Examining the structural changes in Fe2(CO)9 under high external pressures by Raman spectroscopy

Pressure-induced structural changes in di-iron nonacarbonyl [Fe-2(CO)g] were examined by in situ Raman spectroscopy with the aid of a diamond anvil cell. Our results indicate that Fe-2(CO)(9) undergoes a pressure-induced phase transformation at about 0.9 GPa. Upon further compression, another structural transformation is identified at 7 GPa. In the low-pressure phase below 0.9 GPa, the pi back-bonding between metal and carbonyl increases with increasing pressure. In the high-pressure phase above 7 GPa, the combination of high-pressure and laser irradiation induces a change in structure from Fe-2(CO)(9) to Fe-2(CO)(8). Fe-2(CO)(8) appears to adopt a structure with C-2v rather than D-3d or D-2h symmetry. The metal-metal bond is gradually weakened under high pressures, and Fe-2(CO)(8) eventually decomposes by breaking the Fe-Fe bond when compressed up to 17.7 GPa.