Effect of Substituents on the Strength of A−Cl^-(A = Si, Ge, and Sn) Bonds in Hypervalent Systems:  ACl₅^-, ACl₄F^-, and A(CH₃)₃Cl₂^-

Abstract

The gas-phase strengths of the A-Cl(-) bonds in ACl(5)(-), ACl(4)F(-), and A(CH(3))(3)Cl(2)(-) (A = Si, Ge, and Sn) have been determined by measuring thresholds for collision-induced dissociation in a flowing afterglow-tandem mass spectrometer. Bond dissociation energies increase in the order Si < Ge < Sn. Replacement of the three equatorial chlorides with methyl groups weakens the bonds, while replacing one axial chloride with a fluoride strengthens the bonds. Computational results using the B3LYP model with several basis sets parallel the experimental periodic trends, but provide bond dissociation energies lower than experiment by 7-44 kJ mol(-1). MP2 computational results are in better agreement with experiment. The results are consistent with steric hindrance and electrostatic effects playing significant roles in the bonding energetics.

Bond strengths in POCl3−, POCl2−, and PSCl2−

“…The calculated values are 13-39 kJ mol −1 below experiment; the G3 calculations are in better agreement than the B3LYP calculations. B3LYP/aug-cc-pVTZ calculations of P Cl − bond strengths in 10-electron complexes are in good agreement with experiment [14], but corresponding bond strengths in group 14 complexes such as ACl 5 − (A = Si, Ge, Sn) are 10-44 kJ mol −1 lower than experimental results [31]. The reason for these inconsistent results is not clear.…”

Synthesis of Chlorosilicates

“…[5,6] The pentachlorosilicate was detected in the mass spectrum of SiCl 4 . [7] Although the formation of a contact ion pair between SiF 4 and CsF is observed under matrix conditions, SiCl 4 and CsCl show no reaction. [8] The formation of mixed chlorofluorosilicates from chloride salts and SiF 4 , however, has been reported.…”

Electrophilic Aromatic Formylation with Difluoro(phenylsulfanyl) methane

“…We propose that the mechanism proceeds through a short-lived fluoro-(phenylsulfanyl)methylium cation (1a-cation) which is immediately trapped by chloride ion from the SnFCl 4 anion leading to 1b. 25 Under excess Lewis acid, 1b immediately undergoes further fluoride abstraction by either SnFCl 3 or SnCl 4 , resulting in the formation of an α-chloro thionium ion (1c-cation) as an active formylating species. Although fluorine is known to stabilize carbocation centers through back-bonding, under the conditions of excess SnCl 4 the stronger Sn-F bond (Sn-F 414 vs Sn-Cl 323 kJ/mol) drives the reaction to a single α-chloro thionium ion intermediate.…”