These complexes exhibit a TBP geometry with apical trans phosphines, as previously
established by IR data [23] and confirmed by multinuclear NMR analysis [22].
Control experiments indicated that the monosubstituted derivatives
Fe(CO)4(PR3), prepared independently, do not react with PR3 under the conditions
of Eq. (3), and thus are not intermediates in the formation of the disubstituted
derivatives Fe(CO)3(PR3)2 [22].
As may be seen, the necessary reaction time (Eq. (3)) seems to be related to the
Tolman cone angle (h) of the phosphines [24] rather than to their pKa values [25].
As a matter of fact, whereas PPh3 (h=145°) and PCy3 (h=170°) do not react
with K+[HFe(CO)4]− in ethanol at room temperature, P(n-Bu)3 (h=132°) does
react (even below 5 °C) but does not lead to the corresponding
Fe(CO)3[P(n-Bu)3]2 derivative. Under these conditions, the reaction proceeds with
evolution of 2 equiv. of CO, leading to the dihydride H2Fe(CO)2[P(n-Bu)3]2
(Eq. (4)). Similarly, use of PMe2Ph (h=122°) leads to the dihydride
H2Fe(CO)2[PMe2Ph]2 [26].