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1. Exceptionally Long Covalent CC Bonds—A Local Vibrational Mode Study

   https://doi.org/10.3390/molecules26040950  

   Delgado, Alexis Antoinette; Humason, Alan; Kalescky, Robert; Freindorf, Marek; Kraka, Elfi (February 2021, Molecules)

   For decades one has strived to synthesize a compound with the longest covalent C−C bond applying predominantly steric hindrance and/or strain to achieve this goal. On the other hand electronic effects have been added to the repertoire, such as realized in the electron deficient ethane radical cation in its D3d form. Recently, negative hyperconjugation effects occurring in diamino-o-carborane analogs such as di-N,N-dimethylamino-o-carborane have been held responsible for their long C−C bonds. In this work we systematically analyzed CC bonding in a diverse set of 53 molecules including clamped bonds, highly sterically strained complexes such as diamondoid dimers, electron deficient species, and di-N,N-dimethylamino-o-carborane to cover the whole spectrum of possibilities for elongating a covalent C−C bond to the limit. As a quantitative intrinsic bond strength measure, we utilized local vibrational CC stretching force constants ka(CC) and related bond strength orders BSO n(CC), computed at the ωB97X-D/aug-cc-pVTZ level of theory. Our systematic study quantifies for the first time that whereas steric hindrance and/or strain definitely elongate a C−C bond, electronic effects can lead to even longer and weaker C−C bonds. Within our set of molecules the electron deficient ethane radical cation, in D3d symmetry, acquires the longest C−C bond with a length of 1.935 Å followed by di-N,N-dimethylamino-o-carborane with a bond length of 1.930 Å. However, the C−C bond in di-N,N-dimethylamino-o-carborane is the weakest with a BSO n value of 0.209 compared to 0.286 for the ethane radical cation; another example that the longer bond is not always the weaker bond. Based on our findings we provide new guidelines for the general characterization of CC bonds based on local vibrational CC stretching force constants and for future design of compounds with long C−C bonds. 

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2. Dimerization of indenocorannulene radicals: imposing stability through increasing strain and curvature

   https://doi.org/10.1039/D0QO00686F  

   Rogachev, Andrey Yu.; Zhu, Yikun; Zhou, Zheng; Liu, Shuyang; Wei, Zheng; Petrukhina, Marina A. (January 2020, Organic Chemistry Frontiers)

   null (Ed.)

   One-electron reduction of bowl-shaped indenocorannulene, C 26 H 12 , with Rb metal in THF affords [{Rb + (18-crown-6)} 2 (C 26 H 12 –C 26 H 12 ) 2− ]·4THF, as confirmed by single-crystal X-ray diffraction. The product consists of a dimeric σ-bonded dianion (C–C, 1.568(7) Å) having two endo -η 6 coordinated {Rb + (18-crown-6)} moieties (Rb–C, 3.272(4)–3.561(4) Å). The (C 26 H 12 –C 26 H 12 ) 2− dimer represents the first crystallographically confirmed example of spontaneous coupling for indenocorannulene monoanion radicals, C 26 H 12 ˙ − . Comprehensive theoretical investigation of the new dimer confirms the single σ-bond character of the linker and reveals a significant increase of both thermodynamic and kinetic stability of [σ-(C 26 H 12 ) 2 ] 2− in comparison with analogues formed by such π-bowls as corannulene and its dibenzo-derivative. The in-depth computational analysis and direct comparison of the series demonstrates the effect of curvature on radical coupling processes, allowing control over stability and reactivity of bowl-shaped π-radicals. 

