Planar Tetracoordinate Carbons in Allene-Type Structures

Energetics and spectroscopic studies of CNO (-) (H 2 O) n $$ {\mathbf{CNO}}^{\left(\hbox{-} \right)}{\left({\mathbf{H}}_{\mathbf{2}}\mathbf{O}\right)}_{\mathbf{n}} $$ clusters and the temperature dependencies of the isomers: An approach based on a combined recipe of parallel tempering and quantum chemical methods

A system associated with several number of weak interactions supports numerous number of stable structures within a narrow range of energy. Often, a deterministic search method fails to locate the global minimum geometry as well as important local minimum isomers for such systems. Therefore, in this work, the stochastic search technique, namely parallel tempering, has been executed on the quantum chemical surface of theCNO (-) (H 2 O) n $$ {\mathrm{CNO}}^{\left(\hbox{-} \right)}{\left({\mathrm{H}}_2\mathrm{O}\right)}_n $$ system forn = 1 $$ n=1 $$ -8 to generate global minimum as well as several number of local minimum isomers. IR spectrum can act as the fingerprint property for such system to be identified. Thus, IR spectroscopic features have also been included in this work. Vertical detachment energy has also been calculated to obtain clear information about number of water molecules in several spheres around the central anion. In addition, in a real experimental scenario, not only the global but also the local minimum isomers play an important role in determining the average value of a particular physically observable property. Therefore, the relative conformational populations have been determined for all the evaluated structures for the temperature range between 20K and 400K. Further to understand the phase change behavior, the configurational heat capacities have also been calculated for different sizes.

InnTl4-nH+ (n = 0∼4): Tetracoordinate Hydrogen in a Planar Fashion?

The recent report of planar tetracoordinate hydrogen (ptH) in In4H+ is very intriguing in planar hypercoordinate chemistry. Our high-level CCSD(T) calculations revealed that the proposed D4h-symmetric ptH In4H+ is a first-order saddle point with an imaginary frequency in the out-of-plane mode of the hydrogen atom. In fact, at the CCSD(T)/aug-cc-pV5Z/aug-cc-pV5Z-PP level, the C4v isomer, with the H atom located 0.70 Å above the In4 plane, is 0.5 kcal/mol more stable than the D4h isomer. However, given the small perturbation from planarity and essentially barrierless C4v ↔ D4h ↔ C4v transition, the vibrationally averaged structure can still be considered as a planar. Extending our exploration to the InnTl4-nH+ (n = 0-3) systems, we found all these ptH structures, except for In2Tl2H+, to be the putative global minimum. The single σ-delocalized interaction between the central hydrogen atom and InnTl4-n ligand rings proves pivotal in establishing planarity and aromaticity and conferring substantial stability upon these rule-breaking ptH species.

Revisiting the Structure and Bonding in Li5H6- and the Exploration of Reactivity: Planar Pentacoordinate Hydrogen

Recently, Guha and co-workers (Sarmah, K.; Kalita, A.; Purkayastha, S.; Guha, A. K. Pushing The Extreme of Multicentre Bonding: Planar Pentacoordinate Hydride. Angew. Chem. Int. Ed. 2024, e202318741) reported a highly intriguing bonding motif: planar pentacoordinate hydrogen (ppH) in Li5H6-, featuring C2v symmetry in the singlet state with two distinct H-Li (center-ring) bond distances. We herein revisited the potential energy surface of Li5H6- by using a target-oriented genetic algorithm. Our investigation revealed that the lowest-energy structure of Li5H6- exhibits a ppH configuration with very high D5h symmetry and a 1A1' electronic state. We did not find any electronic effect like Jahn-Teller distortion that could be responsible for lowering its symmetry. Moreover, our calculations demonstrated significant differences in the relative energies of other low-lying isomers. An energetically very competitive planar tetracoordinate hydrogen (ptH) isomer is also located, but it corresponds to a very shallow minimum on the potential energy surface depending on the used level of theory. Chemical bonding analyses, including AdNDP and EDA-NOCV, uncover that the optimal Lewis structure for Li5H6- involves H- ions stabilized by the Li5H5 crown. Surprisingly, despite the dominance of electrostatic interactions, the contribution from covalent bonding is also significant between ppH and the Li5H5 moiety, derived from H-(1s) → Li5H5 σ donation. Magnetically induced current density analysis revealed that due to minimal orbital overlap and the highly polar nature of the H-Li covalent interaction, the ppH exhibits local diatropic ring currents around the H centers, which fails to result in a global aromatic ring current. The coordination of Li5H6- with Lewis acids, BH3 and BMe3, instantly converts the ppH configuration to (quasi) ptH. These Lewis acid-bound ptH complexes show high electronic stability and high thermochemical stability against dissociation and, therefore, will be ideal candidates for the experimental realization.

