Zur Möglichkeit der planaren Tetrakoordination von Elementen der IV. Hauptgruppe (C,Si, Ge)

On the Possibility of Planar Tetracoordination of the Fourth Main Group Elements (C, Si, Ge) Semiempirical MO calculations (CNDO/2, EHT) have been used to examine the ability of silicon, carbon, and germanium to form planar tetracoordinated compounds. The calculations have been performed for the tetrahedral ground state structures (T_d – symmetry) as well as for the artificial planar structures (D_4h – symmetry) of the compounds CH₄, SiH₄, GeH₄, and CF₄, SiF₄, GeF₄. A comparison between the tetrahedral and planar species showed, that all planar species are less stable. Furthermore they have larger bond distances and their bonds are stronger polarized. The possibility of the examined compounds to form planar structures increases with growing atomic number of the central atom and with increasing electronegativity of the substituents.     

Structure and stability of B+13 clusters

The structures and energies of B⁺₁₃, observed experimentally to be an unusually abundant species among cationic boron clusters, have been studied systematically with B3LYP/6–31G* density functional theory. The most thermodynamically stable B⁺₁₂ and B⁺₁₃ clusters are confirmed to have planar or quasiplanar rather than globular structures. However, the computed dissociation energies of the 3-dimensional B⁺₁₃ clusters are much closer to the experimental values than those of the planar or quasiplanar structures. Hence, planar and 3-dimensional B⁺₁₃ may both exist. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 203–214, 1998     

ChemInform Abstract: Two‐Dimensional Cu2Si Monolayer with Planar Hexacoordinate Copper and Silicon Bonding.

A new Cu₂Si two‐dimensional monolayer featuring planar hexacoordinate copper and planar hexacoordinate silicon is predicted on the basis of DFT calculations and molecular dynamic simulations.     

Planar Iron Hydride Nanoclusters: Combined Spectroscopic and Theoretical Insights into Structures and Building Principles

The controlled assembly of well-defined planar nanoclusters from molecular precursors is synthetically challenging and often plagued by the predominant formation of 3D-structures and nanoparticles. Herein, we report planar iron hydride nanoclusters from reactions of main group element hydrides with iron(II) bis(hexamethyldisilazide). The structures and properties of isolated Fe₄, Fe₆, and Fe₇ nanoplatelets and calculated intermediates enable an unprecedented insight into the underlying building principle and growth mechanism of iron clusters, metal monolayers, and nanoparticles.     

Theoretical design of planar molecules with a nona- and decacoordinate central atom

The maximum coordination number of the central atom in planar molecules generated by now in mole-cular beams was 8. We made use of the chemical bond model developed for planar boron clusters to check the possibility of existence of planar molecules with the coordination numbers 9 and 10. The objects of or study were the AlB₉ and AlB ₁₀ ⁺ clusters which have local minima corresponding to highly symmetrical D _9h and D _10h structures, respectively. According to our calculations, the highly symmetrical structure of AlB₉ is a global minimum or a low-lying isomer, and, therefore, it holds promise as a new ligand for coordination chemistry. The energy of the highly symmetrical structure of AlB ₁₀ ⁺ with the coordination number 10 is too high, and this structure is hardly synthetically feasible. Thus, 9 is presently the maximum coordination number of an atom in a planar molecule.     

Dichlorido[(S,RS)‐1‐diphenylphosphino‐2‐(ethylsulfanylmethyl)ferrocene]palladium(II)

The reaction of enantiomerically pure planar chiral ferrocene phosphine thioether with bis(acetonitrile)dichloridopalladium yields the title square‐planar mononuclear palladium complex as an enantiomerically pure single diastereoisomer, [PdFe(C₅H₅)(C₂₀H₂₀PS)Cl₂]. The planar chirality of the ligand is retained in the complex and fully controls the central chirality on the S atom. The absolute configuration, viz. S for the planar chirality and R for the S atom, is unequivocally determined by refinement of the Flack parameter.     

Raman spectra and structure of biphenyl isoatomers (the S0, S1, T1 states and the cation and anion radicals)

The Raman and transient resonance Raman spectra of biphenyl (BP) and its perdeuterated analogue (BP-d₁₀) in three different electronic states (S₀, S₁ and T₁) and in two different ionized states (cation and anion) have been recorded in solution. The S₀ Raman spectra have also been measured for the crystalline state. The obtained set of spectra are analysed on the basis of the established vibrational assignments for the ground state of the planar (crystal) and the non-planar (solution) structures. The analysis suggests that BP in solution exists as a twisted structure in the S₁ state, but that it takes planar or nearly planar structures in the T₁, the cationic and the anionic states.     

