A density functional theory study of η² acyl bonding in Fe and Mn carbonyl complexes

The stability of the η²-acyl-Fe(CO)₃X (acyl = CH₃C(O), C₂H₅C(O), C(CH₃)₃C(O); X = H, I, CH₃) complexes relative to the respective η¹-acyl-Fe(CO)₃X complex and their alkyl isomers has been studied using density functional theory (DFT). The effect of an acetyl versus a propionyl group, and the effect of the nature of the substituent, X, on the stability of various isomers is considered. For the CH₃ substituent the effect of a t-valeryl ((CH₃)₃CC(O)) group was also probed. The η²-(CH₃)₃CC(O)Fe(CO)₃CH ₃ complex is the most stable of the iron complexes studied. The effect of the substituents on the energies of the molecular orbitals of the complexes provides insight into the energy ordering of the various isomeric structures. DFT calculations were also performed on the η²-CH₃C(O)Mn(CO)₄ complex and its isomers. Good agreement is achieved with regard to these calculations and available experimental and theoretical data. Predictions are presented for the stability of η²-acyl-Cr complexes.