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

Abstract

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.

Bridged and Unbridged Nickel–Nickel Bonds Supported by Cyclopentadienyl and Phosphine Ligand Sets

“…The successful isolation of the dimeric complexes with both phosphine and cyclopentadienyl-based ligands thus seems to be much more sensitive to the substitution patterns of the cyclopentadienyl ligands than that of the phosphines. This is in contrast to the observation by Figueroa and co-workers of the steric bulk of the L-donor isocyanide ligand determining the structure . The steric demands of the phosphine ligands do not seem to play a major role in the successful preparation of the dimeric species, contrasting the assumptions made in the work by Wilke and co-workers .…”

“…The first example of an unbridged Ni-Ni complex was K 4 [Ni 2 (CN) 6 ], which has a short Ni–Ni bond length of 2.32 Å . Two organometallic complexes were subsequently reported, both supported by cyclopentadienyl and phosphines or bulky isocyanide ligands. , Figueroa’s complex was obtained by reaction of nickelocene with an aryl isocyanide ligand. Notably, reduction of the steric bulk of the aryl group yields a dimeric complex with bridging isocyanides.…”

Probing for Four-Coordinate Zerovalent Iron in a π-Acidic Ligand Field: A Functional Source of FeL4 Enabled by Labile Dinitrogen Binding

“…We interpret this unusual observation as further reflection of the instability of the [Fe­(CO) 2 (CNR) 2 ] fragment and the seeming inability of the encumbering CNAr Tripp2 ligands to provide a stabilizing secondary bonding contact to the zerovalent Fe center via π-arene interactions. As we have shown previously, π-arene interactions from the flanking rings of m -terphenyl isocyanides are commonly encountered when coordinatively unsaturated, low-valent metal centers are generated within these systems. ,− While such interactions are not observed prior to the formation of 2 , it is important to note that re-exposure of this THF-adduct to an N 2 atmosphere regenerates the terminal dinitrogen complex 1 (Scheme ). Accordingly, this substitution indicates that the THF ligand is not electronically well matched for the low-valent Fe center, despite being able to stabilize the complex to ligand redistribution processes.…”

Low-Valent Transition Metalate Anions in Synthesis, Small Molecule Activation, and Catalysis

“…136,137,162 Compounds (111)] served as starting materials for the synthesis of dinuclear or mononuclear complexes featuring either bridging isocyanide ligands or a terminal carbyne, respectively. 136,137 The dinuclear complexes [(μ-CNAr Mes2 ) 2 -{CpCo} 2 ] (112), K[(μ-CNAr Mes2 ) 2 {CpCo} 2 ] (K [113]), and K 2 [(μ-CNAr Mes2 ) 2 {CpCo} 2 ] (K 2 [114]) were obtained by reduction of 108 with increasing equivalents of KC 8 (see Scheme 12). 136 In the solid state, both anionic species, K [113] and K 2 [114], form an ion pair with the potassium counterions and the arenes in the isocyanide ligand.…”