corresponds to the total number of (dþp*NO) electrons) that
allows for a general classification of transition metal nitrosyls
without the need to define an exact electronic structure. Another
complication of the coordination chemistry of NO is that many
transition metal complexes catalyze the oxidative or reductive
transformation of NO into other NOxspecies, most prominently
nitrite (NO 2 ), nitrate (NO 3 ), hyponitrite, nitrous oxide (N 2 O),
or HNO( 32 ).
NO release takes place in the redox reaction backward to the
synthetic procedure induced thermally by UV/visible or even by
near infrared (NIR) radiation. There is also a possibility of the
gaseous NO release by a homolytic cleavage of the N O bond
in some nitrito complexes.
½LnMONO!hv
½ðLnMOþNO 10 ÞSuch a behavior was observed in the case of the [M(TTP)(ONO)]
complexes of Mn(III), Cr(III), and the Mo(TPP)(O)(ONO) complex
of Mo(V) (TPP¼tetraphenylporphyrin)( 33 ).
The special role of transition metal NO complexes, or metal
nitrosyls, in the environment consists in the photoreversibility
of NO binding, as consequence of which the compounds are sig-
nificant photocatalysts enabling NO cycling(34,35). Moreover,
NO is one of the most important signaling and regulatory mole-
cule in all animal and plant organisms. In contact with biofluids,
a main sink for NO occurs through the very fast reactions with
oxymyoglobin, oxyMb, and oxyhaemoglobin, oxyHb, giving
NO 3 . Also bio-relevant is the use of the so-called NO-donor
drugs: N-hydroxyurea ((OH)NHCONH 2 ), hydroxylamine
(NH 2 OH), sodium nitroprusside (Na 2 [Fe(CN) 5 NO]), organic
nitrates and nitrites (first of all glyceryl trinitrate, C 3 H 5 (ONO) 3 ),
etc., which must be metabolized in the body through oxidative or
reductive processes up to NO generation in order to be rendered
physiologically active (32,36– 40 ).
B.1. Iron NO complexes
Most of the target receptors of NO contain iron centers( 34 ).
This behavior illustrates the important role of iron complexes
in the binding, storage, transport, and release of NOxspecies in
biological and environmental processes, controlling the local
atmospheric nitrosity.
It is known since long that aqua complexes of Fe(II) are able to
bind gaseous NO producing reversibly nitrosyl complexes:
304 ZOFIA STASICKA