The dimeric bridging carbonyl complexes [(μ-CO) 2 [CpCo] 2 ] n (n = 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) 2 [CpCo] 2 ] 2− , 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) 2 [CpCo] 2 ] n dimers, including the dianionic member. Structural and spectroscopic analyses of these [(μ-CNAr) 2 [CpCo] 2 ] n dimers, which feature the m-terphenyl isocyanide CNAr Mes2 (Ar Mes2 = 2,6-(2,4,6-Me 3 C 6 H 2 ) 2 C 6 H 3 ), 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 6arene dimers [Co 2 ((η 6 -Mes)(μ-CNAr Mes )) 2 ] n (n = 0, 1+), which are valence isoelectronic to the mono-and dianionic [(μ-CNAr) 2 [CpCo] 2 ] n derivatives. Structural and spectroscopic studies of these η 6 -arene complexes, as well as the related neutral nickel dimer (μ-CNAr Mes ) 2 [CpNi] 2 provide evidence for an electronic structure environment dominated by M→(CN)π* backbonding interactions, rather than direct M−M bonding. This conclusion is supported by DFT-derived molecular orbital analysis on the bridging-isocyanide [(μ-CNAr Mes2 ) 2 [CpCo] 2 ] n dimers.
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