Reactions of the sigma-phenyl complex of Fe(III)-octaphenyltetraazaporphine with nitrous bases

The low-spin pentacoordinated Fe(III)-octaphenyltetraazaporphine sigma-phen yl complex (Ph)FeOPTAP, obtained from the chloro complex (CI)FeOPTAP and ph enylmagnesium bromide, reacts with nitrous bases L [L = pyridines (Py, 4-Me Py, 4-PhPy, 3-CIPy, 3-CNPy, 4-CNPy, or 3, 4-Me2Py), imidazoles (Im, 1-MeIm, and 2-MeIm), and aliphatic amines (Pip, BzNH(2), and c-HxNH(2))] to form t he low-spin hexacoordinated extra complexes (L)(Ph)FeOPTAP. The concentrati onal equilibrium constants for formation of the extra complexes (K-ex) were determined by spectrophotometric titration in toluene. The stability of (L )(Ph)FeOPTAP increases with an increase in the basicity of L. A correlation between logK(ex) and Hammett sigma constants (rho = -1.98) was established for a series of substituted pyridines. It is shown that, along with the co ordination sigma-effect, the ability of L for pi-bonding exerts substantial influence on the stability of the extra complex (L)(Ph)FeOPTAP.