Copper and its compounds are also known to catalyze and
photocatalyze NO transformations( 110 ).

B.4. Cobalt NO complexes

Cobalt is present in human organisms in the form of vitamin
B 12 derivatives, and the physiological actions of NO on vitamin
B 12 derivatives were intensively studied( 54 ). It was reported
that the reduced form of vitamin B 12 , [Cbl(II)], reacts with
gaseous NO to form a stable nitrosyl complex.

½ ŠþCbl IIðÞ NO^ !½ Š!Cbl NOðÞ½ ŠðCbl IIIðÞNO
 22 Þ

The large value of the formation constant for this complex
indicates that the binding of NO by cob(II)alamin is very effi-
cient, and thus may account for the observed physiological
effects. In analogy to porphyrin Co(II) nitrosyl complexes, the
cob(II)alamin nitrosyl adduct was formally described as [Cbl
(III)NO
].
Irradiation of the [Cbl(III)NO
] complex by visible light in
aqueous solution leads to the formation of Cbl(II) and free NO,
after which re-formation of [Cbl(III)NO
] could be observed
(54,60,111).

½ŠCbl IIIðÞNO

!

hvðÞ532 nm

½ ŠþCbl IIðÞ NO!½Cbl IIIðÞNO
Šð 23 Þ

The reaction between aquacobalamin, [Cbl(H 2 O)], and NO at low
pH was studied repeatedly and it is now accepted that under
such conditions nitrous acid impurities are responsible for the
reaction observed between vitamin B 12 and NO( 112 – 118 ).
The reaction of NO with [CoII(TPPS)] (where TPPS¼tetra-
meso-(4-sulfonatophenyl)porphinate) was found to approach
diffusion limited rates, similarly to the reaction with
the [FeII(TPPS)] and [FeII(TMPS)] (TMPS=tetrameso
(sulfonatomesityl)porphinate)( 68 ).
An untypical behavior of gaseous NO in the presence of
[CoII(salen)] nanoparticles was reported. Under these conditions
NO underwent disproportionation generating different NxOy
products ( 119 ).

B.5. NO complexes of other transition metals

Thermally stable chromium(III) dinitrito complexes undergo
photocleavage of the CrO

O bond in coordinated nitrite leading
to the generation of NO and a CrIV¼¼O intermediate.

METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 313