The crystal structure of [Ni(dmf)₆][NiCl₄] and comments on the hydrolysis of coordinated amides and peptides in metal complexes

Solutions of nickel(II) chloride in N,N-dimethylformamide (DMF) in the presence of Et₂O slowly give blue-green crystals of [Ni(dmf)₆][NiCl₄] whose structure has been confirmed by X-ray crystallography. The complex crystallises with three crystallographically distinct [Ni(dmf)₆]²⁺ cations in the asymmetric unit, Ni(1) on a general position and Ni(2) and Ni(3) on centres of symmetry. There are also two unique [NiCl₄]^2- anions, both on general positions. N,N-Dimethylformamide is O-bonded to nickel as coordination via nitrogen would cause the loss of 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) Å]. The C=O bond lengths, which range from 1.219(7) to 1.259(7) Å, are not significantly different from those for uncomplexed dmf (1.232 ± 0.004 Å), suggesting that there is very little polarisation 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)₅dmf]³⁺ to give the formato complex is some 10⁴-fold faster than that of the free ligand. Copper(II)-catalysed amide and peptide bond hydrolysis is subject to ca. 10⁶-fold rate enhancements. The origin of these rate enhancements, which appear to be due to transition state effects rather than ground state effects, is discussed in detail.