Gauging metal Lewis basicity of zerovalent iron complexes via metal-only Lewis pairs

A range of pentacoordinate iron complexes of the form [Fe(CO)(5-n)L-n] (L = neutral donor, n = 0-2) were treated with gallium chloride or bromide, leading to hexacoordinate metal-only Lewis pairs (MOLPs) of the form [L-n(OC)(5-n)Fe -> GaX3] (X = Cl, Br). These complexes were used to gauge the Lewis basicity of the pentacoordinate iron precursors through comparison of their experimentally-and computationally-derived Fe-Ga distances and the degree of pyramidalization at the Ga center. The data indicate that consecutive replacement of CO groups with donor ligands increases the Lewis basicity of [Fe(CO)(5-n)L-n] complexes, with the largest effect seen upon going from n = 0 to n = 1. While the Lewis basicity differences are clear between tri-, tetra-and pentacarbonyl complexes, the difference between the ligands within each group is less clear. A series of transfer experiments were also performed, in which the GaCl3 was transferred consecutively to Fe complexes with increasing numbers of donor ligands.