The gas-phase strengths of the A-Cl- bonds in ACl(5)(-), ACl(4)F(-), and A(CH3)(3)Cl-2(-) (A = Si, Ge, and Sri) 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.