Mononuclear Four-Coordinate Bis-Fluoride Bis-NHC Complexes of Chromium(II), Iron(II), and Cobalt(II).

The reaction between silylamido complexes of Cr(II), Fe(II), and Co(II) and IMes·2HF salt in the presence of IMes (IMes = 1,3-dimesitylimidazol-2-ylidene) led to isolation of Cr(IMes) 2 F 2 ( 2-Cr ), Fe(IMes) 2 F 2 ( 2-Fe ), and Co(IMes) 2 F 2 ( 2-Co ). X-ray structural studies revealed that 2-Cr adopts square planar geometry, while 2-Fe and 2-Co have distorted tetrahedral geometry. Magnetic susceptibility studies of 2-Cr , 2-Fe , and 2-Co were consistent with high-spin complexes, S = 2 for 2-Cr / 2-Fe and S = 3/2 for 2-Co . We demonstrated that fluoride can be successfully exchanged for cyanide and azide using trimethylsilyl cyanide and trimethylsilyl azide ( 3-Fe and 4-Fe ). DFT studies suggest that the preference of 2-Cr to adopt square planar geometry over tetrahedral is due to its d 4 metal center, where four electrons fill the lower-lying d-orbitals.