When this work was initiated, the only reported direct ligand substitution on
[HFe(CO)4]− was the reaction of Na+[HFe(CO)4]− with 13CO [52]. In this case
again, the role of the counterion is very important since Na+[HFe(CO)4]− readily
exchanges carbonyl ligands with 13CO in THF whereas [PPN]+[HFe(CO)4]− does
not. No proven example of direct substitution of a carbonyl ligand by a weaker
p-acceptor ligand has been reported before and disinclination towards thermal ligand
substitution has been reported for [HFe(CO)4]−, V(CO)−6 and CpMo(CO)−5 [53].