I hope all is well. Today, in my course I talked about the 18 electron-rule and gave the students a few problems to practice. I asked the students to determine if the complex Fe2(CO)9 followed the 18 electron rule. So, I go through the example and explain the the M-M bond contributes one electron. A student asked me "Where does the electron come from in the bond? Are you counting an electron twice from Fe?" I explained that I am not counting electron twice from Fe. What is the best way to explain this? I never had a student ask me where does the electron come from before. (Ha) The formation of an M-M single bond provides an extra electron for each iron. Any thoughts on helping me explain this to an undergraduate?