Strong carbon - noble gas covalent bond and fluxionality in hypercoordinate compounds

Thermodynamic, kinetic, and chemical bonding analysis at the coupled cluster level has been carried out for a series of hypercoordinated carbon compounds with formula CH4Ng2+ (Ng = He-Rn). Results show that these compounds could be stable at room temperature and Born-Oppenheimer molecular dynamics simulations (BOMD) in conjunction with activation energies indicate high kinetic stability. In addition, all chemical bonding descriptors agree with a strong C-Ng covalent bond and a bonding pattern similar to that of CH5+. Finally, BOMD simulations showed that these compounds are fluxional, with a continuous formation/breaking of H-H and C-H bonds. To the best of the authors' knowledge, these results represent the first series of fluxional compounds possessing a covalent bond between a main group element and a noble gas atom.

Searching for Systems with Planar Hexacoordinate Carbons

Here, we present evidence that the D2h M2C50/2+ (M = Li-K, Be-Ca, Al-In, and Zn) species comprises planar hexacoordinate carbon (phC) structures that exhibit four covalent and two electrostatic interactions. These findings have been made possible using evolutionary methods for exploring the potential energy surface (AUTOMATON program) and the Interacting Quantum Atoms (IQA) methodology, which support the observed bonding interactions. It is worth noting, however, that these structures are not the global minimum. Nonetheless, incorporating two cyclopentadienyl anion ligands (Cp) into the CaC52+ system has enhanced the relative stability of the phC isomer. Moreover, cycloparaphenylene ([8]CPP) provides system protection and kinetic stability. These results indicate that using appropriate ligands presents a promising approach for expanding the chemistry of phC species.

Structure and Bonding in Planar Hypercoordinate Carbon Compounds

The term hypercoordination refers to the extent of the coordination of an element by its normal value. In the hypercoordination sphere, the element can achieve planar and/or non-planar molecular shape. Hence, planar hypercoordinate carbon species violate two structural rules: (i) The highest coordination number of carbon is four and (ii) the tetrahedral orientation by the connected elements and/or groups. The unusual planar orientations are mostly stabilized by the electronic interactions of the central atom with the surrounding ligands. In this review article, we will talk about the current progress in the theoretical prediction of viable planar hypercoordinate carbon compounds. Primary knowledge of the planar hypercoordinate chemistry will lead to its forthcoming expansion. Experimental and theoretical interests in planar tetracoordinate carbon (ptC), planar pentacoordinate carbon (ppC), and planar hexacoordinate carbon (phC) are continued. The proposed electronic and mechanical strategies are helpful for the designing of the ptC compounds. Moreover, the 18-valence electron rule can guide the design of new ptC clusters computationally as well as experimentally. However, the counting of 18-valence electrons is not a requisite condition to contain a ptC in a cluster. Furthermore, this ptC idea is expanded to the probability of a greater coordination number of carbon in planar orientations. Unfortunately, until now, there are no such logical approaches to designing ppC, phC, or higher-coordinate carbon molecules/ions. There exist a few global minimum structures of phC clusters identified computationally, but none have been detected experimentally. All planar hypercoordinate carbon species in the global minima may be feasible in the gas phase.