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3. One-electron bonds in copper–aluminum and copper–gallium complexes

   https://doi.org/10.1039/D2SC01998A  

   Graziano, Brendan J.; Scott, Thais R.; Vollmer, Matthew V.; Dorantes, Michael J.; Young, Victor G.; Bill, Eckhard; Gagliardi, Laura; Lu, Connie C. (June 2022, Chemical Science)

   Odd-electron bonds have unique electronic structures and are often encountered as transiently stable, homonuclear species. In this study, a pair of copper complexes supported by Group 13 metalloligands, M[N(( o -C 6 H 4 )NCH 2 P i Pr 2 ) 3 ] (M = Al or Ga), featuring two-center/one-electron (2c/1e) σ-bonds were synthesized by one-electron reduction of the corresponding Cu( i ) ⇢ M(III) counterparts. The copper bimetallic complexes were investigated by X-ray diffraction, cyclic voltammetry, electron paramagnetic spectroscopy, and density functional theory calculations. The combined experimental and theoretical data corroborate that the unpaired spin is delocalized across Cu, M, and ancillary atoms, and the singly occupied molecular orbital (SOMO) corresponds to a σ-(Cu–M) bond involving the Cu 4p z and M n s/ n p z atomic orbitals. Collectively, the data suggest the covalent nature of these interactions, which represent the first examples of odd-electron σ-bonds for the heavier Group 13 elements Al and Ga. 

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4. Synthesis, Crystal and Electronic Structure of Ca ₄ CdIn ₂ Ge ₄ : A Quaternary Variant of the Monoclinic Mg ₅ Si ₆ Structure

   https://doi.org/10.1002/zaac.202500006  

   Ghosh, Kowsik; Bobev, Svilen (May 2025, Zeitschrift für anorganische und allgemeine Chemie)

   Reported is the synthesis of a new polar intermetallic phase, Ca₄CdIn₂Ge₄, crystals of which can be readily obtained employing the In‐flux method. The structure and the chemical composition of the new compound are established based on single‐crystal X‐Ray diffraction and energy‐dispersive X‐Ray spectroscopy data. Ca₄CdIn₂Ge₄crystallizes in a monoclinic crystal system with the space groupC2/m(no. 12) with lattice parametersa = 16.7383(12) Å,b = 4.4235(3) Å,c = 7.4322(5) Å, andβ = 106.560(1)°. The structure can formally be classified as a variant of the Mg₅Si₆structure type (Pearson symbolmS22). Considering the InGe and CdGe interactions as mostly covalent, the polyanionic substructure can be rationalized as consisting of ribbons of edge‐shared [InGe₄] tetrahedra connected by Ge₂dimers and bridged by Cd atoms in nearly square‐planar environment. Chemical bonding analysis based on TB‐LMTO‐ASA calculations affirms the notion for covalent character of the GeGe bonding with the dimers. The calculations also show that the bonding in the tetrahedra is more covalent in character than the bonding in square‐planar fragments, with the CaGe interactions being the least covalent among all interactions, though not exactly ionic. 

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5. Clarifying the quantum mechanical origin of the covalent chemical bond

   https://doi.org/10.1038/s41467-020-18670-8  

   Levine, Daniel S.; Head-Gordon, Martin (December 2020, Nature Communications)

   null (Ed.)

   Abstract Lowering of the electron kinetic energy (KE) upon initial encounter of radical fragments has long been cited as the primary origin of the covalent chemical bond based on Ruedenberg’s pioneering analysis of H $${}_{2}^{+}$$ 2 + and H 2 and presumed generalization to other bonds. This work reports KE changes during the initial encounter corresponding to bond formation for a range of different bonds; the results demand a re-evaluation of the role of the KE. Bonds between heavier elements, such as H 3 C–CH 3 , F–F, H 3 C–OH, H 3 C–SiH 3 , and F–SiF 3 behave in the opposite way to H $${}_{2}^{+}$$ 2 + and H 2 , with KE often increasing on bringing radical fragments together (though the total energy change is substantially stabilizing). The origin of this difference is Pauli repulsion between the electrons forming the bond and core electrons. These results highlight the fundamental role of constructive quantum interference (or resonance) as the origin of chemical bonding. Differences between the interfering states distinguish one type of bond from another. 

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