Hard Bends Soft: Bond Angle and Bending Force Constant Predictions for Dihalides, Dihydrides, and Dilithides of Groups 2 and 12

A “hard-bends-soft” rule is proposed as a complement to the HSAB principle. The rule completely separates the unexpected bent structures of some heavy alkaline earth (Ca, Sr, Ba) dihalides and dihydrides from the linear geometries of all of the other group 2 and 12 triatomic molecules. A simple function of chemical softness reproduces the bond angle within ±4° for the group 2 and group 12 dihalides, dihydrides, and dilithides. The bending force constants of the heavy group 2 dihalides and dihydrides correlate linearly with the atomic softness difference. Predictions are made for the bond angles and bending force constants of RaX 2 (X = H and halogens) and MAt 2 (M = group 2 and 12 elements).