Coordination Equilibria Between Seven- and Five-coordinate Iron(II) Complexes

Octahedral, tetrahedral, and square
planar geometries are the most often encountered coordination geometries
for transition metal complexes. In certain cases, coordination equilibria
can exist between different geometries, such as between six- and four-coordinate
geometries in nickel­(II) complexes, which were discovered half a century
ago. Here, we present the first examples of a seven-five coordination
equilibrium. Extensive spectroscopic studies in solution have provided
evidence for a dynamic equilibrium between two iron­(II) complexes,
one with a seven-coordinate pentagonal bipyramidal geometry and one
with a five-coordinate trigonal bipyramidal geometry.