Spin States, Bonding and Magnetism in Mixed-Valence Iron(0)-Iron(II) Complexes**

"Xenophilic" complexes offer metal-metal bonds between disparate metal sites, but the nature of the metal-metal bonding is often unclear. Here, we describe two novel complexes with unsupported Fe-Fe bonds, (LFe)-Fe-x-Fp (L-X = beta-aldiminate or beta-diketiminate; Fp = Fe(CO)(2)Cp), that offer insight into Fe-Fe bonding. Mossbauer, magnetism, and DFT analysis indicate that the most accurate electronic structure description is LFeII <- Fe-0(CO)(2)Cp, in which the Fe(CO)(2)Cp is low-spin iron(0) and acts as an X-type ligand toward the high-spin iron(II) of the LFe fragment. This largely electrostatic interaction has a bond order of only 0.5. The three-coordinate high-spin iron(II) site has large zero-field splitting, and in addition its Mossbauer parameters can be used to rank the Fp(-) "metalloligand" as a donor; it is nearly as strong a donor as phosphides and alkyls.