Structural and electronic comparison of 15- to 17-electron dichloro-complexes of molybdenum and rhenium: electrochemical behaviour and crystal structures of trans-[ReCl 2 (dppe) 2 ]A (A = Cl or BF 4 ; dppe = Ph 2 PCH 2 CH 2 PPh 2 ), trans-[ReCl 2 (dppe) 2 ] and [NBu n 4 ] 2 [trans-MoCl 2 (dppe) 2 ][BF 4 ] 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]33 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 contact by ca. 0.1 Å for each electron added. In contrast, the Re–Cl distances increase by ca. 0.1 Å upon addition of one electron (d5 to d6) but the Mo–Cl distances increase by less than 0.02 Å on addition of an electron (d3 to d4). These results are interpreted mainly in terms of π effects. Reduction of these complexes leads to cleavage of the metal–chloride bond which has been tracked by ramp-clamp voltammetry.