1. 5-ish Slides about Enemark-Feltham Notation
Kyle Grice
DePaul University
November 2018

2. Iron Nitrosyl Complexes
NO ligands are redox non-innocent, which means NO has multiple accessible redox states under common conditions
In addition, Iron has common several redox and spin-states (Fe2+ vs. Fe3+, high spin or low spin).

3. NO (Yes?)
As a neutral molecule, NO is a radical with one unpaired electron
NO has been found to be a neurotransmitter and vasodialator
There has been significant research in NO-releasing molecules (NORMs) for medical applications.
Bond Order: 2.5
Bond Length: 115 pm

4. Basics of Enemark-Feltham
{M-L}X or {Fe-NO}X where X is the sum of the metal valence and ligand π* electrons.
NO+ has no π* electrons, NO has 1, and NO- has 2.
Iron can be high spin or low spin in several oxidation states!
Nitroprusside is a blood-pressure lowering compound:
Enemark-Feltham Notation: {Fe-NO}6

5. How do we get more detailed info?
Often it is hard to determine the exact oxidation state.
information can sometimes be obtained from the NO ligand:
NO+ ligands have shorter bonds because they have formally triple N-O bonds. They tend to be linear.
NO- are bent due to the extra lone pair on the nitrogen.
NO is in-between these two extremes in term s of length and angle
IR frequencies also help:
Linear: 1650 – 1900 cm-1
Bent: 1520 – 1690 cm-1
Linear Fe-N-O bonding and a short N-O distance. The molecule is diamagnetic too!
This is low spin Fe2+ with a NO+ ligand.

6. Beyond Enemark-Feltham
To deconvolute the oxidation state of the iron and the NO, various methods can be used:
Evidence from X-ray structures and IR
DFT calculations
Fe K-edge XAS
Fe Mossbauer