Electronic Structure and FeNO Conformation of Nonheme Iron−Thiolate−NO Complexes: An Experimental and DFT Study

Reactions of NO and CO with Fe(II) complexes of the tripodal trithiolate ligands NS3 and PS3*
yield trigonal-bipyramidal (TBP) complexes with varying redox states and reactivity patterns with respect
to dissociation of the diatomic ligand. The previously reported four-coordinate [FeII(NS3)]- complex reacts
irreversibly with NO gas to yield the S = 3/2 {FeNO}7 [Fe(NS3)(NO)]- anion, isolated as the Me4N+ salt. In
contrast, the reaction of NO with the species generated by the reaction of FeCl2 with Li3PS3* gives a high
yield of the neutral, TBP, S = 1 complex, [Fe(PS3*)(NO)], the first example of a paramagnetic {FeNO}6
complex. X-ray crystallographic analyses show that both [Fe(NS3)(NO)]- and [Fe(PS3*)(NO)] feature short
Fe−N(NO) distances, 1.756(6) and 1.676(3) Å, respectively. However, whereas [Fe(NS3)(NO)]- exhibits
a distinctly bent FeNO angle and a chiral pinwheel conformation of the NS3 ligand, [Fe(PS3*)(NO)] has
nearly C3v local symmetry and a linear FeNO unit. The S = 1 [FeII(PS3)L] complexes, where L = 1-MeIm,
CN-, CO, and NO+, exhibit a pronounced lengthening of the Fe−P distances along the series, the values
being 2.101(2), 2.142(1), 2.165(7), and 2.240(1) Å, respectively. This order correlates with the π-backbonding
ability of the fifth ligand L. The cyclic voltammogram of the [Fe(NS3)(NO)]- anion shows an irreversible
oxidation at +0.394 V (vs SCE), apparently with loss of NO, when scanned anodically in DMF. In contrast,
[Fe(PS3*)(NO)] exhibits a reversible {FeNO}6/{FeNO}7 couple at a low potential of −0.127 V. Qualitatively
consistent with these electrochemical findings, DFT (PW91/STO-TZP) calculations predict a substantially
lower gas-phase adiabatic ionization potential for the [Fe(PS3)(NO)]- anion (2.06 eV) than for [Fe(NS3)(NO)]-
(2.55 eV). The greater instability of the {FeNO}7 state with the PS3* ligand results from a stronger
antibonding interaction involving the metal dz2 orbital and the phosphine lone pair than the analogous orbital
interaction in the NS3 case. The antibonding interaction involving the NS3 amine lone pair affords a relatively
“stereochemically active” dz2 electron, the z direction being roughly along the Fe−N(NO) vector. As a result,
the {FeNO}7 unit is substantially bent. By contrast, the lack of a trans ligand in [Fe(StBu)3(NO)]-, a rare
example of a tetrahedral {FeNO}7 complex, results in a “stereochemically inactive” dz2 orbital and an
essentially linear FeNO unit.