Conformational variation and crystalline phase transformation of low syndiotactic polypropylene films in stretched and stress‐relaxed states

Low syndiotactic polypropylene (sPP; rrrr = 80%) films were isothermally crystallized at 0 °C (sample S₀) and 90 °C (sample S₉₀) for 65 h, respectively. Fourier transform infrared spectroscopy, differential scanning calorimetry, and wide‐angle X‐ray diffraction were used to characterize the structure transformation and orientation behavior of samples S₀ and S₉₀ at both stretched and stress‐relaxed states. It was found that stretching (λ = 0–700%) induces the transformation of the chain conformation from helical to trans‐planar form for both S₀ and S₉₀ films. The stretched S₀ and S₉₀ samples show well oriented trans‐planar chains as well as partially retained helices. Simultaneously, crystalline phase transformation occurs during the stretching and relaxing processes of the investigated sPP samples, i.e., stable form I crystals can be transformed into metastable form III or mesophase by stretching samples, and vice versa. For stretched S₀ sample, form III with trans‐planar conformation, which generally exists in highly stretched sPP, cannot be observed, even at higher strains. For sample S₉₀, however, stretching might induce the formation of both the form III crystals and mesophase with trans‐planar chains; releasing the tension, form III again gets converted into trans‐planar mesophase and form I crystals. In the stretched and stress‐relaxed states of samples S₀ and S₉₀, the difference of the delicate orientation behavior and relative content of chain conformation and crystalline form can be attributed to the different heat‐treating methods of the low syndiotacticity sPP. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2924–2936, 2005     

Normal Coordinate Analysis in Cartesian Space II. Planar Symmetric XYn Molecules

Force constants of planar symmetric XY_n type molecules in the Cartesian space have been derived in terms of the vibrational frequencies. The force constants and normal coordinates of some planar XY₃ molecules with available isotoptic data are worked out. Further study of infrared absorption spectra of square planar XY₄ complex ions will be needed for the present normal coordinate analysis.     

Planar Hypercoordinate Carbons in Alkali Metal Decorated CE32− and CE22− Dianions

After exploring the potential energy surfaces of M_mCE₂^p (E=S−Te, M=Li−Cs, m=2, 3 and p=m-2) and M_nCE₃^q (E=S−Te, M=Li−Cs, n=1, 2, q=n-2) combinations, we introduce 38 new global minima containing a planar hypercoordinate carbon atom (24 with a planar tetracoordinate carbon and 14 with a planar pentacoordinate carbon). These exotic clusters result from the decoration of V-shaped CE₂²⁻ and Y-shaped CE₃²⁻ dianions, respectively, with alkali counterions. All these 38 systems fulfill the geometrical and electronic criteria to be considered as true planar hypercoordinate carbon systems. Chemical bonding analyses indicate that carbon is covalently bonded to chalcogens and ionically connected to alkali metals.     

Li2B12 and Li3B12: Prediction of the Smallest Tubular and Cage‐like Boron Structures

An intriguing structural transition from the quasi‐planar form of B₁₂ cluster upon the interaction with lithium atoms is reported. High‐level computations show that the lowest energy structures of LiB₁₂, Li₂B₁₂, and Li₃B₁₂ have quasi‐planar (C_s), tubular (D_6d), and cage‐like (C_s) geometries, respectively. The energetic cost of distorting the B₁₂ quasi‐planar fragment is overcompensated by an enhanced electrostatic interaction between the Li cations and the tubular or cage‐like B₁₂ fragments, which is the main reason of such drastic structural changes, resulting in the smallest tubular (Li₂B₁₂) and cage‐like (Li₃B₁₂) boron structures reported to date.     

Pseudopotential,MC-SCF and limited CI calculations on nickel-bis-dithiolene

The pseudopotential method is used to perform multiconfigurational (MC) SCF and full valence configuration interaction (CI) calculations on the dianion of nickel-bis-dithiolene, [Ni(S₂C₂H₂)₂]^2?. Both a planar (D_2h) and a near tetrahedral (D_2d) conformation are considered. Charge distributions are calculated, yielding a charge on Ni of 0.22 in the planar and 0.57 in the tetrahedral case. The experimental ground state, the planar ¹Ag state, approaches a 3d⁹ configuration on Ni (the d-population equals 8.87), while the nickel 3d-population of all other calculated states ranges from 8.35 to 8.69. The single d—d excitation spectrum for the planar complex is discussed, and the first transition is computed to be ¹Ag → ¹B_1g. Intramolecular rotation, i.e., one ligand ring rotating with respect to the other, is also discussed. The rotation is proposed to occur via a tetrahedral triplet intermediate since the relevant singlet tetrahedral state is estimated to lie more than 1 eV higher than the triplet intermediate. The discussion of the intramolecular rotation is made difficult by an error in the calculated singlet—triplet splittings. Possible causes of this error are pointed out.     

