Mixed chloride/amine complexes of dimolybdenum(II,II). 2. Reactions of Mo₂Cl₄(NHEt₂)₄ with monodentate and bidentate phosphines. New type of compounds Mo₂Cl₄(diphosphine)₄

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 (l,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(η¹-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(μ-dmpm)₂ (7) takes place. Mo₂Cl₄(η¹-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) Å, b = 20.120(8) Å, c = 19.070(6) Å, and Z = 4; for 5, hexagonal space group P6₄22 with a = 12.396(1) Å, c = 21.960(2) Å, 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) Å (4) and 2.137(1) Å (5) are consistent with quadruple bonding.