Mixed-Valent Dicobalt and Iron–Cobalt Complexes with High-Spin Configurations and Short Metal–Metal Bonds

Cobalt–cobalt and iron–cobalt bonds are investigated in coordination complexes with formally mixed-valent [M₂]³⁺ cores. The trigonal dicobalt tris­(diphenylformamidinate) compound, Co₂(DPhF)₃, which was previously reported by Cotton, Murillo, and co-workers (<i>Inorg. Chim. Acta</i> <b>1996</b>, <i>249</i>, 9), is shown to have an energetically isolated, high-spin sextet ground-state by magnetic susceptibility and electron paramagnetic resonance (EPR) spectroscopy. A new tris­(amidinato)­amine ligand platform is introduced. By tethering three amidinate donors to an apical amine, this platform offers two distinct metal-binding sites. Using the phenyl-substituted variant (abbreviated as L^Ph), the isolation of a dicobalt homobimetallic and an iron–cobalt heterobimetallic are demonstrated. The new [Co₂]³⁺ and [FeCo]³⁺ cores have high-spin sextet and septet ground states, respectively. Their solid-state structures reveal short metal–metal bond distances of 2.29 Å for Co–Co and 2.18 Å for Fe–Co; the latter is the shortest distance for an iron–cobalt bond to date. To assign the positions of iron and cobalt atoms as well as to determine if Fe/Co mixing is occurring, X-ray anomalous scattering experiments were performed, spanning the Fe and Co absorption energies. These studies show only a minor amount of metal-site mixing in this complex, and that FeCoL^Ph is more precisely described as (Fe_0.94(1)Co_0.06(1))­(Co_0.95(1)Fe_0.05(1))­L^Ph. The iron–cobalt heterobimetallic has been further characterized by Mössbauer spectroscopy. Its isomer shift of 0.65 mm/s and quadrupole splitting of 0.64 mm/s are comparable to the related diiron complex, Fe₂(DPhF)₃. On the basis of spectroscopic data and theoretical calculations, it is proposed that the formal [M₂]³⁺ cores are fully delocalized