We report the detailed computational study of flattening of the puckered Si-5 ring by suppression of the pseudo-Jahn-Teller (PJT) effect through coordination of two C1(-) anions to the molecule forming an inverse sandwich dianion [Si5Cl10-2Cl](2-) complex. The PJT effect that causes nonplanarity of the Si5Cl10 structure (C-s,) 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 Si5Cl10 with valent electron pairs of Cl(-)donors suppresses the PJT effect, with the Si5 ring becoming planar (D-5h) 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 [Si5Cl10.2X](2-) (where X = F, CI, 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.