STRUCTURAL AND ELECTRONIC COMPARISON OF 15-ELECTRON TO 17-ELECTRON DICHLORO-COMPLEXES OF MOLYBDENUM AND RHENIUM - ELECTROCHEMICAL-BEHAVIOR AND CRYSTAL-STRUCTURES OF TRANS-[RECL2(DPPE)2]A (A = CL OR BF4, DPPE = PH2PCH2CH2PPH2), TRANS-[RECL2(DPPE)2] AND [NBUN4]2[TRANS-MOCL2(DPPE)2][BF4]3

The molecular structures of the complexes trans-[ReCl2(dppe)2]A 1 (A = Cl or BF4, dppe = Ph2PCH2CH2PPh2), trans-[ReCl2(dppe)2] 2 and [NBun4]2[trans-MoCl2(dppe)2][BF4]3 3 have been determined by X-ray crystallography. The metal-phosphorus and-chloride bond lengths are sensitive to the number of metal d electrons. The M-P (M = Mo or Re) distances contract by ca. 0.1 angstrom for each electron added. In contrast, the Re-Cl distances increase by ca. 0.1 angstrom upon addition of one electron (d5 to d6) but the Mo-Cl distances increase by less than 0.02 angstrom on addition of an electron (d3 to d4). These results are interpreted mainly in terms of pi effects. Reduction of these complexes leads to cleavage of the metal-chloride bond which has been tracked by ramp-clamp voltammetry.