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 alky l iodides (RI; R = Mel Et or Pr) or EtBr was studied by stopped-flow techni ques, which indicate that it involves a fast first alkylation to give trans -[Rc (CN)(CNR)(dppe)(2)] that undergoes a subsequent relatively slow alkyla tion at the cyano-ligand with a rate constant that decreases with the incre ase of the carbon chain length of the R group and with the replacement of i odide by bromide in the organohalide. Sodium iodide inhibits the rates of a lkylation, 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 k inetics 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 tha t 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 monocli nic space group P2(1)/c with a = 11.673(2). b = 13.302(3), c = 17.166(4) An gstrom. (C) 1999 Elsevier Science S.A. All rights reserved.