Toward the Mechanism of Perchlorinated Cyclopentasilane (Si₅Cl₁₀) Ring Flattening in the [Si₅Cl₁₀·2Cl]^2– Dianion

Abstract

We report the detailed computational study of flattening of the puckered Si ring by suppression of the pseudo-Jahn-Teller (PJT) effect through coordination of two Cl anions to the molecule forming an inverse sandwich dianion [SiCl·2Cl] complex. The PJT effect that causes nonplanarity of the SiCl structure (C) results from vibronic coupling of pairs of occupied molecular orbitals (OMOs) and unoccupied molecular orbitals (UMOs). It was shown that filling the intervenient molecular orbitals of puckered SiCl with valent electron pairs of Cl donors suppresses the PJT effect, with the Si ring becoming planar (D) upon complex formation. In this paper, the stabilization energy E(2) associated with donor-acceptor charge transfer (delocalization) was estimated using NBO analysis for all studied inverse sandwich compounds [SiCl·2X] (where X = F, Cl, Br). It was found that the polarizability of the donor ion might significantly affect the stabilization energy value and should be taken into account when choosing the ligands suitable for forming Si-based one-dimensional compounds and other nanoscale materials.

Halide‐Ion Diadducts of Perhalogenated Cyclopenta‐ and Cyclohexasilanes

“…Most key structural parameters of [Si 5 Cl 10 · 2Cl] 2– have been correctly predicted by a recent DFT study . The most significant deviation between the computed and experimentally determined values is evident for the Cl – ··· Cl – distance (calcd: 4.215 Å vs. exp: 4.292 Å).…”

“…Nevertheless, also the DFT calculations unequivocally confirm much farther separated Cl – ions in the Si 5 compared to the Si 6 diadduct. To rationalize the planar conformation of the Si 5 ring in [Si 5 Cl 10 · 2Cl] 2– , the authors offer a similar explanation as previously discussed for the [Si 6 X 12 · 2 X ] 2– diadducts …”

Tetrel Bonding Interactions in Perchlorinated Cyclopenta- and Cyclohexatetrelanes: A Combined DFT and CSD Study

“…In fact, crystallographic studies [ 35 , 36 , 37 ] have shown that perhalogenated cyclohexasilanes can strongly bind electron-rich moieties, such as halide anions or organocyanides (such as acetonitrile). Several theoretical studies [ 38 , 39 , 40 , 41 ] have explored this possibility by theoretically analyzing a series of anion/lone pair-Si inverted sandwiched complexes and confirming their ability to behave as efficient ditopic anion receptors.…”

Mechanism of Charged, Neutral, Mono-, and Polyatomic Donor Ligand Coordination to Perchlorinated Cyclohexasilane (Si6Cl12)

“…Another common property characteristic of both halogenated cyclosilanes is Si 6 (or Si 5 ) ring flattening upon donors’ coordination that was also observed in charged complexes as well as in the neutral ones. The suppression of ring puckering effect as well as the origin of Si hyper-coordination in chlorinated cyclic silanes has been extensively studied. − Using density functional theory (DFT) methodology it was shown that for both chlorinated CPS and CHS the Si ring distortion is presumably a consequence of pseudo-Jahn–Teller (PJT) effect that occurs due to a vibronic coupling between pairs of occupied (OMOs) and unoccupied molecular orbitals (UMOs).…”

“…One of the strongest arguments in favor of PJT effect-driven Si-ring puckering in halogenated cyclosilanes is a possibility of its suppression by coordination of two donor ions (e.g., F, Cl, Br, and I anions) below and above the midring. Thus the DFT modeling has also shown that filling the Si 5 Cl 10 and Si 6 Cl 12 UMOs engaged in vibronic coupling with electron pairs of Cl – donors upon coordination enables complete suppression of the PJT effect (experimentally observed flattening of Si 6 ring) and formation of inverse sandwich compounds. , It was presumed that the formation of the Lewis acid site in the middle of Si 6 ring, on which the apical donors’ electrons are partially accommodated, occurs due to withdrawal of a significant portion of electron density from the Si atoms of the ring by 12 halogens. , …”