Geometric and Electronic Structure Analysis of the Three-Membered Electron-Transfer Series [(μ‑CNR)<sub>2</sub>[CpCo]<sub>2</sub>]<sup><i>n</i></sup> (<i>n</i> = 0, 1–, 2−) and Its Relevance to the Classical Bridging-Carbonyl System

The dimeric bridging carbonyl complexes [(μ‑CO)<sub>2</sub>­[CpCo]<sub>2</sub>]<sup><i>n</i></sup> (<i>n</i> = 0, 1−) have occupied a central position in the understanding of metal–metal bonding interactions when bridging ligands are present. Based on simple electron-counting formalisms, these dimers have been proposed to possess formal Co–Co bond orders of 2 and 1.5, respectively. However, this simple bonding scheme has been contrasted by molecular orbital theory considerations, as well as spectroscopic data that probes M–M bonding interactions generally. While this system has received considerable attention, there has been a long-standing synthetic limitation in that the doubly reduced dianionic dimer, [(μ‑CO)<sub>2</sub>­[CpCo]<sub>2</sub>]<sup>2–</sup>, has not been amenable to isolation, thereby precluding an analysis of ostensible full-integer reduction in a homologous series. Accordingly, herein is presented the synthesis of a homologous, three-membered series of bridging-isocyanide [(μ‑CNAr)<sub>2</sub>­[CpCo]<sub>2</sub>]<sup><i>n</i></sup> dimers, including the dianionic member. Structural and spectroscopic analyses of these [(μ‑CNAr)<sub>2</sub>­[CpCo]<sub>2</sub>]<sup><i>n</i></sup> dimers, which feature the <i>m</i>-terphenyl isocyanide CNAr<sup>Mes2</sup> (Ar<sup>Mes2</sup> = 2,6-(2,4,6-Me<sub>3</sub>C<sub>6</sub>H<sub>2</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>), reveal that this series possesses similar overall properties to the bridging carbonyl counterparts. However, high-resolution X-ray crystallographic studies have revealed important structural differences that were not discernible in older studies of the carbonyl complexes. Also presented is the synthesis of the bridging-isocyanide/η<sup>6</sup>-arene dimers [Co<sub>2</sub>((η<sup>6</sup>-Mes)­(μ‑CNAr<sup>Mes</sup>))<sub>2</sub>]<sup><i>n</i></sup> (<i>n</i> = 0, 1+), which are valence isoelectronic to the mono- and dianionic [(μ‑CNAr)<sub>2</sub>­[CpCo]<sub>2</sub>]<sup><i>n</i></sup> derivatives. Structural and spectroscopic studies of these η<sup>6</sup>-arene complexes, as well as the related neutral nickel dimer (μ‑CNAr<sup>Mes</sup>)<sub>2</sub>­[CpNi]<sub>2</sub> provide evidence for an electronic structure environment dominated by M→(CN)­π* back-bonding interactions, rather than direct M–M bonding. This conclusion is supported by DFT-derived molecular orbital analysis on the bridging-isocyanide [(μ‑CNAr<sup>Mes2</sup>)<sub>2</sub>­[CpCo]<sub>2</sub>]<sup><i>n</i></sup> dimers.