Crystal and molecular structures of Mo2Cl4(PMePh2)4 and Re2Cl4(PMePh2)4

The dropwise addition of SiMe3Cl to a stirred suspension of Mo2(O2CMe)4 and PMePh2 in CH2Cl2 results in the formation of the quadruply bonded dimolybdenum complex Mo2Cl4(PMePh2)41. The triply bonded complex Re2Cl4(PMePh2)42 was synthesised by reaction of PMePh2[NBu4]2[Re2Cl8], and NaBH4 in a MeOH–EtOH slurry and by direct reduction of ReCl5 with Na/Hg in the presence of PMePh2. The crystal structures of Mo2Cl4(PMePh2)4·C6H51a, Re2Cl4(PMePh2)4·C6H62a and Re2Cl4(PMePh2)4·Me2CO 2b have been determined. The complexes were also characterized by 1H and 31P-{1H} NMR spectroscopy. The M2Cl4(PMePh2)4 molecules have qualitatively the same structure in all three cases, with effective S4() symmetry. In 1a the MoMo distance is 2.135(1)A and for 2a and 2b the ReRe distances for the two structural determinations average to 2.257 A. It is shown that in addition to the observed structures of 1 and 2 there are three enantiomeric pairs of isomers obtainable by different combinations of rotations of the PMePh2 ligands about the M–P bonds, all of which are of higher energy. It is suggested that the unstable green isomer of 1 has one of these structures.

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