Effect of substituents on the strength of A-Cl- (A = Si, Ge, and Sn) bonds in hypervalent systems: ACl5-, ACl4F-, and A(CH3)3Cl2-.

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.