A Dimeric Hydride-Bridged Complex with Geometrically Distinct Iron Centers Giving Rise to an = 3 Ground State.

Structural and spectroscopic characterization of the dimeric iron hydride complex [PhB(BuIm)FeH] reveals an unusual structure in which a tetrahedral iron(II) site ( = 2) is connected to a square planar iron(II) site ( = 1) by two bridging hydride ligands. Magnetic susceptibility reveals strong ferromagnetic coupling between iron centers, with a coupling constant of = +110(12) cm, to give an = 3 ground state. High-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy confirms this model. A qualitative molecular orbital analysis of the electronic structure, as supported by electronic structure calculations, reveals that the observed spin configuration results from the orthogonal alignment of two geometrically distinct four-coordinate iron fragments held together by highly covalent hydride ligands.