In the presence of excess proton, the carbonyl on H_2Fe(CO)_4 is activated making the iron center more sensitive to the nucleophilic attack by phosphine. During the reaction of H_2Fe(CO)_4 with phosphine in given conditions, elimination of hydrogen first takes place to form Fe(CO)_4(PR_3), which then reacts with the second phosphine to give trans-Fe(CO)_3(PR_3)_2 in high yield. The process has been proved by the reaction of the intermediate Fe(CO)_4(PPh_3) with PPli3 in the presence of excess proton.

Key words: SUBSTITUTION REACTION, IRON COMPOUNDS, CARBONYL COMPOUNDS, NUCLEOPHILIC REACTION