3‐Cyano‐6‐methyl‐4‐(2‐naphthyl­ethenyl)‐1‐benzo­pyran‐2‐one

In the title compound, C₂₃H₁₅NO₂, the naphthyl unit is planar and the benzo­pyran unit is nearly planar. These two moieties are inclined at an angle of 9.10 (6)° with respect to one another.     

1‐Acetyl‐3‐[1‐(4‐methoxyphenyl)‐2‐oxopropylidene]‐1H‐indol‐2(3H)‐one: a twinned crystal

The crystal of the title compound, C₂₀H₁₇NO₄, which was used for collecting intensity data was twinned. Each of the two crystallographically independent mol­ecules in the asymmetric unit has a planar indole moiety perpendicular to a planar oxo­propyl moiety. The distribution of the bonds at the central C atom joining the oxo­propyl, phenyl and indole substituents is also planar. The packing is stabilized by intermolecular C—H?O interactions, as well as by dipole–dipole and van der Waals interactions.     

Cover Feature: Rare Spin Avoided σ-σ Diradical Planar Tetracoordinate Boron Cluster: A Proto-Star for Planar Pentacoordination (ChemPhysChem 18/2023)

We report a global planar star-like cluster B₃Li₃ featuring three planar tetracoordinate boron centres with a rare spin avoided σ-σ diradical character. The cluster was found to be stable towards dissociation into different fragments. The spin density was found to be localized solely on the three boron atoms in the molecular plane. This spin avoided σ-σ diradical character leads to the extension of the coordination number to yield a neutral B₃Li₃H₃ and a cationic B₃Li₃H₃⁺ cluster with three planar pentacoordinate boron centres in their global minimum structures. The planar geometry of the aninonic B₃Li₃H₃⁻ cluster is slightly higher in energy. The planar global clusters were found to maintain planarity in their ligand protected benzene bound complexes, B₃Li₃(Bz)₃, B₃Li₃H₃(Bz)₃ and B₃Li₃H₃(Bz)₃⁺ with high ligand dissociation energies offering candidature for experimental detection.     

A system with three contiguous planar tetracoordinate carbons is viable: a computational study on a C6H6 isomer

Ab initio and density functional theory calculations indicate that a benzene dication isomer (1: C₆H₆²⁼) with three contiguous planar tetracoordinate carbons is at a minimum on the potential energy surface. The remarkable preference for the planar structure for 1 is traced to the aromatic stabilization present in the three membered ring formed by the three planar tetracoordinate carbon atoms.     

Bis(2,3‐di­methyl­quinoxalinium) tribromo­cuprate(I)

The title compound, (C₁₀H₁₁N₂)₂[CuBr₃], contains layers of planar monoprotonated cations. Isolated trigonal–planar [CuBr₃]^2? anions are hydrogen bonded to cations in adjacent layers, providing three‐dimensional stability to the crystal structure.     

Density functional theory on the pentaatomic planar tetracoordinate carbon molecules CGa3Ge and [CGa3Ge]−

The pentaatomic molecules CGa₃Ge and [CGa₃Ge]^? were studied via density functional theory (DFT). Six planar geometry isomeric structures were gained, and the global minima structures 1 contain planar tetracoordinate carbons. To gain a better understanding about which electronic factors contribute to the stabilization of planar tetracoordinate carbon structures, natural bond orbital (NBO) analysis were calculated. The results show that structures 1 are σ and π aromaticity. This analysis suggests that the presence of 18 valence electrons is crucial for planar geometries to be stable and preferred over tetrahedral structures.     

Theoretical study of aromaticity in inorganic tetramer clusters

Ground state geometry and electronic structure of M ₄ ^2- cluster (M = B, Al, Ga) have been investigated to evaluate their aromatic properties. The calculations are performed by employing the Density Functional Theory (DFT) method. It is found that all these three clusters adopt square planar configuration. Results reveal that square planar M ₄ ^2- dianion exhibits characteristics of multifold aromaticity with two delocalised π-electrons. In spite of the unstable nature of these dianionic clusters in the gas phase, their interaction with the sodium atoms forms very stable dipyramidal M₄Na₂ complexes while maintaining their square planar structure and aromaticity.     

Photoelectrochemical CO2 Reduction by a Molecular Cobalt(II) Catalyst on Planar and Nanostructured Si Surfaces

In the presence of a molecular Co^II catalyst, CO₂ reduction occurred at much less negative potentials on Si photoelectrodes than on an Au electrode. The addition of 1 % H₂O significantly improved the performance of the Co^II catalyst. Photovoltages of 580 and 320 mV were obtained on Si nanowires and a planar Si photoelectrode, respectively. This difference likely originated from the fact that the multifaceted Si nanowires are better in light harvesting and charge transfer than the planar Si surface.