A Combined Spectroscopic and Computational Study of a High-Spin S=7/2 Diiron Complex with a Short Iron-Iron Bond

The nature of the iron-iron bond in the mixed-valent diiron tris(diphenylforamidinate) complex Fe-2(DPhF)3, which was first reported by Cotton, Murillo et al. (Inorg. Chim. Acta 1994, 219, 7-10), has been examined using additional spectroscopic and theoretical methods. It is shown that the coupling between the two iron centers is strongly ferromagnetic, giving rise to an octet spin ground state. On the basis of Mossbauer spectroscopy, the two iron centers, formally mixed-valent Fe(II)Fe(I), are completely equivalent with an isomer shift delta = 0.65 mm s(-1) and quadrupole splitting Delta E-Q = +0.32 mm s(-1). A large, positive zero-field splitting D-7/2 = 8.2 cm(-1) has been determined from magnetic susceptibility measurements. Multiconfigurational quantum studies of the complete molecule Fe-2(DPhF)(3) found one dominant configuration (sigma)(2)(pi)(4)(pi*)(2)(sigma*)(1)(delta)(2)(delta*)(2) which accounts for 73% of the ground-state wave function. By considering all the configurations, an estimated metal metal bond order of 1.15 has been calculated. Finally, Fe-2(DPhF)(3) exhibits weak electronic absorptions in the visible and near-infrared regions, which are assigned as d-d transitions from the doubly occupied metal-metal pi molecular orbital to half-occupied pi*, delta, and delta* orbitals.