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)4 1. The triply bonded complex Re2Cl4(PMePh2)4 2 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.C6H6 1a, Re2Cl4(PMePh2)4.C6H6 2a and Re2Cl4(PMePh2)4.Me2CO 2b have been determined. The complexes were also characterized by H-1 and P-31-{H-1} NMR spectroscopy. The M2Cl4(PMePh2)4 molecules have qualitatively the same structure in all three cases, with effective S4 (4BAR) symmetry. In 1a the Mo = Mo distance is 2.135(1) angstrom and for 2a and 2b the Re = Re distances for the two structural determinations average to 2.257 angstrom. 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.