would occur more readily in THF than in methanol. In fact, the reverse is observed
for P(n-Bu)3 and PMe2Ph (vide infra, Table 2). Actually, in methanol, all the IR
carbonyl absorption bands of K+[HFe(CO)4]− are shifted to higher frequencies
(Table 1). The Fe–CO bonds are weaker, and hence CO substitution occurs more
readily in hydroxylic solvents than in THF.
In other words, in protic media, the back-bonding onto the carbonyl ligands of
K+[HFe(CO)4]− is less important than in THF. This observation suggests an
interaction between K+ and the charge on the [Fe–H]− moiety. Comparison of the
1H NMR hydride chemical shifts of K+[HFe(CO)4]− in CD3OD and in THF-d8
(Table 1) shows that the hydridic character is more pronounced in methanol than
in THF, in agreement with a more important back-bonding to the CO ligands in
the latter (IR data). This observation leads to the proposed representations for
K+[HFe(CO)4]− in THF and in protic media (Fig. 3).