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, L^x Fe-Fp (L^X = β-aldiminate or β-diketiminate; Fp = Fe(CO)₂ Cp), that offer insight into Fe-Fe bonding. Mössbauer, magnetism, and DFT analysis indicate that the most accurate electronic structure description is LFe^II ←Fe⁰ (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 Mössbauer parameters can be used to rank the Fp^- "metalloligand" as a donor; it is nearly as strong a donor as phosphides and alkyls.