Mo2Cl4(diphosphine)(2) were obtained. The crystal structures of the complexes 4 and 5 have been investigated by X-ray diffraction. The crystallographic parameters for them are as follows: for 4, orthorhombic space group Pbcn with a = 7.6015(8) Angstrom, b = 20.120(8) Angstrom, c = 19.070(6) Angstrom, and Z = 4; for 5, hexagonal space group P6(4)22 with a = 12.396(1) Angstrom, c = 22.960(2) Angstrom, and Z = 3. Both structures have a Mo2Cl4P4 core where the phosphorus atoms are trans one to another on each metal center. The Mo-Mo distances of 2.137(1) Angstrom (4) and 2.137(1) Angstrom (5) are consistent with quadruple bonding.