Solvent and incoming ligand effects on the mechanism of substitution reactions of trans-[FeH(L)(DPPE)(2)](+) (L = MeCN), and comparison with the dihydrogen analogue (L = H-2)

Reaction of trans-[FeH(MeCN)(DPPE)(2)](+) with PhCN to form trans-[FeH(PhCN )(DPPE)(2)](+) occurred with a single measurable kinetic step in THF, aceto ne or MeOH solutions (DPPE=Ph2PCH2CH2PPh2). The observed rate constants are independent of the concentration of the nucleophile and do not change very much with the solvent nature. However, the activation parameters are stron gly solvent-dependent, especially Delta V double dagger(;) that changes fro m 20 cm(3) mol(-1) in THF to 35 cm(3) mol(-1) in neat benzonitrile. Kinetic results for the reactions of trans-[FeH(MeCN)(DPPE)(2)](+) with substitute d benzonitriles indicate that kinetic parameters for the substitution of co -ordinated MeCN also change with the nature of the entering ligand. The rea ctions of trans-[FeH(L)(DPPE)(2)](+) complexes (L=MeCN or H-2) with the bid entate ligands C6H4(CN)(2) (1,2-dicyanobenzene) and Me2PCH2CH2PMe2 (DMPE) h ave been also monitored by NMR at variable temperature. While for the aceto nitrile complex and dicyanobenzene, trans-[FeH{C6H4(CN)(2)}(DPPE)(2)](+) is the only product detected with no evidence of the formation of reaction in termediates or side-products, small amounts of free DPPE are detected for t he same reaction with the dihydrogen complex to form the same monosubstitut ed complex. Moreover, whereas the acetonitrile complex does not react with DMPE, free DPPE is the only DPPE-containing product after reaction of trans -[FeH(H-2)(DPPE)(2)](+) with DMPE excess. The whole of the kinetic data and NMR observations are, on the one hand, consistent with a chelate ring-open ing mechanism for the reactions of the dihydrogen complex and, on the other hand with a simple dissociative mechanism for the reactions of the acetoni trile complex. In all cases, the absolute values of Delta V double dagger, for these reactions are larger than usual and also show an unusual dependen ce on the nature of the solvent and the entering ligand.