Back bonding
Advanced Chemical Bonding · Chemistry
Study notes
Free CO stretches at 2143 cm⁻¹ but Fe–CO in haemoglobin at ~1960 cm⁻¹. Explain. Step 1: CO donates its lone pair to Fe (σ donation). Step 2: Fe's filled d orbitals donate BACK into CO's empty π* orbitals. Step 3: π* is antibonding for C–O → C–O weakens, frequency drops. Step 4: But Fe–C strengthens — synergy! IR frequency measures back-bonding strength.