Structural and electronic comparison of 15- to 17-electron dichloro-complexes of molybdenum and rhenium: electrochemical behaviour 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

Abstract

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.