Electronic structure and FeNO conformation of nonheme iron-thiolate-NO complexes: An experimental and DFT study

Jeanet Conradie, Duncan A. Quarless, Hua Fen Hsu, Todd C. Harrop, Stephen J. Lippard, Stephen A. Koch, Abhik Ghosh

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60 Citations (Scopus)


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 [Fe II(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 Li 3PS3* 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-Melm, 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.

Original languageEnglish
Pages (from-to)10446-10456
Number of pages11
JournalJournal of the American Chemical Society
Issue number34
Publication statusPublished - 2007 Aug 29

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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