2.2.1. Nature of the counterion
The IR spectra ofM+[HFe(CO)4]− in THF have been thoroughly studied [38,39].
When M+ is Na+, an interaction has been evidenced between the latter and an
equatorial carbon monoxide ligand (nCO absorption band at 1854 cm−1). This interaction
makes the ‘‘axial’’ carbon monoxide ligand more labile. In contrast, when
the cation is [PPN]+, this interaction no longer occurs, and the complex is much
more resistant than Na+[HFe(CO)4]− to ligand exchange with 13CO [52]. This can
explain the lack of reactivity of [PPN]+[HFe(CO)4]− with phosphanes in most
cases. The reason why [PPN]+[HFe(CO)4]− does react with phosphites in CH2Cl2
is not clear. At last, for what concerns ephosium and valphosium ligands (vide
supra) the question whether ligand substitution occurs before or after cation metathesis
is opened.
With regard to reactions of K+[HFe(CO)4]−, the first ligand substitution generates
the anionic hydrides K+[HFe(CO)3P]−. The course of the reaction then depends
on both the nature of the solvent and the steric and electronic properties of the
phosphane.