We investigate a reaction of boron trichloride (BCl 3 ) with iron(III) hydroxide (Fe(OH) 3 ) by ab initio quantum chemical calculation as a simple model for a reaction of iron impurities in BCl 3 gas. We also examine a reaction with water. We find that compounds such as Fe(Cl)(OBCl 2 ) 2 (OHBCl 2 ) and Fe(Cl) 2 (OBCl 2 )(OHBCl 2 ) are formed while producing HCl and reaction paths to them are revealed. We also analyze the stabilization mechanism of these paths using newly-developed interaction energy density derived from electronic stress tensor in the framework of the Regional DFT (Density Functional Theory) and Rigged QED (Quantum ElectroDynamics).