Pentacoordinate Carbon Atoms in a Ferrocene Dication Derivative—[Fe(Si2-η5-C5H2)2]2+

Pentacoordinate carbon atoms are theoretically predicted here in a ferrocene dication derivative in the eclipsed-(1; C2v), gauche-(2; C2) and staggered-[Fe(Si2-η5-C5H2)2]2+(3; C2h) forms for the first time. Energetically, the relative energy gaps for 2 and 3 range from −3.06 to 16.74 and −2.78 to 40.34 kJ mol−1, respectively, when compared to the singlet electronic state of 1 at different levels. The planar tetracoordinate carbon (ptC) atom in the ligand Si2C5H2 becomes a pentacoordinate carbon upon complexation. The ligand with a ptC atom was predicted to be both a thermodynamically and kinetically stable molecule by some of us in our earlier theoretical works. Natural bond orbital and adaptive natural density partitioning analyses confirm the pentacoordinate nature of carbon in these three complexes (1–3). Although they are hypothetical at the moment, they support the idea of “hypercoordinate metallocenes” within organometallic chemistry. Moreover, ab initio molecular dynamics simulations carried out at 298 K temperature for 2000 fs suggest that these molecules are kinetically stable.

CSinGe4-n2+ (n = 1-3): prospective systems containing planar tetracoordinate carbon (ptC)

Density functional theory (DFT) based calculations have been carried out to explore the potential energy surface (PES) of CSi_nGe_4-n^2+/+/0 (n = 1-3) systems. The global minimum structures in the di-cationic states (1a, 1b, and 1c) contain a planar tetracoordinate carbon (ptC). For the CSi₂Ge₂²⁺ system, the second stable isomer (2b) also contains a ptC with 0.67 kcal mol^-1 higher energy than that of the 1b ptC isomer. The global minima of the neutral and mono-cationic states of the designed systems are not planar. The 1a, 1b, and 1c structures follow the 18 valence electron rule. The relative energies of the low-lying isomers of CSiGe₃²⁺, CSi₂Ge₂²⁺, and CSi₃Ge²⁺ systems with respect to the global minima were calculated using the CCSD(T)/aug-cc-pVTZ method. Ab initio molecular dynamics simulations for 50 ps time indicate that all the global minimum structures (1a, 1b, and 1c) are kinetically stable at 300 K and 500 K temperatures. The natural bond orbital (NBO) analysis suggests strong σ-acceptance of the ptC from the four surrounding atoms and simultaneously π-donation occurs from the ptC center. The nucleus independent chemical shift (NICS) showed σ/π-dual aromaticity. We hope that the designed di-cationic systems may be viable in the gas phase.

Stereoinversion of tetrahedral p-block element hydrides

The potential energy surfaces of 15 tetrahedral p-block element hydrides were screened on the multireference level. It was addressed whether stereoinversion competes against other reactions, such as reductive H₂-elimination or hydride loss, and if so, along which pathway the stereomutation occurs. Importantly, stereoinversion transition structures for the ammonium cation (C_4v) and the tetrahydridoborate anion (C_s) were identified for the first time. Revisiting methane's C_s symmetric inversion transition structure with the mHEAT+ protocol revealed an activation enthalpy for stereoinversion, in contrast to all earlier studies, which is 5 kJ mol^-1 below the C-H bond dissociation enthalpy. Square planar structures were identified lowest in energy only for the inversion of AlH₄ ^-, but a novel stepwise C_s-inversion was discovered for SiH₄ or PH₄ ⁺. Overall, the present contribution delineates essentials of the potential energy surfaces of p-block element hydrides, while structure-energy relations offer design principles for the synthetically emerging field of structurally constrained compounds.