Reversible Complexation of Lewis Bases to Low-Coordinate Fe(II), Co(II), and Ni(II) Amides: Influence of the Metal, Donor Ligand, and Amide Substituent on Binding Constants.

The binding constants of several monodentate Lewis bases (L) having different donor atoms [L = tetrahydrofuran (THF), tetrahydrothiophene (THT), pyridine, 4-(dimethylamino)pyridine (DMAP), PMe, PCy, and 1,3-bis(1,3,5-trimethylphenyl)imidazol-2-ylidene (IMes)] to the linear two-coordinate bis(amido) transition-metal complexes, M{N(SiMe)Dipp} [M = Fe (1), Co (2), and Ni (3)] were determined by electronic absorption spectroscopy. In addition, H NMR spectroscopy was used to study the complexation of M{N(SiMe)Dipp} by THF. The binding constants of the less bulky amido complexes [M{N(SiMe)}] (M = Fe or Co; the nickel species is unstable) were also measured and found to bind THF 10-10 times more strongly than M{N(SiMe)Dipp}. Furthermore, unlike the corresponding -N(SiMe)Dipp complexes, where the iron species are more strongly complexed than those of cobalt, the --N(SiMe) species [Co{N(SiMe)}] displayed a higher binding constant to THF than [Fe{N(SiMe)}], which is in agreement with the stronger association of the [Co{N(SiMe)}] dimer in comparison to that of its iron analogue. The binding strengths of the ligands to 1-3 were found to be in the order of DMAP > pyridine > PMe > PCy ≈ IMes > THF > THT, and those of the respective metals were in the order of Fe > Co > Ni.