The Pi-Complex Formed by Acetylene and HCl

The HCl is polar and the H is acidic, that is, positively charged. The acidic hydrogen approaches the electron density of acetylene's triple bond side on. Note in particular that the acetylene structure is hardly affected by the HCl coordination. Acetylene is still perfectly straight.


Figure. The MP2(full)/6-31G* optimized structure of the pi-complex formed between acetylene and hydrogenchloride.