and by the reaction between NO 2 and O 3 in which the NO 3 radi-
cal is formed that is a key compound in nighttime atmospheric
chemistry. NO 3 reacts with various VOCs and oxidizes them
rapidly ( 28 ).
NO forms complexes with almost all transition metal ions and
these compounds have unique physical and chemical properties
imparted by the presence of the strongly electron-withdrawing
NO ligands( 29 ). The reactions of NO play an important role in
environmental and biological processes, since they are
implicated in a number of diseases coupled to its over- or under-
production. Transition metal complexes can be both efficient NO-
scavengers and NO-donors, and thus belong to the important
agents capable of regulating the NO-level in nature. The most
characteristic feature of the interaction between NO and metal
complexes is the redox character of nitrosyl complex formation
accompanied by reduction or oxidation of the metal center and
coordination of the NOþ or NO ligand, respectively. This is
related to the unique dual nature of the NO molecule that can
act as a ligand and/or as a redox partner due to its radical char-
acter in the ground state. As a consequence, NO is formally
oxidized to NOþby the metal ion that is reduced by one electron
(reductive nitrosylation) as in met-myoglobin, where the FeIII
center binds NO in the form of FeII NOþ. The reaction of
reduced vitamin B 12 with gaseous NO to give CoIIbound to NO
in the form of CoIII NO represents an example of oxidative
nitrosylation ( 30 ).
Redox-active metal ions may assist in the nitrosylation of
organic substrates such as alcohols, amines, and thiols:
NOþMnþþEH!ENOþMð Þþn^1 þHþ ð 9 Þwhere E¼RO, R 2 N, RS. In such reductive nitrosylation
reactions, NO is formally oxidized to NOþby the metal ion that
is reduced by one electron( 31 ).
NO is a fascinating diatomic radical in the context of coordina-
tion chemistry due to its notorious non-innocent behavior in
transition metal complexes. For example, NO adducts of ferrous
iron complexes could have electronic structures that vary all
the way from a Fe(I)NOþto a Fe(III)NOextreme with the
Fe(II)NO (radical) case being intermediate. This distinction is
significant, as it can be expected that NOþ, NO (radical), and
NO will show very different reactivities. However,
characterizing the exact electronic structures of transition metal
nitrosyls is difficult, which led to the establishment of the famous
Enemark–Feltham [FeNO]x notation (the superscript x
METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 303