Kinetic study of the alkylation of cyanide at [NBu4][trans- Re(CN)2(dppe)2].: Crystal structures of [NBu4][trans-Re)CN)2(dppe)2] and trans-[Re(CN)2(dppe)2]

The mechanism of alkylation of [NBu4][trans-Re(CN)(2)(dppr)(2)] (1) by alkyl iodides (RI; R = Mel Et or Pr) or EtBr was studied by stopped-flow techniques, which indicate that it involves a fast first alkylation to give trans-[Rc (CN)(CNR)(dppe)(2)] that undergoes a subsequent relatively slow alkylation at the cyano-ligand with a rate constant that decreases with the increase of the carbon chain length of the R group and with the replacement of iodide by bromide in the organohalide. Sodium iodide inhibits the rates of alkylation, probably by forming ion pairs with trans-[Re(CN)(2)(dppe)(2)](-) as confirmed by the formation of the adducts [Re(CN)(CNM)(dppe)(2)] (hl = Li, Na, Tl or Ag) by reaction of 1 with convenient metal salts and by the kinetics of the reaction between Mel and [Re(CN)(CNNa)(dppe)(2)]. The X-ray molecular structures of [NBu4][trans-Re(CN)(2)(dppe)(2)] and trans-Re(CN)(2)(dppe)(2) confirm. they have pseudo octahedral geometries and indicate that the former crystallizes in the triclinic P (1) over bar space group with a = 17.938(2), b = 18.473(3), c = 20.061(3) A and the latter in the monoclinic space group P2(1)/c with a = 11.673(2). b = 13.302(3), c = 17.166(4) Angstrom. (C) 1999 Elsevier Science S.A. All rights reserved.