Nitric Oxide as a Paramagnetic Ligand

The singlet (S = 0) state lies about 1000 cm above the ground state triplet (S = 1) in the EPR spectrum of free dioxygen. Transitions associated with triplet oxygen in solution are detectable by EPR at low temperatures, but dioxygen complexes with even electron metal centers (e.g., ferroheme) are not generally observable by this method. Usually, only odd electron systems (Kramers systems) are detectable by magnetic resonance. 

Carbon monoxide has 14 electrons, which pair to give a net spin of zero. Carbon monoxide complexes of transition metals, like oxygen complexes, cannot convert an even electron system to an odd electron system. In the case of iron, CO usually binds only to ferrous ions, which have six 3d electrons. As a consequence, CO complexes and O2 complexes with iron-containing proteins are generally not detectable by EPR. 

Nitrous oxide has 15 electrons. Transition metal complexes of NO may be pictured as having spins of S , + where S , is the spin of the metal center alone. 

Since the spin of contributed by the NO may add or subtract from the spin of the metal center, a variety of possibilities are open. 

A characteristic of the magnetic resonance spectra of nitrosyl complexes is the presence of hyperfine interactions between the unpaired electrons and the nuclear spin of the nitrogen. The strength and orientation dependence of these interactions are determined by the geometry of the complex and the localization of unpaired electrons in ligand and metal orbitals. 

---