[HFe(CO)3{P(OPh)3}]− (vide supra). An electrostatic interaction between the
hydride and the ammonium centre is also apparent. Interestingly, this interaction is
still present in solution (CD3CN), as evidenced by a strong (36%) NOE effect
between the methyl groups of the ammonium centre and the hydride.
The series of zwitterionic hydridotricarbonylferrates has also been extended to a
phosphinite–ammonium ligand ‘‘valphosium’’ derived from valinol (Eq. (18)) [43].
(18)
This N,N,N-trimethyldiphenylvalphosium hydridotricarbonylferrate exhibits spectroscopic
properties similar to those of the corresponding ephosium hydridotricarbonylferrate
(Eq. (17)) and similar structural features (in particular the cis H–Fe–P
arrangement) are likely.
Variable-temperature NMR study (CD3CN) of the zwitterionic ephosium and
valphosium hydridotricarbonylferrates showed dependence of their 2JPH coupling
constants with temperature [43]. A similar behaviour has been previously observed
only for (phosphane) hydridotricarbonylferrates having the trans H–Fe–P geometry
(vide supra). These zwitterionic complexes are thus special hydridotricarbonylferrates
displaying both a cis H–Fe–P geometry and a dependence of the 2JPH coupling
constant with temperature: the cis geometry could be dictated by a stabilizing effect
of the electrostatic interaction which could not take place in a trans form because
of the steric hindrance of the equatorial CO ligands. These coupling constant
variations could reveal a variation of the H–Fe–P angles with temperature-dependent
average opening of a seven-membered chelation ring with a labile electrostatic edge
HFe−,N+ [43].