THE CRYSTAL-STRUCTURE OF [NI(DMF)(6)][NICL4] AND COMMENTS ON THE HYDROLYSIS OF COORDINATED AMIDES AND PEPTIDE IN METAL-COMPLEXES

Solutions of nickel (II) chloride in N,N-dimethylformamide (DMF) in th e presence of Et2O slowly give blue-green crystals of [Ni(dmf)(6)][NiC l4] whose structure has been confirmed by X-ray crystallography. The c omplex crystallises with three crystallographically distinct [Ni(dmf)( 6)](2+) cations in the asymmetric unit, Ni(1) on a general position an d Ni(2) and Ni(3) on centres of symmetry. There are also two unique [N iCl4](2-) anions, both on general positions. N,N-Dimethylformamide is O-bonded to nickel as coordination via nitrogen would cause the loss o f the: resonance energy of the amide group. All Ni-O bond lengths are within the expected range for such complexes [2.029(4)-2.084(4) Angstr om]. The C=O bond lengths, which range from 1.219(7) to 1.259(7) Angst rom, are not significantly different from those for uncomplexed dmf (1 .232 +/- 0.004 Angstrom), suggesting that there is very little polaris ation of the C=O bond by nickel(II). Oxygen-bonded dmf undergoes rapid hydrolysis in metal complexes, for example the base hydrolysis of [Co (NH3)(5)dmf](3+) to give the formato complex is some 10(4)-fold faster than that of the free ligand. Copper(II)-catalysed amide and peptide bond hydrolysis is subject to ca. 10(6)-fold rate enhancements. The or igin of these rate enhancements, which appear to be due to transition state effects rather than ground state effects, is discussed in detail .