Ralph Schenker, Høgni Weihe and Hans U. Güdel
Ferromagnetic Exchange in the Ground state and ⁴A₁ Exited State of (Et₄N)₃Fe₂F₉. A Magnetic and Optical Spectroscopic Study
Inorg. Chem. 40, 4319-4326 (2001)                 

Abstract: The synthesis, crystal growth, magnetic susceptibility, and polarized optical absorption spectra in the visible and near UV of (Et₄N)₃Fe₂F₉ are reported. From singel-crystal magnetic susceptibility data and high-resolution absorption spectra in the region of the ⁶A₁ ⁴A₁ spin-flip transition, exchange splittings in the ground and excited states are derived. Ferromagnetic ground and excited state exchange parameters J_GS = -1.55cm^–1 and J_ES = -0.53cm^–1 are determined, respectively, and the relevant orbital contributions to the net exchange are derived from the spectra. The results are compared with those reported earlier for the structurally and electronically analogous [Mn₂X₉]^5- pairs in CsMgX₃:Mn²⁺ (X = Cl^–, Br^–), in which the splittings are antiferromagnetic. This major difference is found to be due to the increased metal charge of Fe³⁺ compared to Mn²⁺, leading to orbital contraction and thus to a strong decrease of the orbital overlaps and hence the antiferromagnetic interactions.